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DSB/SSB Transmitter & Receiver Trainer ST2201 & ST2202

1. Connect the module s output to the external audio input on the ST2201 board and put the audio input select switch in the ext position The input signal to the audio input module may be taken from an external microphone or from a cassette recorder by choosing the appropriate switch setting on the module Refer the user manual for the audio input module for further details Scientech Technologies Pvt Ltd 29 ST2201 amp ST2202 Experiment 2 Objective To calculate modulation index of DSB wave by trapezoidal pattern Procedure 1 Perform the experiment number 1 upto step number 6 2 Now apply the modulated waveform to the Y input of the Oscilloscope and the modulating signal to the X input 3 Press the XY switch you will observe the waveform similar to the one given below V in V max Figure 30 Calculate the modulation index by substituting in the formula Percentage Modulation S max V min 4 Some common trapezoidal patterns for different modulation indices are as shown m lt m msi Figure 31 Scientech Technologies Pvt Ltd 30 ST2201 amp ST2202 Experiment 3 Objective Double Sideband AM Reception Procedure This experiment investigates the reception and demodulation of AM waveforms by the ST2201 ST2202 module Both AM broadcast signals and AM transmissions from ST2201 will be examined and the operation of automatic gain control at the receiver will be investigated
2. To avoid unnecessary loading of monitored signals X10 oscilloscope probes should be used throughout this experiment 1 Position the ST2201 amp ST2202 modules with the ST2201 board on the left and a gap of about three inches between them 2 Ensure that the following initial conditions exist on the ST2201 board a Audio oscillator s amplitude pot in fully clockwise position b Audio input select switch in INT position c Balance pot in balanced modulator amp band pass filter circuit 1 block in full clockwise position d Mode switch in DSB position e Output amplifier s gain pot in full counter clockwise position f TX output select switch in ANT position g Audio amplifier s volume pot in fully counter clockwise position h Speaker switch in ON position p o On board antenna in vertical position and fully extended 3 Ensure that the following initial conditions exist on the ST2102 board RX input select switch in ANT position b R F amplifier s tuned circuit select switch in INT position c R E amplifier s gain pot in fully clock wise position d AGC switch in INT position e Detector switch in diode position f Audio amplifier s volume pot in fully counter clockwise position g Speaker switch in ON position h Beat frequency oscillator switch in OFF position ie On board antenna in vertical position and fully extended 4 Turn on power to the modules Scientech Technologies Pv
3. Amplitude fc fm 1 455MHz Frequency b Final SSB output from balanced modulator and band pass filter circuit 2 Figure 49 Note that since there is a large gap between the upper and lower sidebands a gap of about 910 KHz a band pass filter with a very sharp response is not needed to reject the lower sideband a simple tuned circuit band pass filter is quite sufficient 14 Now examine the output of the balanced modulator amp band pass filter circuit 2 block TP22 and check that the waveform is a good sinewave of frequency approximately 1 45MHz Scientech Technologies Pvt Ltd 51 ST2201 amp ST2202 15 16 17 This indicates that only the upper sideband is being passed by the block Check that the waveform is reasonably good sinusoid for all audio modulating frequencies i e all positions of the audio oscillator s frequency pot If this is not the case it may be that the balance pot in the balanced modulator amp band pass filter circuit 2 blocks needs adjusting to remove any residual carrier component at 1 MHz If a reasonably clean sinewave still cannot be obtained for all audio frequencies then the response of the tuned circuit band pass filter needs adjusting This is achieved by adjusting transformer T4 in the balanced modulator amp bandpass filter circuit 2 block To do this follow the procedure given in chapter adjustment of the transmitter s tuned circuits Once the signal at TP22 is a
4. which transmits at 800 KHz the local oscillator will be running at 1 25 MHz The difference frequency is 1 255MHz 800 MHz 455 KHz Scientech Technologies Pvt Ltd 21 ST2201 amp ST2202 If the radio is now re tuned to receive a different station being broadcast on 700 KHz the tuning control re adjusts the RF amplifier to provide maximum gain at 700 KHz and the local oscillator to 1 155 MHz The difference frequency is still maintained at the required 455 KHz This frequency difference therefore remains constant regardless of the frequency to which the radio is actually tuned and is called the intermediate frequency IF Local oscillator frequency Amplitude IF frequency RF frequency 0 455 800 1255 Frequency kHz Figure 21 Note In Figure 21 the local oscillator output is shown larger than the IF and RF frequency components this is usually the case However there is no fixed relationship between the actual amplitudes Similarly the IF and RF amplitudes are shown as being equal in amplitude but again there is no significance in this Image Frequencies In the last section we read we could receive a station being broadcast on 700 KHz by tuning the local oscillator to a frequency of 1 155 MHz thus giving the difference IF frequency of required 455 KHz What would happen if we were to receive another station which was broadcasting on a frequency of 1 61 MHz This would also mix with the
5. 545 KHz Since these two sidebands are separated by a wide frequency range the filter design is not critical and a simple parallel tuned circuit is sufficient The 1 455 MHz output signal then only requires amplification before transmission lower sideband lower sideband filtered out filtered out x x A J on Audio Balanced pees mod EN input ee gt 2 ace eat Modulator Bandpass Filter 2 isb pee ae n n approx Approx 455kHz 1MHz Figure 41 Scientech Technologies Pvt Ltd 44 ST2201 amp ST2202 Experiment 5 Objective Signal Sideband AM Generation Procedure This experiment investigates the generation of signal sideband SSB amplitude modulated waveforms using the ST2201 module To avoid unnecessary loading of monitored signals X10 oscilloscope probes should be used throughout this experiment 1 Ensure that the following initial conditions exist on the board a Audio input select switch in INT position b Mode switch in SSB position c Output amplifier s gain pot in fully clockwise position d Speaker switch in OFF position Turn on power to the ST2201 board Turn the audio oscillator block s amplitude pot to its fully clockwise MAX position and examine the block s output TP14 on an oscilloscope This is the audio frequency sine wave which will be used as out modulating signal Note that the sine wave s frequency can be adjusted from about 300Hz to approximately 3 4 KHz
6. Figure 46 FREQUENCY RESPONSE SUPPRESSED OF CERAMIC BANDPASS Amplitude CARRIER FILTER CENTERED ON 455kHz I I 1 1 I I I UPPER SIDEBAND fc fm fc fm Frequency fc a Modulating frequency fm 300Hz pot in MIN position Scientech Technologies Pvt Ltd 48 ST2201 amp ST2202 A FREQUENCY RESPONSE SUPPRESSED OF CERAMIC BANDPASS Amplitude CARRIER Pa FILTER CENTERED ON 455kHz LOWER SIDEBAND UPPER SIDEBAND fc fm fc fc fm Frequency b Modulating frequency fm 3 4 KHz pot in MAX position Figure 46 Notice that since the upper sideband cuts rising edge of the filter s frequency response when fm 300 Hz there will be a certain amount of signal attenuation when the frequency pot is in its MIN position 11 Note that by passing only the upper side band of frequency fc fm all we have actually done is to shift out audio modulating signal of frequency fm up in frequency by an amount equal to the carrier frequency fc This is shown in Figure 47 Amplitude AUDIO FREQUENCY RANGE oz TO 3 4KHZ fm Frequency a Range of frequencies available from audio oscillator j Amplitude 3 SHIFTED FREQUENCY RANGE fc fm Frequency b Corresponding range of output frequencies from ceramic band pass filter block Figure 47 Scientech Technologies Pvt Ltd 49 ST2201 amp ST2202 12 13 With the audio o
7. Lower Sideband Upper Sideband Amplitude A _f O 0 997 0 9985 0 9995 1 0005 1 0015 1 003 Frequency MHz Figure 10 The Power in Sidebands The modulated carrier wave that is finally transmitted contains the original carrier and the sidebands The carrier wave is unaltered by the modulation process and contains at least two thirds of the total transmitted power The remaining power is shared between the two sidebands The power distribution depends on the depth of modulation used and is given by Total power carrier power 1 N 2 Where N is the depth of modulation The greater the depth of modulation the greater is the contained within the sidebands The highest usable depth of modulation is 100 above this the distortion becomes excessive Since at least twice as much power is wasted as is used this form of modulation is not very efficient when considered on a power basis The good news is that the necessary circuits at the transmitter and the receiver are simple and in expensive to design and construct The double sideband transmitter The transmitter circuits produce the amplitude modulated signals which are used to carry information over the transmission to the receiver The main parts of the transmitter are shown in Figure 11 In Figure 11 amp 12 we can see that the peak to peak voltage in the AM waveform increase and decrease in sympathy with the audio signal Scientech Technologies Pvt Ltd 1
8. RF amplifier gain control to its maximum position fully clockwise Set the AGC switch to the INT position Set the detector switch to the diode position Switch the beat frequency oscillator to the OFF position Switch on the Power Supply Using your dual trace oscilloscope use channel 1 to monitor the audio output signal at TP39 and to act as the trigger input for the oscilloscope Scientech Technologies Pvt Ltd 59 ST2201 amp ST2202 6 10 11 12 13 14 15 16 17 18 Adjust the volume pot in the output stage of the receiver until a sound is just audible then tune into the audio signal generated by the ST2201 fine tuning for the strongest possible signal may be required Adjust the volume pot in the output stage of the receiver to provide a 4 V peak to peak signal On the transmitter set the gain control in the output amplifier to minimum full counter clockwise Monitor the output from the AGC circuit at TP1 At the moment the voltage present is zero volts Slowly and carefully increase the amplitude of the transmitted signal by increasing the gain pot in the transmitter output amplifier as you observe the increasing sinewave at the receiver output at TP39 and the AGC signal at TP1 The receiver output will slowly increase without any change of AGC level This is the delay mentioned in the audio amplifier Further increases in transmitter level will cause the AGC volta
9. band of frequencies around 455 KHz it is not suitable for direct transmission to the receiver To overcome the problem this narrow band of frequencies must be shifted up so that it falls within the AM broad cast band This frequency shifting operation is performed by the balanced modulator amp band pass filter circuit 2 block which contains a balanced modulator followed by a tuned circuit The operation is performed in two stages 1 By amplitude modulating at 1MHz carrier sinewave with the output from the ceramic band pass filter and balancing out the carrier component This is shown in Figure 48 RANGE OF OUTPUT Amplitude FREQUENCIES OBTAINABLE FROM CERAMIC BANDPASS FILTER i 455kHz Frequency a Spectrum of output from ceramic band pass filter block Scientech Technologies Pvt Ltd 50 ST2201 amp ST2202 SUPPRESSED CARRIER Amplitude LOWER SIDEBAND UPPER SIDEBAND fe fm fc 1MHz fc fm 545kHz 1 455MHz Frequency b Spectrum obtained by modulating 1MHz carrier with output from ceramic band pass filter Figure 48 2 By passing the Upper Side and blocking the Lower Sideband using a tuned circuit band pass filter as shown in Figure 49 SUPPRESSED RESPONSE OF CARRIER TUNED CIRCUIT Amplitude A f FILTER UPPER SIDEBAND SIDEBAND fc fm fe 1MHz fc fm 545kHz 1 455MHz Frequency a Rejection of lower side band with tuned circuit band pass filter
10. be in INT position b Mode switch in DSB position c Output amplifier s gain potentiometer in full clockwise position d Speakers switch in OFF position Turn on power to the ST2201 board Turn the audio oscillator block s amplitude pot to its full clockwise MAX position and examine the block s output TP14 on an oscilloscope This is the audio frequency sine wave which will be as our modulating signal Note that the sine wave s frequency can be adjusted from about 300 Hz to approximately 3 4 KHz by adjusting the audio oscillator s frequency potentimeter Note also that the amplitude of this audio modulating signal can be reduced to zero by turning the Audio oscillator s amplitude present to its fully counter clockwise MIN position Return the amplitude present to its max position Turn the balance pot in the balanced modulator amp band pass filter circuit 1 block to its fully clockwise position It is this block that we will use to perform double side band amplitude modulation Monitor in turn the two inputs to the balanced modulator amp band pass filter circuits 1 block at TP1 and TP9 Note that a The signal at TP1 is the audio frequency sinewave from the audio oscillator block This is the modulating input to our double sideband modulator b Test Point 9 carries a sine wave of 1MHz frequency and amplitude 120mVpp approx This is the carrier input to our double sideband modulator Scientech Technol
11. by adjusting the audio oscillator s frequency pot Note That the amplitude of this audio modulating signal can be reduced to zero by turning the audio oscillator s pot to its fully counter clockwise MIN position Leave the amplitude pot on its full clockwise position and adjust the frequency pot for an audio frequency of 2 KHz approx mid way To achieve signal sideband amplitude modulation we will utilize the following three blocks on the ST2201 module a Balanced modulator b Ceramic band pass filter c Balanced modulator amp band pass filter circuit 2 We will now examine the operation of each of these blocks in detail Monitor the two inputs to the balanced modulator block at TP15 and TP6 noting that a The signal TP15 is the audio frequency sinewave from the audio oscillator block This is the modulating input to the balanced modulator block b The signal at TP6 is a sinewave whose frequency is slightly less than 455 KHz It is generated by the 455 KHz oscillator block and is the carrier input to the balanced modulator block Scientech Technologies Pvt Ltd 45 ST2201 amp ST2202 6 Next examine the output of the balanced modulator block at TP17 together with the modulating signal at TP15 trigger the oscilloscope on the modulating signal Check that the waveforms are as shown Figure 42 Figure 42 Note that it may be necessary to adjust the balanced modulator block s balance pot in ord
12. eae eee Amplitude a eee ee ver TO ae PEPEE NRE 0 Frequency Notice that thelkHz signal is no longer present Figure 9 There are two new components 1 Carrier frequency fc plus the information frequency called the upper side frequency fc fm 2 Carrier frequency fc minus the information frequency called the lower side frequency fc fm The resulting signal in this example has a maximum frequency of 1001 KHz and a minimum frequency of 999 KHz and so it occupies a range of 2 KHz This is called the bandwidth of the signal Notice how the bandwidth is twice the highest frequency contained in the information signal Sidebands If the information signal consisted of range of frequencies each separate frequency will create its own upper side frequency and lower side frequency As an example let us imagine that a carrier frequency of 1 MHz is amplitude modulated by an information signal consisting of frequencies 500Hz 105 KHz and 2 KHz As each modulating frequency produces its own upper and lower side frequency there is a range of frequencies present above and below the carrier frequency All the upper side frequencies are grouped together and referred to as the upper sideband USB and all the lower side frequencies from the lower sideband LSB This amplitude modulated wave would have a frequency spectrum as shown in Figure 10 Scientech Technologies Pvt Ltd 12 ST2201 amp ST2202 1MHz 4 Carrier
13. electromagnetic wave A vertical transmitting antenna results in a vertically polarized wave and a horizontal one would result in a horizontally polarized em wave Scientech Technologies Pvt Ltd 18 ST2201 amp ST2202 DSB Receiver The em wave from the transmitting antenna will travel to the receiving antenna carrying the information with it me ae Oe AA Antenna Wie er Loudspeaker Local Oscillator DSB Receiver Figure 18 We will continue to follow our information signal as it passes through the receiver The Receiving Antenna The receiving antenna operates in the reverse mode to the transmitter antenna The electromagnetic wave strikes the antenna and generates a small voltage in it Ideally the receiving antenna must be aligned to the polarization of the incoming signal so generally a vertical transmitting antenna will be received best by using a vertical receiving antenna The actual voltage generated in the antenna is very small usually less than 50 millivolts and often only a few microvolts The voltage supplied to the loudspeaker at the output of the receiver is upto ten volts We clearly need a lot of amplification Radio Frequency RF Amplifier The antenna not only provides very low amplitude input signals but it picks up all available transmissions at the same time This would mean that the receiver output would include all the various stations on top of each other which would make it
14. frequency is then converted into sound by the receiver s audio amplifier block To demodulate out incoming SSB signal we tune the Receiver s local oscillator so that the output frequency range form IF amplifier 2 is slightly below the 455 Scientech Technologies Pvt Ltd 56 ST2201 amp ST2202 12 KHz BFO frequency as shown in part a of the last diagram such that the difference frequency generated by the product detector is the same as the original transmitter audio modulating frequency Then as the frequency of the transmitter s modulating signal changes the output from the product detector should follow it Monitor the output of ST2202 s beat frequency oscillator block TP50 and note that this carries a sinewave of 455 KHz On the ST2201 2 receiver adjust the volume pot so that the receiver s output is clearly audible Note If desired headphones may be used instead of the on board loudspeaker To use the headphones simply plug the headphone jack into the audio amplifier block s headphones socket and put the speaker switch in the OFF position The volume from the headphones is still controlled by the block s volume pot Slowly turn the tuning dial and notice that the tone at the receiver s output changes This is because the frequency of the output signal from IF amplifier 2 changes as the dial is turned Product detector s output as the tuning dial is turned On the ST2201 module turn the volume pot in
15. is at while continuing to monitor TP3 turn the balance pot in the balanced modulator amp Scientech Technologies Pvt Ltd 67 ST2201 amp ST2202 band pass filter circuit 1 block in a clock until the monitored waveform is as shown in Figure 56 tr PMMA Figure 56 The amplitude of this waveform is minimum at points A B C etc in Figure 48 455 KHz oscillator tuned circuit On the ST2201 board monitor TP15 and TP17 triggering on TP15 check that the audio input select switch is in the INT position In the audio oscillator block turn the amplitude maximum position amp frequency pot to its centre position Adjust the balance pot in the balanced modulator block until the waveform at TP17 is as shown in Figure 55 above taking care that adjacent peaks of the waveform s envelop have the same amplitude Next examine TP15 and TP20 on the ST2201 board again triggering the oscilloscope from TPI5 Tune transformer T2 in the 455 KHz oscillator block until the waveform at TP20 is also as shown in Figure 52 Note the overall amplitude of the waveform then very slowly turn T2 clockwise until the overall amplitude is one sixth 1 6 of what it was The 455 KHz oscillator s tuned circuit should now be correctly adjusted Vary frequency of Audio oscillator low to high Drop should be symmetrical If not adjust T2 very slightly Balanced modulator amp bandpass filter circuit 2 tuned circuit Check that the aud
16. link and a receiver and in the case of speech this can be achieved by a length of cable with a microphone and an amplifier at one end and a loudspeaker and an amplifier at the other FIO Microphone Communication link a wire in this example mig Loudspeaker Amplifier Simple Communication System Figure 4 For long distances or for when it is required to send signals to many destinations at the same time it is convenient to use a radio communication system Scientech Technologies Pvt Ltd 9 ST2201 amp ST2202 Amplitude Modulation AM The method that we are going to use is called amplitude modulation As the name suggest we are going to use the information signal to control the amplitude of the carrier wave As the information signal increases in amplitude the carrier wave is also made to increase in amplitude Likewise as the information signal decreases then the carrier amplitude decreases By looking at Figure 5 below we can see that the modulated carrier wave does appear to contain in some way the information as well as the carrier We will see later how the receiver is able to extract the information from the amplitude modulated carrier wave SESAN Information signal ae Amplitude Modulator Carrier wave input A Depth of Modulation Modulated carner wave Figure 5 The amount by which the amplitude of the carrier wave increases and decreases depends on the amplitude
17. modulation is applied the signal generator carrier level is set at a convenient arbitrary level and the manual volume control of the receiver is adjusted to give the standard test output The modulation frequency is then varied over the audio range keeping degree of modulation constant A graph is then plotted in the ratio of actual output in volts to the output at 400 c s against modulation frequency as shown in Figure 54 Figure 54 Scientech Technologies Pvt Ltd 63 ST2201 amp ST2202 Experiment 8 Objective To plot selectivity curve for radio receiver Procedure 1 Setting on ST2202 a Set the detector in diode mode b AGC on c Set the volume control full clockwise Apply AM signal with 400 Hz modulating frequency and 30 modulation taken from AM generator into Rx input socket Set the input carrier frequency to suitable value that lies within the AM band 525 KHz 1600 KHz Also set signal level to 100mV Tune the Receiver using tuning control Also adjust gain potentiometer provided in R F amplifier section of ST2202 so as to get unclipped demodulated signal at detector s output output of audio amplifier Note the voltage level at receiver s final output stage i e audio amplifier s output on CRO voltage at resonance Vr Now gradually offset the carrier frequency in suitable steps of 5 KHz or 10 KHz below and above the frequency adjusted in step 2 without changing the tuning of receive
18. overloading does not occur a Turn the gain pot in ST2201 s output amplifier block so that the pots arrowhead is horizontal and pointing to the left This ensures that the amplitude of the transmitted SSB signal is small b On the ST2202 module fine tune the tuning dial until the amplitude of monitored signal at TP28 is at its greatest c Adjust the gain pot in ST2202 s RF amplifier block until the amplitude of the monitored signal is about 2 volts pk pk d Repeat steps 2 and 3 There should now be no risk of the ST2202 receiver overloading For SSB reception the following blocks of the receiver operate in the same way as they did for the reception of double sideband AM signals R F Amplifier Local Oscillator Mixer e I F Amplifier 1 LF Amplifier 2 Since we have already discussed the operation of these blocks we will only concern ourselves with how we demodulate the SSB signal from IF amplifier 2 The receiver s beat frequency oscillator BFO produces a sinewave at the IF frequency of 455 KHz This 455 KHz sine wave is input to the receiver s product detector block where it is mixed with the SSB from I F amplifier The actual frequency of the output signal from I F amplifier 2 will lie within a limited range of frequencies which lie in the region of 455 KHz The output signal can be varied over this limited range of frequencies by adjusting the frequency of the transmitter s modulating signal f
19. position Compare this signal with that at the diode detector s output TP31 and note how the audio amplifier block s low pass filter has cleaned up the audio signal You may notice that the output from the audio amplifier block tp39 is inverted with respect to the signal at the output of the diode detector TP31 this inversion is performed by the audio power amplifier IC and in no way affects the sound produced by the receiver Scientech Technologies Pvt Ltd 35 ST2201 amp ST2202 14 15 16 Now that we have examined the basic principles of operation of the ST2202 receiver for the reception and demodulation of AM broadcast signals we will try receiving the AM signal from the ST2201 transmitter Presently the gain of ST2201 s output amplifier block is zero so that there is no output from the Transmitter Now turn the gain pot in ST2201 s output amplifier block to its fully clockwise maximum gain position so that the transmitter generates an AM signal On the ST2201 module examine the transmitter s output signal TP13 together with the audio modulating signal TP1 triggering the scope with the signal Since ST2201 TX output select switch is in the ANT position the AM signal at tp13 is fed to the transmitter s antenna Prove this by touching ST2201 s antenna and nothing that the loading caused by your hand reduces the amplitude of the AM waveform at TP13 The antenna will propagate this AM signa
20. therefore follows or tracks the RF amplifier frequency This will prove to be very useful as we will see in the next section Mixer The mixer performs a similar function to the modulator in transmitter We may remember that the transmitter modulator accepts the information signal and the carrier frequency and produces the carrier plus the upper and lower sidebands The mixer in the receiver combines the signal from the RF amplifier and the frequency input from the local oscillator to produce three frequencies 1 A difference frequency of local oscillator frequency RF signal frequency 2 A sum frequency equal to local oscillator frequency RF signal frequency 3 A component at the local oscillator frequency Mixing two signals to produce such components is called a heterodyne process When this is carried out at frequencies which are above the audio spectrum called supersonic frequencies the type of receiver is called a super heterodyne receiver It is not a modern idea having been invented in the year 1917 From RF amplifier To IF amplifier we Mixer iw AAN From local oscillator Figure 20 In the section the local oscillator we read how the local oscillator tracks the RF amplifier so that the difference between the two frequencies is maintained at a constant value In ST2201 amp ST2202 this difference is 455 KHz approximately As an example if the radio is tuned to receive a broadcast station
21. use at one time it is possible that there may be interference between near by transmitters if antenna propagation is used To eliminate this problem use a cable between each transmitter receiver pair connecting it between ST2201 s TX output socket and ST2202 s RX input socket If you do this make sure that the transmitter s TX output select switch and the receiver s RX input select switch are both in the SKT position then follow the steps below as though antenna propagation were being used 8 On the ST2202 module monitor the output of the IF amplifier 2 block TP28 and turn the tuning dial until the amplitude of the monitored signal is at its greatest Check that you have tuned into the SSB signal by turning ST2201 s Scientech Technologies Pvt Ltd 54 ST2201 amp ST2202 10 11 amplitude pot in the audio oscillator block to its MIN position and checking that the monitored signal amplitude drops to zero This should occur at about 85 95 Return the amplitude pot to its MAX position Since the incoming SSB signal contains no carrier component the receiver s AGC circuit cannot make use of incoming carrier amplitude in order to control the receiver s gain This means that the receiver s AGC circuit cannot be used for SSB reception and must be switched off Consequently it is very important to avoid overloading the receiver by transmitting an SSB signal which is too large for the receiver to handle To ensure that
22. 3 ST2201 amp ST2202 On Se ee Information Signal E Audio Oscillator Antenna N Output Modulator Amplifier wi AM Waveform Amplified Output Signal WA AM Transmitter System Carrier Generator Figure 11 To emphasize the connection between the information and the final waveform a line is sometimes drawn to follow the peaks of the carrier wave as shown in Figure 12 This shape enclosed by a dashed line in out diagram is referred to as an envelope or a modulation envelope AT NAA TN Information signal Amplitude modulated wave The envelope A P sit ge Ali si 7 Figure 12 It is important to appreciate that it is only a guide to emphasize of the AM waveform Information Signal We have a choice of information signals on ST2201 We can use the signal provided in the audio oscillator or audio signal by connecting microphone to external audio input and keeping the audio input select switch in ext position In test situations it is more satisfactory to use a simple sinusoidal information signal since its attributes are known and of constant value We can then measure various characteristics of the resultant AM waveform such as the modulation depth for example Such measurements would be very difficult if we were using a varying signal from an external source such as a broadcast station Scientech Technologies Pvt Ltd 14 ST2201 amp ST2202 Carrier Wave Th
23. DSB SSB Transmitter amp Receiver Trainer ST2201 amp ST2202 Operating Manual Ver 1 1 An ISO 9001 2000 company SCI N Cl Technologies pvt ita 94 101 Electronic Complex Pardeshipura Indore 452010 India Tel 91 731 2570301 02 4211100 Fax 91 731 2555643 email info scientech bz Website www scientech bz Toll free 1800 103 5050 A T V T V Rheinland Group CERTIFICATE The Certification Body for Quality Management Systems of T V Rheinland India Pvt Ltd certifies in accordance with T V Rheinland Group procedures that Scientech Technologies Pvt Ltd Unit 1 94 101 Electronics Complex Pardeshi Pura Indore 452 010 Madhya Pradesh India Unit 2 90 91 Electronics Complex Pardeshi Pura Indore 452 010 Madhya Pradesh India has established and applies a quality management system for Design Manufacture of Electronic Test amp Measuring Instruments Training Products for Electrical amp Electronics Education and Providing Technology Training An audit was performed Report No 07930 Proof has been furnished that the requirements according to DIN EN ISO 9001 2000 are fulfilled The certificate is valid until 2010 11 20 Certificate Registration No 85 100 001 07930 QMS 010 Bangalore 2007 11 21 The certification Body of The validity of this certificate is subject to ry as agreed TUV Rheinland India Pvt Ltd in the Contract The Validity of the Certi an be v
24. P3 is known as a double side suppressed carrier DSBSC waveform and its frequency spectrum is as shown in Figure 29 Scientech Technologies Pvt Ltd 28 ST2201 amp ST2202 Amplitude CARRIER REMOVED LOWER UPPER SIDEBAND SIDEBAND 1MHz fm 1MHz 1MHz im Frequency Frequency Spectrum of DSBSC Wave Form Figure 29 Note that now only the two sidebands remain the carrier component has been removed 9 Change the amplitude and frequency of the modulating audio signal by adjusting the audio oscillator block s amplitude and frequency pots and note the effect that these changes on the DSBSC waveform The amplitudes of the two sidebands can be reduced to zero by reducing the amplitude of the modulating audio signal to zero Do these by turning the amplitude present to its MIN position and note that the monitored signal becomes a D C level indicating that there are now no frequency components present Return the amplitude pot to its MAX position 10 Examine the output from the output amplifier block TP13 together with the audio modulating signal at TP1 triggering the scope with the audio modulating signal Note that the DSBSC waveform appears amplified slightly at TP13 as we will see later it is the output amplifier s output signal which will be transmitted to the receiver 11 By using the microphone the human voice can be used as the modulating signal instead of using ST2201 s audio oscillator block
25. a list of all the separate sinusoidal waveforms that were contained within the complex waveform these are called components or frequency components This can be seen in Figure 2 VC AC Ae Ae el Oe Figure 2 Only four of the components of the audio signal in Figure 1 are shown in Figure 2 The actual number of components depends on the shape of the signal being considered and could be a hundred or more if the waveform was very complex The third way is to display all the information on a diagram Such a diagram shows the frequency spectrum It is a graph with amplitude plotted against frequency Each separate frequency is represented by a signal vertical line the length of which represents the amplitude of the sinewave Such a diagram is shown in Figure 3 below Scientech Technologies Pvt Ltd 8 ST2201 amp ST2202 Note that nearly all speech information is contained within the frequency range of 300 Hz to 3 4 KHz Amplitude 0 300Hz 3 4kHz Frequency Figure 3 Although an oscilloscope will only show the original complex waveform it is important for us to remember that we are really dealing with a group of sinewaves of differing frequencies amplitudes and phases A Simple Communication System Once we are out of shouting range of another person we must rely on some communication system to enable us to pass information The essential parts of any communication system are transmitter a communication
26. anced modulator amp band pass circuit 2 block If the waveform at TP13 is still not good sinewave at higher modulating frequencies then if it is likely that the frequency of ST220 s 455 KHz oscillator block needs adjusting To do this follow the procedure given in chapter adjustment of the transmitter s tuned circuits 6 Turn ST2201 s amplitude pot in the audio oscillator block to its full counter clockwise minimum amplitude position and note that amplitude of the monitored output signal from ST2201 at TP13 drops to zero This illustrates that the SSB waveform contains no carrier if the amplitude of the modulating audio signal drops to zero so does the amplitude of the transmitted SSB signal In ST2201 s audio oscillator block return the amplitude pot to its fully clockwise MAX position and put the frequency pot in its midway position 7 We will now transmit the SSB waveform to the ST2202 receiver Since ST2201 s TX output select switch is in the ANT position the SSB signal at TP13 is fed to the transmitter s antenna Prove this by touching ST2201 s antenna and noting that the loading caused by your hand reduces the amplitude of the SSB waveform at TP13 The antenna will propagate this SSB waveform over a maximum distance of about 1 4 ft We will now attempt to receive the propagated SSB waveform with the ST2202 board by using the receiver s on board antenna Note If more than one ST2201 transmitter receiver system is in
27. check that its frequency varies as the tuning dial is turned Re time the receiver to a radio station 8 The operation of the mixer stage is basically to shift the wanted signal down to the IF frequency irrespective of the position of the tuning dial This is achieved in two stages a By mixing the local oscillator s output sinewave with the output from the R F amplifier block This produces three frequency components The local oscillator frequency f sig IF The sum of the original two frequencies f sum 2 f sig IF The difference between the original two frequencies f diff f sig IF f sig IF These there frequency components are shown in Figure 33 l LOCAL OSCILLATOR Amplitude FREQUENCY IF Fsig IF 2Fsig IF Frequency Figure 33 b By strongly attenuating all components except the difference frequency IF this is done by putting a narrow bandwidth band pass filter on the mixer s output Scientech Technologies Pvt Ltd 33 ST2201 amp ST2202 10 The end result of this process is that the carrier frequency of the selected AM station is shifted down to 455 KHz the IF Frequency and the sidebands of the AM signal are now either side of 455 KHz Note that since the mixer s band pass filter is not highly selective it will not completely remove the local oscillators and sum frequency components from the mixer s output this is the case particularly with the local oscillator compone
28. cilloscope the oscilloscope input channels should be AC coupled unless otherwise indicated Ensure that X10 probes are used throughout A frequency measurements Take care not to turn any inductor s core past its end stop as this may also result in damage 1 MHz crystal oscillator tuned circuit Monitor tp9 on ST2201 while using a trimmer tool to adjust transformer T3 in the 1MHz crystal oscillator block By carefully tuning T3 throughout its range of adjustment check that the monitored signal is a D C Level at both ends of the adjustment range and a small amplitude high frequency sinewave in the middle of the range Tune T3 so that it is in the center of the sinewave region Check that the monitored sinewave has amplitude of approximately 120mV peak to peak and a frequency of 1MHz Balanced modulator amp band pass filter circuit 1 tuned circuit On the ST2201 board put the audio input select switch in the INT position then turn the audio oscillator block s amplitude pot to its maximum position fully clockwise Put the mode switch in the DSB position then turn the balance pot in the balanced modulator amp band pass filter circuit 1 block to its minimum position fully counter clockwise Monitor Test Points 1 amp 3 triggering the oscilloscope with the TP1 signal that the waveform appears as shown in Figure 55 BRA ONG ee Oe Ue O Figure 55 Tune transformer T1 until the amplitude of the waveform on TP3
29. d Test Points provided onboard Built in DC Power Supply Fully documented student work book amp operating manual 8 Switched faults Crystal controlled carrier frequency On board audio modulator carrier frequency generation antenna amp speaker Compact size Scientech Technologies Pvt Ltd 4 ST2201 amp ST2202 Audio Oscillator Audio Output Modulators Carrier Frequency Transmitter Output Switched Faults Power Supply Test Points Power Consumption Interconnections Dimensions mm Weight Technical Specifications With adjustable Amplitude amp Frequency 300Hz 3 4 KHz Audio amplifier with speaker 2 nos Balanced modulators with Band pass filter 1 MHz 1 MHz Crystal controlled On board output amplifier Gain Adjustable 1 DSB 1 MHz 2 SSB 1 455 MHz Connected to Antenna Cable 8 Nos 230V 10 50Hz 27 4VA approximately 4 mm Banana sockets W 419 x H90 x D 255 2 8Kgs Approximately Scientech Technologies Pvt Ltd ST2201 amp ST2202 Features A self contained Trainer Functional blocks indicated on board mimic Input output and Test Points provided onboard Built in DC Power Supply Fully documented student work book amp operating manual 8 Switched Faults On board audio modulators detectors amplitude limiter amp filter circuits Effect of noise on the detection of FM signal may be investigated Co
30. d we can say that since both sidebands carry the same information there is no point in transmitting both of them It makes no difference which sideband is removed but in most systems the lower sideband is normally eliminated We can simply transmit a single sideband as shown in Figure 36 and by comparing the power use with Figure 34 we can see a considerable power saving Power No lower side frequency No carrier Side frequency power 320W Upper side frequency The total power is now only 320W Frequency Figure 36 The bandwidth of an SSB system is equal to the range of frequencies present in the information waveform where as a DSB signal has a bandwidth twice as wide as the highest frequency component in the information signal This also means a greatly reduced bandwidth for the system In Figure 36 we are transmitting just a single frequency Scientech Technologies Pvt Ltd 41 ST2201 amp ST2202 The SSB Transmitter The design of the SSB transmitter is accomplished in two stages First we generate a DSBSC signal and then remove the lower sideband to achieve the final SSB result Generating the DSBSC Signal To do this we use a balanced modulator The principle of this circuit is shown in Figure 37 Information Signal Sidebands and carrier ate generated Balanced Modulator ______ ___ DSBSC Output 4 Carrier inverted and fed back in to cancel Carrier Wave Figur
31. dial until the tone generated at the transmitter is also clearly audible at the receiver this should be when the tuning dial is set to about 55 65 and adjust the receiver s volume pot until the tone is at a comfortable level Check that you are tuned into the transmitter s output signal by varying ST2201 s frequency pot in the audio oscillator block and nothing that the tone generated by the receiver changes Scientech Technologies Pvt Ltd 36 ST2201 amp ST2202 The ST2201 2202 receiver is now tuned into AM signal generated by the ST2201 transmitter Briefly check that the waveforms at the outputs of the following receiver blocks are as expected R F Amplifier TP12 Mixer TP20 L F Amplifier 1 TP24 LF Amplifier 2 TP28 Diode Detector TP31 Audio Amplifier TP39 17 By using the microphone the human voice can be used as transmitter s audio modulating signal instead of using ST2201 s audio oscillator block Use DSB and not DSBSC Connect the microphone s output to the external audio input on the ST2201 board and put the audio input select switch in the EXT position An overview of DSB Transmission A double sideband transmission was the first method of modulation developed and for broadcast stations is still the most popular Indeed for medium and long range broadcast stations is still the most popular Indeed for medium and long range broadcast stations it is the only system in use The reaso
32. e 37 Internally the balanced modulator generates the AM waveform which includes the carrier and both sidebands It then offers the facility to feed a variable amount of the carrier back into the modulator in anti phase to cancel the carrier output In this way we can balance out the carrier to suppress it completely leaving just the required DSBSC waveform From DSBSC to SSB The DSBSC signal consists of the two sidebands one of which can be removed by passing them through a band pass filter On the ST2201 this is achieved as shown in Figure 38 Audio input approx 300Hz 3 4kHz Center frequency 455kHz Balanced Seine Output frequency Modulator t aer f close to 455kHz DSBSC SSB signal signal Input frequency approx 453kHz Figure 38 Scientech Technologies Pvt Ltd 42 ST2201 amp ST2202 The inputs to the balanced modulator comprise the audio inputs from the audio oscillator which extend from 300Hz to 3 4 KHz and the carrier input On the ST2201 board this carrier oscillator although marked as 455 KHz actually needs to operate at a frequency which is a little less than this around 453 KHz Why is this It is to ensure that the upper sideband can pass through the ceramic band pass filter but the lower sideband cannot pass through In Figure 39 below the upper sideband can be seen to be within the pass band of the ceramic filter but the lower sideband is outside and will therefore be r
33. e back to enjoy the next few seconds of broadcast until the drift starts again The frequency control necessary to ensure that the re inserted carrier stays at exactly the correct value regardless of changes of temperature vibration etc would make the receiver too complex and expensive for domestic use For this reason DSBSC is very seldom used Overall the waste of transmitted power to send the carrier is less expensive than the additional cost of perhaps several million high quality receivers Such receivers are used for professional and amateur communications but are expensive between ten and a hundred times the cost of a standard radio receiver Scientech Technologies Pvt Ltd 40 ST2201 amp ST2202 Experiment 4 Objective Study of Diode Detector Procedure 1 Connect and make the settings as per experiment number 2 2 Observe the signal flow from the input of diode detector to anode of diode D6 at its cathode after the filter and at the output at TP31 3 Vary the preset R45 in the diode detector block while observing the output of diode detector 4 You can see the variations in the detected output when you change the RC time constant of the filter formed by R45 and C32 Signal Side Band Transmission SSB This is just taking the previous reasoning to its ultimate conclusion If we don t really need the carrier we can leave it out and save power This is DSBSC transmission Just one step further an
34. e carrier wave must meet two main criteria It should be of a convenient frequency to transmit over the communication path in use In a radio link transmissions are difficult to achieve at frequencies less than 15 KHz and few radio links employ frequencies above 10GHz Outside of this range the cost of the equipment increases rapidly with very few advantages Remember that although 15 KHz is within the audio range we cannot hear the radio signal because it is an electromagnetic wave and our ears can only detect waves which are due to changes of pressure The second criteria is that the carrier wave should also be a sinusoidal waveform because a sinusoidal signal contains only a single frequency and when modulated by a signal frequency will give rise to just two side frequencies the upper and the lower side frequencies However if the sinewave were to be a complex wave containing many different frequencies each separate frequency component would generate its own side frequencies The result is that the overall bandwidth occupied by the transmission would be very wide and on the radio would cause interference with the adjacent stations In Figure 13 a simple case is illustrated in which the carrier only contains three frequency components modulated by a single frequency component Even so we can see that the over all bandwidth has been considerably increased Carrier Amplitude 0 gt Frequency Total A sinusoidal Carrier Wave bandwid
35. e external audio input on the ST2201 board and put the audio input select switch in the EXT position The input signal to the audio input select may be taken from an external microphone supplied with the module of from a cassette recorder by choosing the appropriate switch setting on the module Refer the user manual for the audio input module for further details Scientech Technologies Pvt Ltd 52 ST2201 amp ST2202 Objective Experiment 6 Single Sideband AM Reception Procedure This experiment investigates the reception and demodulation of the single sideband amplitude modulated waveforms generated by ST2201 using the ST2202 receiver module To avoid unnecessary loading of monitored signals X10 oscilloscope probes should be used throughout this experiment Position the ST2201 amp ST2202 modules with the ST2201 board the left and a gap of about three inches between them 1 Ensure that the following initial conditions exist on the ST2201 board sg m Audio oscillator s amplitude pot in full clockwise position Audio input select switch in INT position Mode switch in SSB position Output amplifier s gain pot in full clockwise position TX output select switch in ANT position Audio amplifier s volume pot in full counter clockwise position Speaker switch in ON position On board antenna in vertical position and fully extended Ensure that the following initial conditions ex
36. ect switch in ANT position Gain pot in the RF amplifier block in its midway position arrowhead on pot pointing towards the top of the board On board antenna upright and extended 2 Set up the signal generator so that its output is sinewave of amplitude 50m V peak to peak and trial the output lead so that it runs close to the antenna without touching it Monitor TP12 the output from the RF amplifier block and follow the steps below a Turn the vernier tuning dial the position 25 that is so that the dial s pointer is midway between the 20 and 30 marks on the scale adjust the signal generator for and output frequency of 615 KHz 1 KHz and tune transformer T1 until the amplitude of the monitored signal is a maximum Turn the tuning dial position to 75 adjust the signal generator for an output frequency of 1220 KHz 1 KHz and tune trimmer capacitor TC1 until the amplitude of the monitored signal is a maximum Repeat steps i amp ii Finally return the RF amplifier s gain pot to its maximum setting full clockwise Adjustment of local oscillator tuned circuit a b d Monitor the frequency at TP40 the output of the local oscillator block and follow the steps below Turn the tuning dial to position O and tune transformer T5 in the local oscillator block until the monitored frequency is 980 KHz 1 KHz Turn the tuning dial to position 100 adjust the signal generator for an output frequency o
37. ejected The sideband frequencies are quite close to each other and a good quality ceramic filter is required A ceramic filter passes only a narrow range of frequencies with a sharp cut off outside of its pass band Suppressed Frequency response of the camer ceramic bandpass filter 453kHz centered on 455kHz Amplitude A A i 1 i 1 1 1 0 Lower Upper Frequency sideband sideband Figure 39 Transmitting the SSB Signal So far we have got an SSB signal but it is at a frequency around 455 KHz This is too low since we need it to be within the medium wave band if we are to hear it on out receiver We need to shift or translate the signal to a higher frequency We know how to do this We simply pass it through a balanced modulator and filter out the unwanted frequency In the ST2201 transmitter we use the 1MHz carrier for the AM transmission Our SSB signal Center frequency at approx 455kHz Ve 1 455MHz V Balanced Bandpass Output Modulator Filter Amplifier Carrier signal at 1MHz New sidebands Output frequency either side of close to 1 455MHz 1MHz Figure 40 Scientech Technologies Pvt Ltd 43 ST2201 amp ST2202 In Figure 40 our SSB signal that we have just generated is combined with a 1 MHz carrier signal to produce a new DSBSC signal This signal will now have two new sidebands one around 1MHz 455 KHz 1 455 MHz and the other at 1MHz 455 KHz
38. enna and adjusting the intensity of input voltage until standard outputs obtained at resonance for various carrier frequencies Sensitivity is expressed in microvolt A sensitivity curve is shown in Figure 52 Figure 52 Scientech Technologies Pvt Ltd 62 ST2201 amp ST2202 Selectivity Selectivity is expressed in the form of a curve that give the carrier signal strength with standard modulation that is required to produce the standard test output plotted as a function off resonance of the test signal a l IE sa 20 o wv KLOCIOLES OF RESOMMMCE Figure 53 The receiver is tuned to the desired frequency and manual volume control is set for maximum value At standard modulation the signal generator is set at the resonant frequency of the receiver The carrier output of the signal generator is varied until the standard test output is obtained At the same tuning of receiver the frequency of signal generator is varied above and below the frequency to which the receiver is tuned For every frequency the signal generator voltage applied to the receiver input is adjusted to give the standard test output from the receiver The data are plotted in Figure 53 Fidelity Fidelity is the term expressing the behavior of receiver output with modulation frequency of input voltage To obtain a fidelity curve the carrier frequency of the signal generator adjusted to resonance with the receiver standard 400 cycles
39. er to ensure that the peaks of TP17 s waveform envelope labeled A B C etc in the above diagram all have equal amplitude You will recall that the waveform at TP17 was encountered in the previous experiment this is a double sideband suppressed carrier DSBSC AM waveform and it has been obtained by amplitude modulating the carrier sinewave at TP6 of frequency fc with the audio frequency modulating signal at TP15 of frequency fm and then removing the carrier component from the resulting AM signal by adjusting the balance pot The frequency spectrum of this DSBSC waveform is shown in Figure 43 mot Amplitude capes REMOVED LOWER UPPER SIDEBAND SIDEBAND rk ee er fc fm fc fm Frequency Figure 43 Scientech Technologies Pvt Ltd 46 ST2201 amp ST2202 7 The DSBSC output from the balanced modulator block is next passed on to the ceramic filter block whose purpose is to pass the upper sideband but block the lower sideband We will now investigate how this is achieved First note that the ceramic band pass filter has a narrow pass band centered around 455 KHz It was mentioned earlier that the frequency of the carrier input to the balanced modulator block has been arranged to be slightly less than 455 KHz In fact the carrier chosen so that whatever the modulating frequency fm the upper sideband at fc fm will fall inside the filter s pass band while the lower sideband at fc fm always falls outside Consequent
40. erate a a e a Deu ee a toutes 1 No A ASSMAN Uae 25 525 5 Passe yeahs eae ee eG See 1 No Accessories for ST2202 Ty Path Cord TO araa e aaa r a o hens onsen vas AAE TE EAEN aS 2 Nos 2 Matis Cord EEE E E E E ESN 1 No Ss Headphone aruna ean aa i Sieh eee 1 No 4 Manual eiea E E A a GA each acta lat 1 No Scientech Technologies Pvt Ltd 72
41. erified under tuy com with the Identification No 9105027653 are tp 04 11 NABCB 00 www tuv com ST2201 amp ST2202 DSB SSB Transmitter amp Receiver Trainer ST2201 amp ST2202 Table of Contents 1 ST2201 Features 4 2 ST2201 Technical Specifications 5 3 ST2202 Features 6 4 ST2202 Technical Specifications 7 5 Frequency components of Human voice 8 6 A Simple Communication System 9 7 Amplitude Modulation 10 8 Carrier Wave 15 9 Modulators 16 10 Output Amplifier 16 11 DSB Receiver 19 12 Automatic Gain Control 25 e Experiment 1 26 Double Sideband AM Generator e Experiment 2 30 To calculator modulation index of DSB wave by Trapezoidal Pattern e Experiment 3 31 Double Sideband AM Reception e Experiment 4 41 Study of Diode Detector 13 Single Side Band Transmission 41 e Experiment 5 45 Signal Sideband AM Generator e Experiment 6 53 Single Sideband AM Reception e Experiment 7 59 Operation of the Automatic Gain Control Circuit AGC 14 Receiver Characteristics Selectivity Sensitivity Fidelity 61 e Experiment 8 64 To plot Selectivity Curve for Radio Receiver e Experiment 9 65 To plot Sensitivity Curve for Radio Receiver e Experiment 10 66 To plot Fidelity Curve for Radio Receiver 14 Adjustment of Tuned Circuits 67 15 Warranty 16 List of Accessories Scientech Technologies Pvt Ltd 3 ST2201 amp ST2202 Features A self contained Trainer Functional blocks indicated on board mimic Input output an
42. eshold value a voltage is applied to the RF and IF amplifiers in such a way as to decrease their gain to prevent overload As soon as the incoming signal strength decreases such that the mean DC voltage level is reduced below the threshold the RF and IF amplifiers return to their normal operation AGC Off At low signal strength the AGC circuit has no This part of the transmission effect will overload the receiver and cause distortion Threshold level occcccccccc rererere ee eeeh ees age ta voy aaant Aad A a D n N AL OO AGC On The AGC has limited the amplification to prevent overload and distortion Threshold level Figure 25 Recommended testing instruments for experimentation 1 20MHz dual trace oscilloscope 201 2 Switchable probe 3 Function Generator 1 MHz 4 Frequency Counter 10 MHz preferable Scientech Technologies Pvt Ltd 25 ST2201 amp ST2202 Experiment 1 Objective Double Sideband AM Generation Procedure This experiment investigates the generation of double sideband amplitude modulated AM waveforms using the ST2201 module By removing the carrier from such an AM waveforms the generation of double sideband suppressed carrier DSBSC AM is also investigated To avoid unnecessary loading of monitored signals X10 oscilloscope probes should be used throughout this experiment 1 Ensure that the following initial conditions exist on the board a Audio input select switch should
43. f 1220 KHz 1 KHz and tune trimmer capacitor TC 2 until the monitored frequency is 2060 KHz 2 KHz Repeat Steps i amp ii Scientech Technologies Pvt Ltd 69 ST2201 amp ST2202 Adjustment of mixer and IF amplifier tuned circuits 1 Ensure that the beat frequency oscillator switch is in the OFF position and the AGC switch is in the OUT position Set the signal generator up for a sinewave output of amplitude 0 1 V peak to peak and frequency 455 KHz 0 5 KHz Connect the signal generator to TP14 ST2201 s mixer block and insert switched fault 3 which switches OFF the local oscillator Next follow the steps below a Monitor TP28 output of IF amplifier 2 and tune transformer T2 in the mixer block until the amplitude of the monitored signal is a maximum b Tune transformer T3 in the IF amplifier 1 block until the amplitude of the monitored signal is a maximum c Tune transformer T4 in the IF amplifier 2 block until the amplitude of the monitored signal is once again at a maximum d Repeat steps i ii amp iii e Finally remove switched fault 3 Adjustment of beat frequency oscillator tuned circuit 1 3 Put the beat frequency oscillator switch in the ON position and monitor TP46 the output from the beat frequency oscillator Tune transformer T6 until the frequency of the monitored sinewave is 455 KHz 0 5 KHz Finally return the beat frequency oscillator swi
44. frequency amplitudes of the two sidebands can be reduced to zero by reducing the amplitude of the modulating audio signal to zero Do this by turning the amplitude pot to its MIN position and note that the signal at TP3 becomes an un modulated sine wave of frequency 1 MHz indicating that only the carrier component now remains Return the amplitude pot to its maximum position Now turn the balance pot in the balanced modulator amp band pass filter circuit 1 block until the signal at TP3 is as shown in Figure 28 4 Figure 28 The balance pot varies the amount of the 1 MHz carrier component which is passed from the modulator s output By adjusting the pot until the peaks of the waveform A B C and so on have the same amplitude we are removing the carrier component altogether We say that the carrier has been balanced out or suppressed to leave only the two sidebands Note that once the carrier has been balanced out the amplitude of TP3 s waveform should be zero at minimum points X Y Z etc If this is not the case it is because one of the two sidebands is being amplified more than the other To remove this problem the band pass filter in the balanced modulator amp band pass filter circuit 1 block must be adjusted so that it passes both sidebands equally This is achieved by carefully trimming transformer T1 until the waveform s amplitude is as close to zero as possible at the minimum points The waveform at T
45. ge decrease Notice how the increased input signal from the transmitter is largely offset by the AGC circuit to maintain a reasonably constant level of signal at TP39 Switch off the Power Supply We can now observe the operation of the AGC circuit under real conditions Remove the cable connecting ST2201 the transmitter is not going to be used in this part of the investigation Fully extend the receiver antenna and set it up vertically Switch the RX input select to ANT antenna and adjust the gain control in the RF amplifier to its minimum value fully counter clockwise The volume control in the audio amplifier should be set at its midpoint Switch on the Power Supply Use a channel 1 to monitor the audio output signal at TP39 and to act as the trigger input for the oscilloscope and monitor the output from the AGC circuit at TP1 Tune the receiver until a broadcast station is heard If no station is heard over the whole tuning range RF amplifier gain can be increased slightly before re tuning It is likely that the volume from the speaker will be quite low and the AGC output signal will be showing no response Now slowly increase the RF amplifier gain observing the effects on the loudspeaker volume and the signals on the oscilloscope Using a low strength weak input signal touch the antenna and notice the improved reception You are actually acting as an antenna and picking up additional signal to reinforce the receptio
46. ghout its range Note that for most audio frequencies the waveform is a good clean sinewave indicating that the lower sideband has been totally rejected by the filter For low audio frequencies you may notice that the monitored signal is not such a pure sinusoid This is because the upper and lower sidebands are now very close to each other and the filter can no longer completely remove the lower sidebands are now very close to each other and the filter can no longer completely remove lower sideband Nevertheless the lower sideband s amplitude is sufficiently small compared with the upper sideband that its presence can be ignored Since the upper sideband dominates for all audio modulating frequencies we say that single sideband SSB amplitude modulation has taken place Note If the monitored waveform is not a good sinewave at higher modulating frequencies i e when the frequency pot is near the MAX position then it is likely that the frequency of the 455 KHz oscillator needs to be trimmed To do this follow the procedure given in chapter adjustment of the transmitter s tuned circuits Note that there is some variation in the amplitude of the signal at the filter s output TP20 as the modulating frequency changes This variation is due to the frequency response of the ceramic band pass filter and is best explained by considering the spectrum of the filter s input signal at the MIN and MAX positions of the frequency pot as shown in
47. he carrier component at the output of the balanced modulator block from being balanced out by the block s balance pot so that a DSBSC waveform cannot be obtained at TP17 This is achieved by shorting the SIG pin of the 1496 pin 4 to 0 volts Fault shorts together the input TP18 and output TP19 of the ceramic filter in the ceramic bandpass filter block allowing both sidebands of the balanced modulator block s output signal to reach TP20 Fault disables the output of the balanced modulator amp bandpass filter circuit 2 block at TP22 by shorting the bias input pin 5 of the 1496 to 0 volts ST2202 switched faults 1 Fault disable the R F amplifier block by open circuiting the transistor s base bias chain This cause the bias voltage on the transistor s base at TP10 to drop to o volts Fault disable the output from the mixer block TP20 by open circuiting the 1 K emitter resistor of the modulating transistor Open circuit fault stops the local oscillator from working by removing the bias voltage at TP30 from the transistor s base Open circuit fault disables the output from the diode detector block TP31 by removing the D C bias at TP30 from the diode s anode Fault disables the output from IF amplifier 1 block TP24 by shorting the transistor s emitter TP23 to the 12 volts supply Fault disable the product detector block by shorting the base of the block s output transistor at TP34 to Ovol
48. ill cause the harmonic relationship between notes to be lost This makes SSB useless for transmitting music Scientech Technologies Pvt Ltd 58 ST2201 amp ST2202 Objective Experiment 7 Operation of the Automatic Gain Control Circuit AGC Procedure To avoid unnecessary loading of the circuits signals X10 oscilloscope probes should again be used during this practical exercise Remember to take this into account when measuring signal amplitudes 1 Position ST2201 amp ST2202 modules with the ST2201 board on the left and gap of about three inches between them 2 OnST2201 set the following initial conditions a b C Audio input select switch to INT position Mode switch to DSB Speaker switch to OFF In the audio oscillator the amplitude and the frequency pot should be set to maximum full clockwise In the balanced modulator amp band pass filter circuit 1 the balance pot should be set to maximum full clockwise In the output amplifier increase the gain to its maximum value full clockwise The TX output select should be set to SKT socket Connect a cable from the TX output socket on ST2201 to the RX input socket on ST2202 3 OnST2202 set the following initial conditions d e f In the audio amplifier switch the speaker to ON RX input select switch to SKT socket In the RF amplifier switch the tuned circuit select to INT internal position and increase the
49. impossible to listen to any one transmission The receiver circuits generate noise signals which are added to the wanted signals We hear this as a background hiss and are particularly noticeable if the receiver is tuned between stations or if a weak station is being received The RF amplifier is the first stage of amplification It has to amplify the incoming signal above the level of the internally generated noise and also to start the process of selecting the wanted station and rejecting the unwanted ones Selectivity A parallel tuned circuit has its greatest impedance at resonance and decreases at higher and lower frequencies If the tuned circuit is included in the circuit design of an amplifier it results in an amplifier which offers more gain at the frequency of resonance and reduced amplification above and below this frequency This is called Selectivity Scientech Technologies Pvt Ltd 19 ST2201 amp ST2202 Selectivity of the amplifier Amplifier 3 gain 2 Dead cet ieee I ASS A AEE AE E 0 Cee EAN eee TEE eS EAEE EENETI EEEE O m ee E Frequency gt kHz Strength of received 10mV stations 0 800 810 820 830 840 ena KAZ 50 7 We have tuned the l receiver to this Signal 404 station strength after the amplifier in mV 800 810 820 830 840 Frequency gt kHz Figure 19 The radio receiver is tuned to a frequency of 820 KHz and at this frequency the amplifier provides a gain of fi
50. ing antenna which is really an AC current Since an electric current always has a magnetic field associated with it an alternating magnetic field is produced The overall effect is that the output amplifier has produced alternating electric and magnetic fields around the antenna The electric and magnetic fields spread out as an electromagnetic wave at the speed of light 3 x 10 meters per second For maximum efficiency the antenna should be of a precise length The optimum size of antenna for most purposes is one having an overall length of one quarter of the wave length of the transmitted signal This can be found by Vv Ls f Where v speed of light u wave length and f frequency in Hertz In the case of the ST2201 the transmitted carrier is 1 MHz and so the ideal length of antenna is 3x 10 u 1x 10 u 300m One quarter of this wavelength would be 75meters about 245 feet We can now see that the antenna provided on the ST2201 is necessarily less than the ideal size Polarization If the transmitting antenna is placed vertically the electrical field is vertical and the magnetic field is horizontal as seen in Figure 15 16 amp 17 If the transmitting antenna is now moved by 90 to make it horizontal the electrical field is horizontal and the magnetic field becomes vertical By convention we use the plane of the electric field to describe the orientation or polarization of the em
51. io input select switch is in the INT position and turn the audio oscillator block s amplitude and frequency pots to their maximum positions full clockwise Put the mode switch in the SSB position and monitor TP22 the output from the balanced modulator amp bandpass filter circuit 2 block Tune transformer T4 in the balanced modulator amp bandpass filter circuit 2 until the monitored signal has maximum amplitude Adjust the following pot until the monitored signal is a good clean sine wave 1 Balance pot in the balanced modulator block 2 Balance pot in the balanced modulator amp band pass filter circuit 2 block Now fine tune T4 so that the monitored sinewave s amplitude is once again a maximum Scientech Technologies Pvt Ltd 68 ST2201 amp ST2202 Adjustment of ST2202 Tuned circuits This section describes how to adjust ST2202 s tuned circuits for correct operation Where signals are to be monitored with an oscilloscope the oscilloscope input channels should be AC coupled unless otherwise indicated Ensure that X10 probes are used throughout frequency counter should be used for all frequency measurements Adjustment of R F amplifier tuned circuit 1 Ensure that the following initial conditions exist on the ST2201 board a b c f Beat frequency oscillator switch OFF position AGC switch in OUT position Tuned circuit select switch in RF amplifier in INT position RX input sel
52. ise side of the minimum frequency position Scientech Technologies Pvt Ltd 57 ST2201 amp ST2202 13 14 On the ST2202 module monitor the output of the product detector block at TP37 together with the output of the audio amplifier block TP39 triggering the scope with the later signal Note There will be no signal at TP39 if the audio amplifier s volume pot is in its fully counter clockwise minimum position Vary the frequency of the Transmitter s audio modulating signal by adjusting the audio oscillator s frequency pot on the ST2201 module Note There will be no signal at TP39 if the audio amplifier s volume pot is in fully counter clockwise minimum position Also try briefly reducing the amplitude of the Transmitter s modulating signal to zero by turning the audio oscillator s amplitude pot fully clockwise and note that the receiver s output amplitude also drops to zero With the receiver s tuning dial adjusted for correct demodulation of the transmitted SSB signal you may notice that there is a slight drift in the tone generated by the Receiver oscillator circuits leading to changes in the difference frequency produced by the product detector Oscillator drift is a serious problem in SSB communication since it shifts all the frequency components which make up the Receiver s audio output signal by the same amount If we try to use our SSB communications system to transmit music then oscillator drift w
53. ist on the ST2202 board ae p sm mo m 0 RX input select switch in ANT position R F amplifier s tuned circuit select switch in INT position R F amplifier s gain pot in full clockwise position AGC switch in out position Detector switch in product position Audio amplifier s volume pot in fully counter clockwise position Speaker switch in ON position Beat frequency oscillator switch in ON position On board antenna in vertical position and fully extended Turn on power to the modules On the ST2201 module examine the transmitter s output signal TP13 and make sure that this is a good SSB waveform by checking that the signal is a reasonably good sinewave Scientech Technologies Pvt Ltd 53 ST2201 amp ST2202 Note The amplitude of the transmitter s output signal will change as the pot is tuned also the monitored sinewave may be slightly less pure at low modulating frequencies These characteristics are due to the fact that the ceramic bandpass filter is not a perfect filter and they will have negligible effect on the quality of the Receiver audio output If the monitored waveform is not a good sinewave at higher modulating frequencies i e when the frequency pot is approximately in centre try adjusting the balance pots in the following two blocks in order to ensure that the 455 KHz and 1 MHz carrier components have been completely balanced out a Balanced modulator block and b Bal
54. itude modulation broadcast receivers the definition of sensitivity has been standardized as amplitude of carrier voltage modulated 30 at 400 cycles which when applied to the receiver input terminals through a standard dummy antenna will develop an output of 0 5 watt in a resistance load of appropriate value substituted for the loud speaker Selectivity The selectivity of a radio receiver is that characteristic which determines the extent to which it is capable of differentiating between the desired signal and signal of other frequencies Fidelity This is defined as the degree with which a system accurately reproduces at its output the essential characteristics of signals which is impressed upon its input Scientech Technologies Pvt Ltd 61 ST2201 amp ST2202 Determination of receiver characteristics A laboratory method for the measurement of receiver characteristics is shown in Figure 51 We use here an artificial signal to represent the voltage that is induced in the receiving antenna This artificial signal is applied through dummy antenna which in association antenna with which the receiver is to be used Substituting the resistance load of proper value for the loudspeaker and measuring the audio frequency power determine the receiver output Standard Signal Generator Figure 51 Sensitivity Sensitivity is a determined by impressing different RF voltages in series with a standard dummy ant
55. l amount of amplitude modulation can be detected if any This is because there are many unwanted frequencies getting through to the amplifier output which tend to drown out the wanted AM Signal You may notice that the waveform itself drifts up and down on the scope display indicating that the waveform s average level is changing This is due to the operation of the AGC circuit which will be explained later 7 The next stage of the receiver is the mixer stage which mixes the R F amplifier s output with the output of a local oscillator The Frequency of the local oscillator is also tuned by means of the tuning dial and is arranged so that its frequency is always 455 KHz above the signal frequency that the R F amplifier is tuned to This fixed frequency difference is always present irrespective of the position of the tuning dial and is arranged so that its frequency is always 455 KHz above the signal frequency that the R F amplifier is tuned to This fixed frequency difference is always present irrespective of the position of the tuning dial and is known as the intermediate frequency IF for short This frequency relationship is shown below for some arbitrary position of the tuning dial Scientech Technologies Pvt Ltd 32 ST2201 amp ST2202 R F AMPLIFIER LOCAL OSCILLATOR Amplitude OUTPUT OUTPUT I F 455kHz fsig fsig IF Frequency Figure 32 Examine the output of the local oscillator block and
56. l over a maximum distance of about 1 4 feet We will now attempt to receive the propagated AM waveform with the ST2201 ST2202 board by using the receiver s on board antenna Note If more than one ST2201 transmitter receiver system is in use at one time it is possible that there may be interference between nearby transmitters if antenna propagation is used To eliminate this problem use a cable between each transmitter receiver pair connecting it between ST2201 s TX output socket and ST2201 ST2202 s RX input socket If you do this make sure that the transmitter s TX output select switch and the receiver s RX input select switch are both in the SKT position then follow the steps below as though antenna propagation were being used On the ST2201 module turn the volume pot in the audio amplifier block clockwise until you can hear the tone of the audio oscillator s output signal from the loudspeaker on the board Note If desired headphones may be used instead of the loudspeaker on the board To use the headphones simply plug the headphone jack into the audio amplifier block s headphones socket and put the speaker switch in the OFF position The volume from the headphones is still controlled by the block s volume pot Turn the volume pot to the full counter clockwise minimum volume position On the ST2201 ST2202 receiver adjust the volume pot so that the receiver s output can be clearly heard Then adjust the receiver s tuning
57. local oscillator frequency of 1 155 MHz to produce the required IF frequency of 455 KHz This would mean that this station would also be received at the same time as our wanted one at 700 KHz Station 1 Frequency 700 KHz Local oscillator 1 155 MHz IF 455 KHz Station 2 Frequency 1 61 MHz oscillator 1 155 MHz IF 455 KHz An image frequency is an unwanted frequency that can also combine with the Local Oscillator output to create the IF frequency Scientech Technologies Pvt Ltd 22 ST2201 amp ST2202 Notice how the difference in frequency between the wanted and unwanted stations is twice the IF frequency In the ST2201 ST2202 it means that the image frequency is always 910 KHz above the wanted station This is a large frequency difference and even the poor selectivity of the RF amplifier is able to remove the image frequency unless it is very strong In this case it will pass through the receiver and will be heard at the same time as the wanted station Frequency interactions between the two stations tend to cause irritating whistles from the loudspeaker Intermediate Frequency Amplifier IF Amplifiers The IF Amplifier in this receiver consists of two stages of amplification and provides the main signal amplification and selectivity Operating at a fixed IF frequency means that the design of the amplifiers can be simplified If it were not for the fixed frequency all the amplifiers may need to be tunable acros
58. ly the upper sideband will suffer little attenuation while the lower sideband will be heavily attenuated to such an extent that it can be ignored This shown in the frequency spectrum in Figure 44 FREQUENCY RESPONSE OF CERAMIC BANDPASS FILTER CENTERED ON 455KHZ SUPPRESSED CARRIER Amplitude LOWER SIDEBAND UPPER SIDEBAND fc fm fc fc fm Frequency Figure 44 8 Monitor the output of the ceramic band pass filter block at TP20 together with the audio modulating signal at TP15 using the later signal to trigger the oscilloscope Note that the envelope of the signal at TP20 now has fairly constant amplitude as shown in Figure 45 t p 1S OPN I Ne se ee i ae Figure 45 Scientech Technologies Pvt Ltd 47 ST2201 amp ST2202 10 If the amplitude of the signal at TP20 is not reasonably constant adjust the balance pot in the balance modulator block to minimize variations in the signal s amplitude If the constant amplitude waveform still cannot be obtained then the frequency of the 455 KHz oscillator needs to be trimmed To do this follow the procedure given in chapter adjustment of the transmitter s tuned circuits Now trigger the oscilloscope with the ceramic band pass filter s output signal TP20 and note that the signal is a good clean sinewave indicating that the filter has passed the upper sideband only Next turn the audio oscillator block s frequency pot throu
59. mpact size Scientech Technologies Pvt Ltd 6 ST2201 amp ST2202 Technical Specifications Construction Frequency Range Intermediate Frequency IF Input Circuits Tuning Receiving media Detector Circuits Audio Output Automatic Gain Control Switched Faults Power Supply Test Points Power Consumption Interconnections Dimensions mm Weight Super heterodyne 980 KHz to 2 060 MHz 455 KHz RF amplifier Mixer Local oscillator Beat frequency oscillator IF amplifier 1 IF amplifier 2 With variable capacitor ganged dial marking on board ANWR WN H Telescopic antenna RF cable 1 Diode detector DSB 2 Product detector SSB Audio amplifier with speaker amp headphone Switchable 8 Nos 230V 10 50Hz 50 Nos 3VA approximately 4mm Banana sockets W 419 x H90 x D255 2 8Kg Approximately Scientech Technologies Pvt Ltd ST2201 amp ST2202 Frequency Components of Human Voice When we speak we generate a sound that is very complex and changes continuously so at a particular instant in time the waveform may appear as shown in Figure 1 below Amplitude gt time Figure 1 However complicated the waveform looks we can show that it is made of many different sinusoidal signals added together To record this information we have a choice of three methods The first is to show the original waveform as we did in Figure 1 The second method is to make
60. n Scientech Technologies Pvt Ltd 60 ST2201 amp ST2202 19 At moderate signal levels touching the antenna will cause the signal strength to increase sufficiently to cause the AGC to be activated In fact with a moderate to high level of input signal moving your hand closet to the antenna can be seen to affect the levels on the oscilloscope 20 Tune into a strong broadcast station and increase the RF amplifier gain control to maximum full clockwise The volume control in the audio amplifier can be reduced to a comfortable sound level On your oscilloscope the AGC circuit should be producing high control voltages to reduce the gain of the amplifiers 21 Switch the AGC OFF and listen to the loudspeaker output The signal will sound very distorted due to the receiver amplifiers being overloaded 22 Switch the AGC back on again and notice how effective it is in preventing overloading 23 Switch off your Power Supply and oscilloscope Receiver Characteristics The important characteristics of receivers are sensitivity selectivity amp fidelity described as follows Sensitivity The sensitivity of radio receiver is that characteristic which determines the minimum strength of signal input capable of causing a desired value of signal output Therefore expressing in terms of voltage or power sensitivity can be defined as the minimum voltage or power at the receiver input for causing a standard output In case of ampl
61. n for such widespread use is that the receiver design can be very simple and reliable None of the characteristics are particularly critical so reception is still possible even in adverse conditions In this context a broadcast is information transmitted for entertainment or information and available for use by anyone with a receiver It never requires a response or acknowledgement for the receiving station So in many ways it is similar to a newspaper or magazine which is published and distributed to anyone who is interested in reading a copy Radio is also used for communications in which the signal is addressed to a receiving station or a group of stations Using the written word this would correspond to a private letter or perhaps business or military information being exchanged For this type of communication other systems are used one of which is investigated in this chapter As we will see there are two serious drawbacks to the DSB AM system Scientech Technologies Pvt Ltd 37 ST2201 amp ST2202 DSB is Wasteful of Power The first problem is to do with the power distribution in a DSB amplitude modulated wave 4 Power Carrier power 1000W Side frequency power 320W Side frequency power 320W Lower side frequency Carrier Upper side frequency Frequency The total power being transmitted is 1000 320 320 1640W Figure 34 How much of the DSB AM wave is really needed The whole purpose of the modulatio
62. n system is to transfer information from one place to another How efficiently does it to achieve this We are transmitting two sidebands and a carrier The carrier contains no useful information at all and yet contains over half the total power This is clearly a waste Even the sidebands can be improved We can remember that combining the information signal and the carrier gave rise to an upper and a lower sideband each of which contains a copy of the information being transmitted There is no necessity to send two copies of the same information so this is a waste of power and bandwidth Scientech Technologies Pvt Ltd 38 ST2201 amp ST2202 Double Sideband Suppressed Carried Transmission DSBSC If we avoid using the carrier frequency shown in Figure we would save ourselves 1KW of the transmitted power Power No camer Side frequency power 320W Side frequency power 320W Lower side freauencv Upper side freauency Frequency The total power being transmitted is now reduced to 640W Figure 35 Well the carrier has done its job in the modulator That is where we needed it to move or translate the audio signals up to radio frequency values which can be radiated by the antenna This shifting or translating of frequencies is the main function of a modulator At the transmitter the carrier can easily be removed by a band stop filter designed to eliminate the carrier frequency whilst allowing the two sidebands to be tran
63. ncy amp output level Scientech Technologies Pvt Ltd 65 ST2201 amp ST2202 Experiment 10 Objective To plot fidelity curve for radio receiver Procedure 1 Setting on ST2202 a Set the detector in diode mode b AGC on c Set the volume control fully clockwise Apply AM signal of 100mV with 400Hz modulating signal and 30 modulation into Rx input Select a suitable carrier frequency that lies within AM Band 525 KHz 1600 KHz Tune the ST2202 receiver to that frequency using tuning control Also adjust gain potentiometer provided in R F amplifier section so as to get unclipped demodulated signal at detector s output Note the demodulated signal level Vr at the final output stage i e output of audio amplifier on CRO for the applied AM signal with 400Hz modulating signal Now vary the modulating signal frequency over audio range 300 Hz 3 KHz in suitable steps say 100Hz Note the corresponding output level Vi at the output of audio amplifier on CRO Tabulate readings as under Carrier frequency Modulating frequency Output Voltage Relative response 20 log Vi Vr dB Plot the graph between modulating frequency and relative response Scientech Technologies Pvt Ltd 66 ST2201 amp ST2202 Adjustment of Tuned Circuits This section describes how to adjust ST2201 s tuned circuits for correct operation Where signals are to be monitored with an os
64. nt which is much larger in amplitude than the sum and difference components Examine the output of the mixer block TP20 with an a c coupled oscilloscope channel and note that the main frequency component present changes as the tuning dial is turned This is the local oscillator component which still dominates the mixer s output in spite of being attenuated by the mixer s band pass filter Tune in to a strong broadcast station again and note that the monitored signal shows little if any sign of modulation This is because the wanted component which is now at the IF frequency of 455 KHz is still very small in component which is now at the IF frequency of 455 KHz is still very small in comparison to the local oscillator component What we need to do now is to preferentially amplify frequencies around 455 KHz without amplifying the higher frequency local oscillator and SUM components This selective amplification is achieved by using two IF amplifier stages IF amplifier 1 and IF amplifier 2 which are designed to amplify strongly a narrow band of frequencies around 455 KHz without amplifying frequencies on either side of this narrow band These IF amplifiers are basically tuned amplifiers which have been pre tuned to the IF frequency they have a bandwidth just wide enough to amplify the 455 KHz carrier and the AM sidebands either side of it Any frequencies outside this narrow frequency band will not be amplified Examine the out
65. of a 455 KHz carrier and the A M sidebands either side of it carrying the wanted audio information The next step is extract this audio information from the amplitude variations of the signal at the output of IF amplifier 2 This operation is performed by the diode detector block whose output follows the changes in the amplitude of the signal at its input To see how this works examine the output of the diode detector block TP31 together with the output from IF amplifier 2 at tp28 Note that the signal at the diode detector s output Follows the amplitude variations of the incoming signal as required e Contains some ripple at the IF frequency of 455 KHz and The signal has a positive DC offset equal to half the average peak to peak amplitude of the incoming signal We will see how we make use of this offset later on when we look at automatic gain control AGC The final stage of the receiver is the audio amplifier block contains a simple low pass filter which passes only audio frequencies and removes the high frequency ripple from the diode detector s output signal This filtered audio signal is applied to the input of an audio power amplifier which drives on board loudspeaker and the headphones if these are used The final result is the sound you are listening to The audio signal which drives the loudspeaker can be monitored at TP39 providing that the audio amplifier block s volume pot is not in its minimum volume
66. of the information signal and is called the depth of modulation The depth of modulation can be quoted as a fraction or as a percentage Percentage modulation Toron x 100 V max V min Here is an example Figure 6 Scientech Technologies Pvt Ltd 10 ST2201 amp ST2202 In above Figure 6 we can see that the modulated carrier wave varies from a maximum peak to peak value of 10 volts down to a minimum value of 6 volts Inserting these figure in the above formula we get 10 6 Percentage modulation z x 100 10 6 4 x100 16 25 or 0 25 The Frequency Spectrum Assume a carrier frequency fc of 1 MHz and amplitude of say 5 volts peak to peak The carrier could be shown as Amplitude EEN 0 1MHz Frequency Figure 7 If we also have a 1 KHz information signal or modulating frequency fm with amplitude of 2V peak to peak it would look like this Amplitude 2V Carrier Information Signal 0 1kHz 1MHz Frequency Figure 8 When both signals have passed through the amplitude modulator they are combined to produce an amplitude modulated wave Scientech Technologies Pvt Ltd 11 ST2201 amp ST2202 The resultant AM signal has a new frequency spectrum as shown in Figure 9 inserting changes that occurs as a result of the modulation process 1 The original 1 KHz information frequency has disappeared 2 The 1 MHz carrier is still present and is unaltered Sit E
67. ogies Pvt Ltd 26 ST2201 amp ST2202 6 Next examine the output of the balanced modulator amp band pass filter circuit 1 block at tp3 together with the modulating signal at TP1 Trigger the oscilloscope on the TP1 signal Check that the waveforms as shown in Figure 26 t p 1 em er a wa a Figure 26 The output from the balanced modulator amp band pass filter circuit 1 block at TP3 is a double sideband AM waveform which has been formed by amplitude modulating the 1MHz carrier sinewave with the audio frequency sinewave from the audio oscillator The frequency spectrum of this AM waveform is as shown below in Figure 27 where fm is the frequency of the audio modulating signal Amplitude C RRER LOWER SIDEBAND UPPER SIDEBAND 1MHz fm 1MHz 1MHz fm Frequency Figure 27 7 To determine the depth of modulation measure the maximum amplitude Vmax and the minimum amplitude V min of the AM waveform at TP3 and use the following formula Percentage Modulation YE hin max Vmin Where V max and V min are the maximum and minimum amplitudes shown in Figure 26 Scientech Technologies Pvt Ltd 27 ST2201 amp ST2202 8 Now vary the amplitude and frequency of the audio frequency sinewave by adjusting the amplitude and frequency present in the audio oscillator block Note the effect that varying each pot has on the amplitude modulated waveform The amplitude and
68. put of IF amplifier 1 at TP24 with an a c coupled oscilloscope channel and note that a The overall amplitude of the signal is much larger than the signal amplitude at the mixer s output indicating that voltage amplification has occurred b The dominant component of the signal is now at 455 KHz irrespective of any particular station you have tuned into This implies that the wanted signal at the IF frequency has been amplified to a level where it dominates over the unwanted components c The envelope of the signal is modulated in amplitude according to the sound information being transmitted by the station you have tuned into Scientech Technologies Pvt Ltd 34 ST2201 amp ST2202 11 12 13 Examine the output of IF amplifier 2 TP28 with an a c coupled oscilloscope channel noting that the amplitude of the signal has been further amplified by this second IF amplitude of the signal has been further amplified by this second IF amplifier stage IF amplifier 2 has once again preferentially amplified signals around the IF frequency 455 KHz so that a The unwanted local oscillator and sum components from the mixer are now so small in comparison that they can be ignored totally b Frequencies close to the I F frequency which are due to stations close to the wanted station are also strongly attenuated The resulting signal at the output of IF amplifier 2 TP28 is therefore composed almost entirely
69. r while maintaining the input signal level Now record the signal level at output of audio amplifier for different input carrier frequency on CRO i e voltage off resonance Vi Tabulate the readings as under Carrier Frequency Output Voltage Ratio 20 log Vi Vr dB Plot the curve between ratio and carrier frequency Scientech Technologies Pvt Ltd 64 ST2201 amp ST2202 Experiment 9 Objective To plot sensitivity curve for radio receiver Procedure 1 Setting on ST2202 a Set the detector in diode mode b AGC on c Set the volume control fully clockwise Apply AM signal with 400Hz modulating signal and 30 modulation taken from AM generator into Rx input socket Set the input carrier frequency so as to lie within the AM Band 525 KHz 1600 KHz Also tune the detector to that carrier frequency using tuning control You will hear atone Set the input AM level to 100mV Also adjust the gain potentiometer provided in R F amplifier section of ST2202 so as to get unclipped demodulated signal at detectors output Record input carrier frequency amp signal level at the final output stage i e output of audio amplifier observed on CRO Change the input carrier frequency amp also tune the receiver to that frequency amp repeat step 4 Tabulate the collected readings as under Carrier frequency Output pp Plot the graph between carrier freque
70. reasonably good sinewave for all audio frequencies we have achieved our objective of shifting up the narrow range of output frequencies from the ceramic band pass filter block which were around 455 KHz so that they are now around 1 455 MHz As a result they now fall within the AM broadcast range of 525 KHz to 1 60MHz and will be detectable by the ST2202 receiver When the modulating audio signal is swept over its entire range a range of 3 4 KHz 300Hz 3 1 KHz the SSB waveform at TP22 sweeps over the same frequency range So single sideband modulation has simply served to shift our range of audio frequencies up so they are centered around 1 455MHz Monitor the 1 455 MHz SSB signal at TP22 together with the audio modulating signal TP15 triggering the scope with the later Reduce the amplitude of the audio modulating signal to zero by means of the audio oscillator block s amplitude pot and note that the amplitude of the SSB signal also drops to zero as expected Return the amplitude pot to its MAX position Examine the final SSB output at TP22 together with the output from the output amplifier block TP13 Note that the final SSB waveform appears amplified slightly at TP13 As we still see later it is the output signal which will be transmitted to the receiver By using the microphone the human voice can be used as the audio modulating signal instead of using ST2201 s audio oscillator block Connect the microphone to th
71. rom center slightly lower and slightly higher In addition the position of the limited range of frequencies from IF amplifier 2 will depend on the exact frequency of the receiver local oscillator output If the Scientech Technologies Pvt Ltd 55 ST2201 amp ST2202 oscillator s frequency is varied slightly from its present frequency this range of frequencies can be moved both above and below 455 KHz This is illustrated in Figure 50 Amplitude LOCAL OSCILLATOR FREQUENCY DECREASED SLIGHTLY 455kHz Frequency a Frequency range of IF amplifier 2 s output with slightly reduced local oscillator frequency Amplitude LOCAL OSCILLATOR FREQUENCY INCREASED SLIGHTLY 455kHz Frequency b Frequency range of IF amplifier 2 s output with slightly increased local oscillator frequency Figure 50 The product detector block mixes the output from the BFO with the output from I F amplifier block mixing process results in the generation of two new frequency components a component whose frequency is the sum of the two input frequencies e a component whose frequency is the difference between the two input frequencies A low pass filter at the output of the product detector rejects all frequencies except the difference frequency Consequently any slight difference in frequency between the BFO s output and I F amplifier 2 s output will result in and audio frequency at the product detector s output This audio
72. s the whole range of incoming RF frequencies and it would be difficult to arrange for all the amplifiers to keep in step as they are re tuned In addition the radio must select the wanted transmission and reject all the others To do this the band pass of all the stages must carefully controlled Each IF stage does not necessarily have the same band pass characteristics The overall response is important Again this is something which is much more easily achieved without the added complication of making them tunable At the final output from the IF amplifiers we have a 455 KHz wave which is amplitude modulated by the wanted audio information The selectivity of the IF amplifiers has removed the unwanted components generated by the mixing process Diode Detector The function of the diode detector is to extract the audio signal from the signal at the output of the IF amplifiers It performs this task in a very similar way to a half wave rectifier converting an AC input to a DC output Figure 22 shows a simple circuit diagram of the diode detector Input Output OV Figure 22 Scientech Technologies Pvt Ltd 23 ST2201 amp ST2202 In Figure 22 the diode conducts every time the input signal applied to its anode is more positive than the voltage on the top plate of the capacitor When the voltage falls below the capacitor voltage the diode ceases to conduct and the voltage across the capacitor leaks away until the next time
73. scillator block s frequency pot roughly in its midway position arrowhead pointing towards the top turn the block s amplitude pot to its MIN position and note that the amplitude of the signal at the ceramic band pass filter s output TP20 drops to zero This highlights one on the main advantages of SSB amplitude modulation if there is no modulating signal then the amplitude of the SSB waveform drops to zero so that no power is wasted Return the amplitude pot to its MAX position You will recall that we have used a ceramic band pass filter to pass the wanted upper sideband but reject the unwanted lower sideband which was also produced by the amplitude modulation process We used this type of filter because it passes the upper sideband yet has a sufficiently sharp response to strongly attenuate the lower sideband which is close by However there is a disadvantage of this type of filter is the range of frequencies that the filter will pass is fixed during the filter s manufacture and cannot subsequently be altered The particular filter we are using has a pass band centered on 455 KHz and this is why we have arranged for the wanted upper sideband to also be at about 455 KHz As we will see in later experiments the ST2201 ST2202 receiver will accept radiofrequency signals in the AM broadcast band i e signals which fall in the frequency range of 525 KHz However since the SSB output from the ceramic band pass filter occupies a narrow
74. smitted At the receiver the carrier must be re inserted to produce the modulation envelope to enable the detector to extract the information signal And here lies the problem The carrier has to be re inserted at exactly the correct frequency to reproduce the original AM waveform within a few hertz If it is not there are serious problems with reception Take a situation in which the upper and lower side frequencies are spaced 4 KHz either side of the carrier at 600 4 596 KHz amp 600 4 604 KHz Now let s assume that the receiver carrier were to be re inserted at an incorrect value of 601 KHz This would result in a spacing of only 3 KHz between the carrier and the upper side frequency and 5 KHz between the carrier and the lower side frequency What effect would this have Scientech Technologies Pvt Ltd 39 ST2201 amp ST2202 Remembering our previous exercise in which we created an AM envelop by plotting a graph we can see that these incorrect side frequency spacing will give rise to a badly deformed modulation envelop and hence a distorted output sound which makes speech sound like Donald Duck With this type of transmission the receiver would be carefully tuned in to the correct frequency and the station would be received A few moments later the reinserted carrier frequency would drift slowly off tune and Donald Duck would re appear We would have to reach over and retune the radio and settl
75. t Ltd 31 ST2201 amp ST2202 5 On the ST2202 module slowly turn the audio amplifier s volume pot clockwise until sounds can be heard from the on board loudspeaker Next turn the vernier tuning dial until a broad cast station can be heard clearly and adjust the volume control to a comfortable level Note If desired headphones supplied with the module may be used instead of the on board loudspeaker To use the headphones simply plug the headphone jack into the audio amplifier block s headphones socket and adjust controlled block s volume pot 6 The first stage or front end of the ST2202 AM receiver is the R F amplifier stage This is a wide bandwidth tuned amplifier stage which is tuned into the wanted station by means of the tuning dial Once it has been tuned into the wanted station the R F amplifier having little selectivity will not only amplify but also those frequencies that are close to the wanted frequency As we will see later these nearby frequencies will be removed by subsequent stages of the receiver to leave only the wanted signal Examine the envelope of the signal at the R F amplifier s output at TP12 with an a c coupled oscilloscope channel Note that a The amplifier s output signal is very small in amplitude a few tens of millivolts at the most This is because one stage of amplification is not sufficient to bring the signal s amplitude up to a reasonable level b Only a very smal
76. tch to the OFF position ST2201 switched faults This section lists the faults on the ST2201 and ST2202 modules There are 8 faults switches on each modules 1 Fault prevents the 1MHz oscillator from oscillating by disconnecting the tuned circuits primary winding from the 12volts supply Fault disables the output of the balanced modulator amp bandpass filter circuit 1 block at TP3 to become a double sideband suppressed carrier DSBSC signal irrespective of the position of the block s balance pot The fault disconnects the balance pots slider from the 12 volt supply Fault cause the output frequency from the balanced modulator amp bandpass filter circuit 1 block at TP3 to become a double sideband suppressed carrier DSBSC signal irrespective of the position of the block s balance pot The fault disconnects the balance pots slider from the 12volt supply Scientech Technologies Pvt Ltd 70 ST2201 amp ST2202 4 Fault cause the output frequency from the audio oscillator block at TP14 to drop to 150Hz irrespective of the block s frequency pot position The fault disconnects the 56K resistor in the frequency pots divider chain from 0 volts so that the FM sweep input to the 8038 pin 8 is pulled up to 12 volts Fault stops the 455 KHz oscillator by shorting out the 18K resistor the transistor s base bias chain This cause the bias on the transistor s base TP6 to drop to 0 volts Fault prevents t
77. th Carrier Amplitude Ht tt _ g Frequency Total bandwidth If the carrier contained several frequencies each would produce its own side frequencies Figure 13 Scientech Technologies Pvt Ltd 15 ST2201 amp ST2202 Modulator In this circuit the amplitude of the carrier is increased and decreased in sympathy with the incoming information signal S ai e ab Information Signal Carrier Wave AM Waveform AM Modulation Process Figure 14 The modulated signal is now nearly ready for transmission If the modulation process has given rise to any unwanted frequency components then a band pass filter can be employed to remove them Output Amplifier This amplifier is used to increase the strength of the signal before being passed to the antenna for transmission The output power contained in the signal and the frequency of transmission are the two main factors that determine the range of the transmission The Antenna An electromagnetic wave such as a light ray consists of two fields an electric field and magnetic field These two fields are always as right angles to each other and move in a direction which is at right angles to both the magnetic and the electric fields this is shown in Figure 15 16 amp 17 Figure 15 shows the electric fields moving out from the antenna In this example the electric field is vertical because the antenna is positioned vertically in the direction shown by y Fig
78. the audio amplifier block clock wise until you can hear the tone of the audio oscillator s output signal in addition to the tone from the ST2202 board With the receiver s tuning dial on the counter clockwise side of the minimum frequency position i e using dial positions lower than the minimum frequency position find the position where the two tones are approximately the same Then turn the frequency pot in ST2201 s audio oscillator block throughout its range noting that the frequency of the tone generated by ST2202 remains close to that generated by ST2201 for all pot positions Demodulation of the SSB signal has now been achieved so the volume pot in the transmitter s audio amplifier block can now be returned to its full counter clockwise minimum position Note If the tuning dial is tuned on the clockwise side of the minimum frequency position rather than the counter clockwise side a position will still be found where the transmitter and receiver tones are approximately the same However if the transmitter s audio frequency is then increased the receiver s audio frequency will decrease and vice versa The reason for this is that the frequency of IF amplifier 2 s output is now above the BFO frequency instead of below it converting all high frequency components in the transmitter s modulating waveform into low frequency components and vice versa Consequently SSB demodulation is not achieved with tuning dial on the clockw
79. the input signal is able to switch it on again See Figure 23 Waveform at the Capacitor discharges output of the detector Diode conducts and capacitor charges AM waveform at the Voy input of the detector Figure 23 The result is an output which contains three components 1 The wanted audio information signal 2 Some ripple at the IF frequency 3 A positive DC voltage level The Audio Amplifier At the input to the audio amplifier a low pass filter is used to remove the IF ripple and a capacitor blocks the DC voltage level Figure 24 shows the result of the information signal passing through the diode detector and audio amplifier The input to the diode detector from the last IF amplifier VS EEEE Output of diode detector includes a DC level the audio signal ripple at IF frequency ea ace Output after filtering Figure 24 The remaining audio signals are then amplified to provide the final output to the loudspeaker Scientech Technologies Pvt Ltd 24 ST2201 amp ST2202 Automatic Gain Control AGC The AGC circuit is used to prevent very strong signals from overloading the receiver It can also reduce the effect of fluctuations in the received signal strength The AGC circuit makes use of the mean DC voltage level present at the output of the diode detector If the signal increases the mean DC voltage level also increases IF the mean DC voltage level exceeds a predetermined thr
80. ts Fault shorts to 0 volts the AGC control input to the R F amplifier and IF amplifier 1 blocks at TP1 and TP2 disabling both blocks Fault shorts the inverting input pin 2 of the audio amplifier block s LM 386 power amplifier IC to 0 volts so that there is no audio output from the block Scientech Technologies Pvt Ltd 71 ST2201 amp ST2202 Warranty 1 We guarantee the product against all manufacturing defects for 24 months from the date of sale by us or through our dealers Consumables like dry cell etc are not covered under warranty 2 The guarantee will become void if a The product is not operated as per the instruction given in the operating manual b The agreed payment terms and other conditions of sale are not followed c The customer resells the instrument to another party d Any attempt is made to service and modify the instrument 3 The non working of the product is to be communicated to us immediately giving full details of the complaints and defects noticed specifically mentioning the type serial number of the product and date of purchase etc 4 The repair work will be carried out provided the product is dispatched securely packed and insured The transportation charges shall be borne by the customer List of Accessories Accessories for ST2201 L Patent Cord lO rae e eaves Sea e a ENAR a ENE 2 Nos Zes Mans COnd ass 5 5 55 el Boao n eh eR OE 1 No Si _ Microphone 2 se eseh evel a
81. ure 16 shows magnetic field is always at right angles to the electric field so in this case it is positioned horizontally in the direction shown by x Figure 17 shows an electromagnetic wave both fields exist together and they move at the speed of light in a direction that is at right angles to both fields shown by the arrow labeled z Scientech Technologies Pvt Ltd 16 ST2201 amp ST2202 Figure 15 This shows the electric field moving out from the antenna In this example the electric field is vertical because the antenna is positioned vertically in the direction shown by y x lt Magnetic Field Antenna Figure 16 The magnetic field is always at right angles to the electric field so in this case it is positioned horizontally in the direction shown by x Antenna bie Electromagnetic peu Wave XL j z Figure 17 In an electromagnetic wave both fields exist together and they move at the speed of light in a direction that is at right angles to both fields shown by the arrow labeled z The antenna converts the power output of the output amplifier into an electromagnetic wave Scientech Technologies Pvt Ltd 17 ST2201 amp ST2202 How does it do this The output amplifier causes a voltage to be generated along the antenna thus generating a voltage difference and the resultant electric field between the top and bottom This causes an alternating movement of electrons on the transmitt
82. ve Assuming the incoming signal has an amplitude of 10 mV as shown its output at this frequency would be 5 x 10mV 50mV The stations being received at 810 KHz and 830 KHz each have a gain of one With the same amplitude of 10m V this could result in outputs of 1 x 10mV 10mV The stations at 800 KHz and 840 KHz are offered a gain on only 0 1 approx This means that the output signal strength would be only 0 1 x 10mV ImV The over all effect of the selectivity is that the incoming signals each have the same amplitude the outputs vary between 1mV and 50mV so we can select or tune the amplifier to pick out the desired station The greatest amplification occurs at the resonance frequency of the tuned circuit This is sometimes called the center frequency In common with nearly all radio receivers ST2202 adjusts the capacitor value by means of the tuning control to select various signals Scientech Technologies Pvt Ltd 20 ST2201 amp ST2202 The Local Oscillator This is an oscillator producing a sinusoidal output similar to the carrier wave oscillator in the transmitter In this case however the frequency of its output is adjustable The same tuning control is used to adjust the frequency of both the local oscillator and the center frequency of the RF amplifier The local oscillator is always maintained at a frequency which is higher by a fixed amount then the incoming RF signals The local oscillator frequency

DSB/SSB Transmitter & Receiver Trainer ST2201 & ST2202

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DSB/SSB Transmitter &amp; Receiver Trainer ST2201 &amp; ST2202

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DSB/SSB Transmitter & Receiver Trainer ST2201 & ST2202