Frequency Modulation and Demodulation Trainer ST2203 Operating
Contents
1.2. Off condition b Audio amplifier block s amplitude potentiometer in fully clockwise maximum position c Audio oscillator block s frequency potentiometer in fully counter clockwise minimum position d Amplitude present in the mixer amplifier block in fully clockwise position e VCO switch in phase locked loop detector block in Off position 2 Make the connections shown in figure 29 3 Turn on power to the ST2203 module 4 Initially we will use the varactor modulator to generate our FM signal since this is the more linear of the two frequency modulators as far as its frequency voltage characteristic is concerned To select the varactor modulator put the reactance varactor switch in the varactor position Ensure that the varactor modulator s carrier frequency potentiometer is in the midway position 5 The varactor modulator to frequency modulate a 455 KHz carrier sine wave is now using the audio oscillator s output signal which appears at TP1 As we saw earlier this FM waveform appears at the FM output socket from the mixer amplifier block You will probably need to have an X expansion control on your Oscilloscope 6 Now monitor audio input signal to the varactor modulator block at TP14 together with the output form the quadrature detector block at TP46 triggering the Oscilloscope The signal at TP46 should contain three components ST2203 Scientech Technologies Pvt Ltd 38 a A positi
3. by open circuiting the non supply end of 56 K bias resistor R38 Causes the block s output amplitude at TP68 to drop to 0 volts peak to peak 5 Fault shorts out the 1 K feedback resistor between the output pin 1 and the inverting input pin 2 of the reactance modulator block s driver op amp 34084 a IC 1 This prevents the reactance modulator s output at TP13 from being frequency modulated by the signal applied to the audio input socket 6 Fault shorts the base of the mixer amplifier s modulating transistor Tr7 to 0 V This causes the output amplitude from the mixer amplifier block at TP34 to drop to 0 Vpp irrespective of the position of the block s amplitude potentiometer 7 Fault shorts TP48 in the Foster Seeley ratio detector block to 0 volts This prevents any signal from appearing across T6 s resonant circuit between TP49 amp 51 and disables the outputs from the block for both foster Seeley and ratio modes of operation 8 Fault shorts out the phase locked loop detector block s frequency adjust potentiometer VR 7 This increases the free running frequency of the Voltage Controlled Oscillator VCO to approximately 500 KHz preventing the phase locked loop from locking into the incoming 455 KHz FM carrier irrespective of the setting of the frequency adjust potentiometer Consequently the block s output TP60 no longer contains a component at the original audio modulating signal frequency ST2203
4. component at the detector s output and also blocks the DC offset voltage Consequently the signal at the output of the low pass filter amplifier block at TP73 should be very closely resemble the original audio making signal if not then slowly adjust the frequency adjust potentiometer of PLL block ST2203 Scientech Technologies Pvt Ltd 45 6 Adjust the audio oscillator block s amplitude and frequency potentiometer and compare the original audio signal with the final demodulated signal 7 We can investigate the effect of noise on the system by following the procedure given in earlier experiments The only change will be that we will use phase locked loop detector instead of quadrature or detuned resonant circuit ST2203 Scientech Technologies Pvt Ltd 46 Figure 35 ST2203 Scientech Technologies Pvt Ltd 47 Foster Seeley Detector The last two demodulators to be considered employ the phase shift that often accompanies a change in frequency in an AC circuit The Foster Seeley circuit is shown in figure 36 At first glance it looks rather complicated but it becomes simpler if we consider it a bit at a time Figure 36 When the input signal is un modulated We will start by building up the circuit a little at a time To do this we can ignore many of the companies Figure 37 shows only the parts which are in use when the FM input signal is un modulated Figure 37 We may recognize immediately that it consist
5. important for us to remember that we are really dealing with a group of sine waves of differing frequencies amplitudes and phases A Simple Communication System Once we are out of shouting range from another person we must rely on some communication system to enable us to pass information The only essential parts of any communication system are a transmitter a communication 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 Simple Communication System Figure 4 For long distances or when it is required to send signals to many destinations at the same time it is convenient to use a radio communication system One of the alternative systems is frequency modulation in which the information signal is used to control the frequency of the carrier wave This works equally well and in some respects better than the amplitude modulation The frequency of the carrier is made to increase as the voltage in the information signal increases and to decrease in frequency as it reduces The larger the amplitude of the information signal the further the frequency of the carrier signal is shifted from ST2203 Scientech Technologies Pvt Ltd 8 its starting point The frequency of the information signal determines how many times in a second this change in frequency occurs Notice in figure 5 that the modulation pro
6. Adjustment 59 17 Switched Faults 62 18 Warranty 63 19 List of Accessories 63 ST2203 Scientech Technologies Pvt Ltd 4 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 operating manual amp work book 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 Compact size ST2203 Scientech Technologies Pvt Ltd 5 Technical Specifications Audio Oscillator With adjustable amplitude amp frequency 300 Hz 3 4 KHz Two Types of FM Modulator 1 Reactance Modulator with carrier frequency adjustment 2 Varactor Modulator with carrier frequency adjustment Mixer Amplifier Allows FM input signal to be amplitude modulated by a noise input prior to demodulation with gain adjustment Transmitter Output Frequency 455 KHz Five Types of FM Demodulator 1 Detuned Resonant Detector 2 Quadrature Detector 3 Foster Seeley Detector 4 Ratio Detector 5 Phase Locked Loop Detector Low Pass Filter Amplifier 3 4 KHz cut off frequency with adjustable gain Amplitude Limiter 1 No Switched Faults 8 Nos Power Supply 230 V 10 50Hz Test Points 74 Nos Po
7. Frequency Modulation and Demodulation Trainer ST2203 Operating Manual Ver 1 1 An ISO 9001 2000 company 94 101 Electronic Complex Pardesipura Indore 452010 India Tel 91 731 2570301 02 4211100 Fax 91 731 2555643 e mail info scientech bz Website www scientech bz Toll free 1800 103 5050 ST2203 Scientech Technologies Pvt Ltd 2 ST2203 Scientech Technologies Pvt Ltd 3 Frequency Modulation and Demodulation Trainer ST2203 Table of Contents 1 Features 4 2 Technical Specifications 5 3 Frequency Components of Human Voice 6 4 A Simple Communication System 7 5 Advantage of FM 9 6 Disadvantage of FM 10 7 FM Transmitter 10 8 Varactor Modulator 13 Experiment 1 17 Study of Frequency Modulation using Varactor modulator 9 Reactance Modulator 20 Experiment 2 22 Study of Frequency Modulation Using Reactance Modulator 10 Demodulation of FM signal 25 11 Detuned Resonant Circuit Detector 27 Experiment 3 29 Study of Operation of Detuned Resonant Circuit 12 Quadrature Detector 35 Experiment 4 37 Study of Operation of Quadrature Detector 13 Phase Locked Loop Detector 42 Experiment 5 44 Study of Operation of Phase Locked Loop Detector 14 Foster Seeley Detector 47 Experiment 6 51 Study of Operation of Foster Seeley Detector 15 Ratio Detector 54 Experiment 7 56 Study of Operation of Ratio Detector 16 Coil
8. Scientech Technologies Pvt Ltd 63 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 1 4mm Patch Cord 8 2 Nos 2 4mm Patch Cord 16 2 Nos 3 Mains Cord 1 No 4 e Manual 1 No Updated 09 02 2009
9. diagram for the reactance modulator is given at the end of operating manual If you wish follow this circuit diagram and examine the test points in the reactance modulator block to make sure that you fully understand how the circuit is working 14 By using the optional audio input module the human voice can be used as the audio modulating signal instead of using ST2203 s audio oscillator block If you have an audio input module connect the module s output to the audio input socket in the modulator circuit s block The input signal to the audio input module may be taken from an external microphone supplied with the module or from a cassette recorder by choosing the appropriate switch setting on the modules ST2203 Scientech Technologies Pvt Ltd 24 Figure 19 ST2203 Scientech Technologies Pvt Ltd 25 Demodulation on FM Signals A FM receiver is very similar to an AM receiver The most significant change is that the demodulator must now extract the information signal from a frequency rather than amplitude modulated wave FM Receiver Figure 20 The basic requirement of any FM demodulator is therefore to convert frequency change into change in voltage with the minimum amount of distortion To achieve this it should ideally have a linear voltage frequency characteristic similar to that shown in figure 21 A demodulator can also be called a discriminator or a detector ST2203 Scientech Technologies Pv
10. is very similar to the construction of a capacitor ST2203 Scientech Technologies Pvt Ltd 13 Varactor Diode Figure 10 By increasing the reverse biased voltage the width of the insulating region can be increased and hence the capacitance value decreases This is shown in figure 11 Low voltage applied Narrow non conducting region More capacitance Increased voltage applied Wider non conducting region Less capacitance Operation of Varactor Diode Figure 11 If the information signal is applied to the varactor diode the capacitance will therefore be increased and decreased in sympathy with the incoming signal Recommended testing instruments for experimentation 1 Scientech 20 MHz Dual Trace Oscilloscope 201 or equivalent 2 Switchable Probe X1 X10 Varactor Modulator The variations in capacitance form part of the tuned circuit that is used to generate the FM signal to be transmitted Have a look at the varactor modulator shown in figure 12 ST2203 Scientech Technologies Pvt Ltd 14 Figure 12 We can see the tuned circuit which sets the operating frequency of the oscillator and the varactor which is effectively in parallel with the tuned circuit Two other components which may not be immediately obvious are C1 and L1 C1 is a DC blocking capacitor to provide DC isolation between the oscillator and the collector of the transmitter L1 is an RF choke which allows the information signal to pass
11. of two envelope detectors like half wave rectifiers being fed from the center tapped coil L2 With reference to the center tap the two voltages V1 and V2 are in anti phase as shown by the arrows The output ST2203 Scientech Technologies Pvt Ltd 48 voltage would be zero volts since the capacitor voltages are in anti phase and are equal in magnitude After adding two capacitors The next step is to add two capacitors and see their effect on the phase of the signals See figure 38 Figure 38 L1 and L2 are magnetically tightly coupled and by adding C3 across the centre tapped coil they will form a parallel tuned circuit with a resonance frequency equal to the un modulated carrier frequency Capacitor C5 will shift the phase of the input signal by 90 with reference to the voltage across L1 and L2 The voltages are shown as Va and Vb in the phasor diagram given in figure 39 Using the input signal Vfm as the reference the phasor diagrams now look the way shown in figure 39 Circuit diagram Phasor diagram Figure 39 ST2203 Scientech Technologies Pvt Ltd 49 The complete circuit By looking back at figure 36 we can see that there are only two components to be added C4 and L3 C4 is not important It is only a DC blocking capacitor and has negligible impedance at the frequencies being used But what it has to do is to supply a copy of the incoming signal across L3 The entire incoming signal is dropped across L3 be
12. output Consequently the signal at the output of the Low Pass Filter amplifier block at TP73 should very closely resemble the original audio modulating signal 9 Monitor the audio input to the varactor modulator at TP14 and the output of the Low Pass Filter amplifier block at TP73 and adjust the gain potentiometer in the Low Pass Filter amplifier block until the amplitudes of the monitored audio waveforms are the same 10 Adjust the audio oscillator block s amplitude and frequency potentiometer and compare the original audio signal with the final demodulated signal 11 We can investigate the effect of noise on the system by following the procedure given in earlier chapters by merely substituting quadrate detector by Foster Seeley Detector ST2203 Scientech Technologies Pvt Ltd 53 Figure 42 ST2203 Scientech Technologies Pvt Ltd 54 Ratio Detector At first glance it appears to be the same as the Foster Seeley Detector There are few modifications that have provided a much improved protection from noise The circuit diagram is given in figure 43 Diode D2 has been reversed so that the polarity of the voltage across C2 will be as shown in the figure When the carrier is un modulated the voltages across C1 and C2 are equal and additive The audio output is taken across C2 or R2 Capacitor C6 is a large electrolytic capacitor It charges to this voltage Owing to the long time constant of C6 the total voltag
13. relationship with the frequency of the information signal or with the frequency deviation or it seems anything else FM is unlike AM in this respect FM Transmitter The block diagram is shown in figure 7 below FM Transmitter Figure 7 ST2203 Scientech Technologies Pvt Ltd 11 The audio oscillator supplies the information signal and could if we wish can be replaced by a microphone and AF amplifier to provide speech and music instead of the sine wave signals that we are using with ST2203 The FM modulator is used to combine the carrier wave and the information signal much in the same way as in the AM transmitter The only difference in this case is that the generation of the carrier wave and the modulation process is carried out in the same block It is not necessary to have the two processes in same block but in our case it is The output amplifier increases the power in the signal before it is applied to the antenna for transmission just as it did in the corresponding block in the FM transmitter The only real difference between the AM and FM transmitters are the modulations so we are only going to consider this part of the transmitter We are going to investigate two types of modulator they are called the varactor modulator and the reactance modulator How do this modulators work The basic idea is quite simple and both modulations function in the same way They both include a RF oscillator to generate the carrier and these oscil
14. voltage level 3 Phases less than 90 result in an increased DC voltage level As the phase changes the DC voltage level moves up and down and re creates the audio signal A low pass filter is included to reduce the amplitude of any high frequency ripple and blocks the DC offset Consequently the signal at the output closely resembles the original input signal The characteristic as shown in figure 28 is straight to cause very little distortion to the final audio output ST2203 Scientech Technologies Pvt Ltd 36 Figure 28 ST2203 Scientech Technologies Pvt Ltd 37 Experiment 4 Objective Study of Operation of Quadrature Detector Procedure This experiment investigates how the quadrature detector block on the ST2203 module performs frequency demodulation The operation of this detector circuit will be described in detail and its sensitivity to noise on the incoming FM signal will be investigated The on board amplitude limiter will then be used to remove any amplitude modulations due to noise before they reach the detector This allows the student to draw conclusions as to whether it is necessary to precede this type of detector with an amplitude limiter state in a practical FM receiver 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 ST2203 module a All Switch Faults in
15. ST2202 board a All Switched Faults in Off condition b Amplitude potentiometer in mixer amplifier block in fully clockwise position c VCO switch in phase locked loop detector block in Off position 2 Make the connections as shown in figure 13 3 Switch On the power 4 Turn the audio oscillator block s amplitude potentiometer to its fully clockwise position and examine the block s output TP1 on an Oscilloscope This is the audio frequency sine wave which will be used as our modulating signal Note that the sine wave s frequency can be adjusted from about 300Hz to approximately 3 4 KHz by adjusting the audio oscillator s frequency potentiometer Note also that the amplitude of this modulating signal is adjusted by audio oscillator amplitude potentiometer Leave the amplitude potentiometer in minimum position 5 Connect the output socket of the audio oscillator block to the audio input socket of the modulator circuit s block 6 Set the reactance varactor switch to the varactor position This switch selects the varactor modulator and also disables the reactance modulator to prevent any interference between the two circuits 7 The output signal from the varactor modulator block appears at TP24 before being buffered and amplified by the mixer amplifier block any capacitive loading e g due to Oscilloscope probe may slightly affect the modulators output frequency In order to avoid this problem we
16. ST2203 Scientech Technologies Pvt Ltd 29 Experiment 3 Objective Study of Operation of Detuned Resonant Circuit Procedure This experiment investigates how the detuned resonant circuit detector block on the ST2203 module performs frequency demodulation The operation of this detector circuit will be described in detail and its sensitivity to noise on the incoming FM signal will be investigated The on board amplitude limiter will then be used to remove any amplitude variations due to noise before they reach the detector This allows the student to draw conclusions as to whether it is necessary to precede this type of detector with an amplitude limiter stage in a practical FM receiver 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 ST2203 module a All Switched Faults in Off condition b Audio amplifier block s amplitude potentiometer in fully clockwise maximum position c Audio amplifier block s frequency potentiometer in fully counter clockwise position d Amplitude potentiometer in the mixer amplifier block in fully clockwise position e VCO switch in phase locked loop detector block in Off position 2 Make the connections as shown in figure 25 3 Switch on the power to the ST2203 module 4 Initially we will use the varactor modulator to ge
17. The input signal is converted into a square wave and together with the VCO output forms the two inputs to an Exclusive OR gate Remember that the Exclusive OR gate provides an output whenever the two inputs are different in value and zero output whenever they are the same Figure 33 shows the situation when the FM input is at its un modulated carrier frequency and the VCO output is of the same frequency and 900 out of phase This provided an output from the Exclusive OR gate with an on off ratio of unity and an average voltage at the output of half of the peak value as shown Now let us assume that the FM signal at the input decreases in frequency see figure 34 The period of the squared up FM signal increases and the mean voltage level from the Exclusive OR gate decreases The mean voltage level is both the demodulated output and the control voltage for the VCO The VCO frequency will decrease until its frequency matches the incoming FM signal Figure 34 ST2203 Scientech Technologies Pvt Ltd 44 Experiment 5 Objective Study of Operation of Phase Locked Loop Detector Procedure This experiment investigates how the phase locked loop detector block on the ST2203 module performs frequency demodulation The operation of this detector circuit will be described in detail and its sensitivity to noise on the incoming FM signal will be investigated On board amplitude limiter will then used to remove any amplitude modulati
18. at TP40 should contain three components A positive DC offset voltage A sine wave at the same frequency as the audio signal all TP14 A high frequency ripple component of small amplitude Check that the audio frequency component is a reasonable sine wave If it is not it is likely that the centre frequency of the varactor modulator s FM output needs adjusting slightly To do this trim transformer T2 in the varactor modulator block in accordance with the instructions given in chapter Adjustment of ST2203 s tuned circuits 8 The low pass filter amplifier block strongly attenuates the high frequency ripple component at the detector s output and also blocks the DC offset voltage Consequently the signal at the output of the low pass filter amplifier block at TP73 should very closely resemble the original audio modulating signal Monitor the input TP69 and output TP73 of the low pass filter amplifier block triggering on TP 73 and note how the quality of the detector s output signal has been improved by low pass filtering Note also that the DC offset has been removed 9 Monitor the audio input to the varactor modulator at TP14 and the output of the low pass filter amplifier block at TP73 and adjust the gain potentiometer in the low pass filter amplifier block until the amplitudes of the two monitored audio waveforms are the same 10 Adjust the audio oscillator block s amplitude and frequency potentio
19. been replaced by the variable capacitor shown dotted In the next part the two supply lines are connected together We can justify this by saying that the output of the DC power supply always includes a large smoothing capacitor to keep the DC voltages at a steady value ST2203 Scientech Technologies Pvt Ltd 21 Figure 18 This large capacitor will have a very low reactance at the frequencies being used in the circuit less than a milliohm We can safely ignore this and so the two supply lines can be assumed to be joined together Remember that this does not affect the DC potentials which remain at the normal supply voltages If the two supply voltages are at the same AC potential the actual points of connection do not matter and so we can redraw the circuit as shown in the third part Operation of the Reactance Modulator If required reference can be made to figure 17 1 The oscillator and tuned circuit provide the un modulated carrier frequency and this frequency is present on the collector of the transistor 2 The capacitor and the resistor provide the 90 phase shift between the collector voltage and current This makes the circuit appear as a capacitor 3 The changing information signal being applied to the base has the same effect as changing the bias voltage applied to the transistor and this would have the effect of increasing and decreasing the value of this capacitance 4 As the capacitance is effective
20. cause C1 and C2 also have negligible impedance If we return to the envelope detector section we now have two voltages being applied to each diode One is V1 or V2 and the other is the new voltage across L3 which is equal to Vfm This part of the diagram and the associated phasor diagram are shown in figure 40 below Circuit diagram Phasor diagram Figure 40 ST2203 Scientech Technologies Pvt Ltd 50 When the input Frequency changes If the input frequency increased above its un modulated value the phasor of Va would fall below 90 due to the parallel tuned circuit becoming increasingly capacitive The phasor representing V1 and V2 would move clockwise as shown in figure 41 This would result in a larger total voltage being applied across D1 and a reduced voltage across D2 Since the capacitor C1 would now charge to a higher voltage the final output from the circuit would be a positive voltage Figure 41 Conversely if the frequency of the FM input signal decreased below the un modulated value the phase shift due to capacitor C5 increases above 90 as the parallel tuned circuit becomes slightly inductive This causes the voltage across diode D2 to increase and the final output from the demodulator becomes negative The effect of noise is to change the amplitude of the incoming FM signal resulting in a proportional increase and decrease in the amplitude of diode voltages VD1 and VD2 and the difference in voltage is the demod
21. cess does not affect the amplitude Figure 5 ST2203 Scientech Technologies Pvt Ltd 9 Advantages of FM There are three advantages of frequency modulation for a communication system Electrical noise alters the amplitude but not the signal frequency Figure 6 1 In the last section we saw that the information signal controlled the frequency of the carrier but had no effect on its amplitude Now when any transmission is affected by electrical noise the noise signal is superimposed on the transmitted signal as shown in figure 6 below In an AM System the demodulator is designed to respond to changes in amplitude of the received signal but in a FM receiver the demodulator is only watching for changes in frequency and therefore ignores any changes in amplitude Electrical noise thus has little or no effect on a FM communication system 2 The bandwidth of the FM signal is very wide compared with an AM transmission Typical broadcast bandwidths are in the order of 250 KHz This allows a much better sound quality so signals like music sound significantly better if frequency modulation is being used 3 When an FM demodulator is receiving an FM signal it follows the variations in frequency of the incoming signal and is said to lock on to the received at the same time The receiver lock on to the stronger of the two signals and ignores the other This is called the capture effect and it means that we can listen to an FM sta
22. dule In the Foster Seeley ratio detector block select the Foster Seeley detector by putting the switch in the Foster Seeley position 5 Initially we will use the varactor modulator to generate our FM signal since this is the more linear of the two modulators as fast as its frequency voltage characteristic is concerned To select the varactor modulator put the reactance varactor switch in the varactor position Ensure that the varactor modulator s carrier frequency potentiometer is in the midway position 6 The audio oscillator s output signal which appears at TP1 is now being used by the varactor modulator to frequency modulate a 455 KHz carrier sine wave As we saw earlier this FM waveform appears at the FM output socket from the mixer amplifier block You will probably need to have an X expansion control on your Oscilloscope ST2203 Scientech Technologies Pvt Ltd 52 7 Now monitor the audio input signal to the varactor modulator block at TP14 together with the Foster Seeley output from the Foster Seeley ratio detector block at TP52 triggering the Oscilloscope on TP14 The signal at TP52 should contain two components A sine wave at the same frequency as the audio signal at TP14 A High frequency ripple component of small amplitude 8 The Low Pass Filter amplifier strongly attenuates this high frequency ripple component and blocks any small DC offset voltage that might exist at the detector s
23. e across R1 amp R2 remains virtually constant at all times In fact it just acts as a power supply or a battery The important thing to note is that it keeps the total voltage of C 1 C 2 at a constant value Figure 43 The generation of the voltage across the diodes Dl and D2 are by exactly the same process as we met in the Foster Seeley Detector Indeed even the changes in voltage occur in the same way and for the same reasons For convenience the resulting phasor diagrams are repeated here in figure 44 ST2203 Scientech Technologies Pvt Ltd 55 Figure 44 An un modulated FM signal will result in equal voltages across R1 and R2 The voltage across R2 is the output from the circuit If frequency of the FM signals increases the voltage across R1 will increase and that across R2 will decrease Conversely if the frequency of the FM signals decreases the voltage across R1 will decrease and that across R2 will increase The final demodulated audio output voltage is taken across R2 and this voltage changes continuously to follow the frequency variations of the incoming FM signal Since the sum of the voltages across R1 and R2 remains constant The ratio of the voltage across R2 to this total voltage changes with the FM signal s frequency It is this changing voltage ratio that gives the ratio detector its name Reducing the Effect of Electrical Noise This is the real purpose of C6 If the amplitude of the FM input signal suddenly i
24. er in fully counter clockwise minimum position d Amplitude potentiometer in the mixer amplifier block in fully clockwise position e VCO switch in phase locked loop detector block in Off position 2 Make connections as in figure 45 3 Turn on power to the module 4 Now monitor the audio input signal to the varactor modulator block at TP14 together with the ratio output from the Foster Seeley ratio detector block at TP53 triggering the Oscilloscope on TP14 The signal at TP53 should be contain two main components A positive DC offset voltage A sine wave at the same frequency as the audio signal at TP14 but shifted in phase by 180 Note that the amount of high frequency ripple present on the signal is very small this is due to the smoothing effect of the large output capacitor 5 The Low Pass Filter amplifier block removes the DC offset voltage at the detector s output and strongly attenuates any residual high frequency ripple that may be present Consequently the signal at the output of the low pass filter amplifier block at TP73 should very closely resemble the original audio modulating signal Monitor the input TP73 and output TP73 of the low pass filter amplifier block triggering on TP73 and note how the two signals differ ST2203 Scientech Technologies Pvt Ltd 57 Figure 45 ST2203 Scientech Technologies Pvt Ltd 58 6 Monitor the audio input to the varactor modu
25. he peak to peak amplitude of the noise output at TP73 this measurement will be valuable in allowing us to compare the detuned resonant circuit with other types of FM detector as far as susceptibility to amplitude modulation is concerned 15 To overcome the problem of the detuned resonant circuit detector s susceptibility to noise we can connect an amplitude limiter block between the FM output and the input to the detuned resonant circuit The amplitude limiter removes amplitude variations from the FM output signal so that the input signal to the detuned resonant circuit detector has constant amplitude Reconnect the ST2203 Scientech Technologies Pvt Ltd 32 amplitude limiter block between the mixer amplifier block and the detuned resonant circuit block as shown in figure 26 at the end 16 Monitor the amplitude limiter s output at TP68 triggering the Oscilloscope from TP5 the noise input form the signal generator Note that the amplitude modulations due to the noise input have been removed Remove the Oscilloscope probe from TP68 and put it on TP73 the output form the low pass filter amplifier block Note that the amplitude of any remaining noise component at TP73 is now minimal 17 Return the audio oscillator blocks amplitude potentiometer to its maximum position and monitor TP73 triggering the Oscilloscope on the audio modulating input at TP14 Note that amplitudes now have no effect on the final audio output Th
26. is shows how an amplitude limiter can be used in a practical FM receiver to remove amplitude variations caused by noise before they reach the detector 18 By using the optional audio input module and audio output module the human voice can be used as the audio modulating signal instead of using ST2203 s audio oscillator block If you have these modules make the following connections Output of audio input module to audio input socket in ST2203 s modulator circuits block Output of ST2203 s low pass filter amplifier block to input socket of audio output module Refer the user manuals for the audio input module ST2108 and audio output module ST2109 for further details of how to use them 19 Throughout this experiment frequency modulation has been performed by ST2203 s varactor modulator block Equally using the reactance modulator block may perform frequency modulation If you wish to repeat any of the above experimentation with the reactance modulator simply put the reactance varactor switch in the reactance position Note However that the linearity of the reactance modulator is not as good as that of the varactor modulator This means that when the reactance modulator is used some distortion of the demodulated audio signal may be noticeable at the detector s output if the amplitude of the audio modulating signal is too large 20 Finally make sure that you fully understand the working of the detuned res
27. lator at TP14 and the output of the low pass filter amplifier block at TP73 and adjust the gain potentiometer in the low pass filter amplifier block until the amplitudes of the monitored audio waveforms are the same 7 Adjust the audio oscillator block s amplitude and frequency potentiometer and compare the original audio signal with the final demodulated signal 8 We can investigate the effect of noise on the system by following the procedures given in earlier chapters by substituting the quardrature detector by ratio detector ST2203 Scientech Technologies Pvt Ltd 59 Coil Adjustment This chapter describes how to adjust ST2203 tuned circuits for correct operation Where signals are to be monitored with an Oscilloscope the scope s input channels should be AC coupled unless otherwise indicated Ensure that X10 Oscilloscope probes are used throughout a frequency counter should be used for all frequency measurements Use the trimming tool supplied with the ST2203 module for trimming inductors Never use a screwdriver as this may damage the inductor s core Also take care not to turn any inductor s core past its end stop as this may also result in damage Reactance modulator tuned circuit transformer T1 Put the reactance varactor Switch in the reactance position and then turn the mixer amplifier block s amplitude potentiometer to its fully clockwise position Turn the reactance modulator block s carrier frequenc
28. lators employ a parallel tuned circuit to determine the frequency of operation The frequency of resonance depends on the value of the inductance and capacitance This extra capacitance will reduce the frequency of resonance Figure 8 ST2203 Scientech Technologies Pvt Ltd 12 Adding an additional capacitor in parallel will cause the total capacitance to increase and this will result in a decrease in the resonance frequency If you feel that a reminder of the formula may be helpful the approximate frequency of resonance is given by Hz LC 2 1 f Where L is the inductance in Henrys and C is the capacitance in Farads The tuned circuit is part of the oscillator used to generate the carrier frequency so the capacitance changes then so will the carrier frequency This is demonstrated in figure 9 Figure 9 To produce a frequency modulated carrier all we have to do is to find a way of making the information signal increase and decrease the size of the capacitance and hence control the carrier frequency In the following sections we will look to see two ways of achieving this First by using a device called a varactor diode and then by using a transistor Varactor Diode The varactor diode is a semiconductor diode that is designed to behave as a voltage controlled capacitor When a semiconductor diode is reverse biased no current flows and it consists of two conducting regions separated by non conducting region This
29. ly in parallel with the tuned circuit the variations in value will cause the frequency of resonance to change and hence the carrier frequency will be varied in sympathy with the information signal input ST2203 Scientech Technologies Pvt Ltd 22 Experiment 2 Objective Study of Frequency Modulation Using Reactance Modulator Procedure This experiment investigates how ST2203 s reactance modulator circuit performs frequency modulation This circuit modulates the frequency of a carrier sine wave according to the audio signal applied to its modulating output 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 ST2203 Module a All Switch Faults in Off condition b Amplitude potentiometer in the mixer amplifier block in fully clockwise c VCO switch in phase locked loop detector block in Off position 2 Make the connections as shown in figure 19 3 Turn on power to the ST2203 module 4 Turn the audio oscillator block s amplitude potentiometer to its fully clockwise Maximum positions and examines the block s output TP1 on an Oscilloscope This is the audio frequency sine wave which will be used 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 potentiome
30. meter and compare the original audio signal with the final demodulated signal You may notice that the demodulated output suffers attenuation as the audio modulating frequency is increased This is caused by low pass filtering which takes place in the detuned resonant circuit s envelope detector and in the low pass filter amplifier block In spite of this high frequency limitation to the range of audio frequencies which can be received the bandwidth of the system is perfectly adequate for normal speech communication ST2203 Scientech Technologies Pvt Ltd 31 In the audio oscillator block put the amplitude potentiometer in its maximum position and the frequency potentiometer in its Minimum position 11 We will now investigate the effect of noise on the system Adjust the external signal generator for a sinusoidal output of amplitude 100m Vpp and frequency 2 KHz this will be our noise input Connect the output of the signal generator to the noise input socket in ST2203 s modulator circuit s block Then monitor the noise input at TP5 and the FM output at TP34 triggering the Oscilloscope on TP5 Note that the FM signal is now being amplitude modulated by the noise input in addition to being frequency modulated by the audio input from the audio oscillator block The amplitude modulations simulate the effect that transmission path noise would have on the amplitude of the FM waveform reaching the receiver This allows us to i
31. modulator circuit s block Monitor the noise input at TP5 and the FM output at TP34 triggering the Oscilloscope on TP5 Note that the FM signal is now being amplitude modulated by the noise input in addition to being frequency modulated by the audio input from the audio oscillator block The amplitude modulations simulate the effect that transmission path noise would have on the amplitude of the FM waveform reaching the receiver This allows us to demodulated audio signal 11 Monitor the audio modulating signal at TP14 and the output of the low pass filter amplifier block at TP73 triggering the Oscilloscope from TP14 12 Remove the Oscilloscope probe form TP73 and place it on TP46 the output form the quadrature detector block Note that the small noise component is still visible 13 Turn the audio oscillator block s amplitude potentiometer to its MIN position so that no frequency modulation takes place Then monitor the noise input at TP5 and the output from the low pass filter amplifier block at TP73 triggering the Oscilloscope from TP5 ST2203 Scientech Technologies Pvt Ltd 39 14 To reduce the effect of amplitude variations even further we can connect an amplitude limiter block between the FM output and the input to the quadrature detector The amplitude limiter removes amplitude variations from the FM output signal so that the input signal to the quadrature detector has constant amplitude Reco
32. monitor the buffered FM output signal the mixer amplifier block at TP34 8 Put the varactor modulator s carrier frequency potentiometer in its midway position and then examine TP34 Note that it is a sine wave of approximately 1 2 Vpp centered on 0V This is our FM carrier and it is un modulated since the varactor modulators audio input signal has zero amplitude ST2203 Scientech Technologies Pvt Ltd 18 9 The amplitude of the FM carrier at TP34 is adjustable by means of the mixer amplifier block s amplitude potentiometer from zero to its potentiometer level Try turning this potentiometer slowly anticlockwise and note that the amplitude of the FM signal can be reduced to zero Return the amplitude potentiometer to its fully clockwise position 10 Try varying the carrier frequency potentiometer and observe the effects 11 Also see the effects of varying the amplitude and frequency potentiometer in the audio oscillator block 12 Turn the carrier frequency potentiometer in the varactor modulator block slowly clockwise and note that in addition to the carrier frequency increasing there is a decrease in the amount of frequency deviation that is present 13 Return the carrier frequency potentiometer to its midway position and monitor the audio input at TP6 and the FM output at TP34 triggering the Oscilloscope on the audio input signal Turn the audio oscillator s amplitude potentiometer throughout its range of adju
33. n ST2203 Scientech Technologies Pvt Ltd 60 Detuned resonant circuit tuned circuit transformer T4 Turn the audio oscillator block s amplitude potentiometer to its fully clockwise position Note The position of the reactance varactor switch and adjust the selected modulator s carrier frequency potentiometer until the monitored sine wave s frequency is 455 KHz 0 5 KHz Make the following connections 1 Output of audio oscillator block to audio input of modulator circuits block 2 FM output of mixer amplifier block to input of de tuned resonant circuit block Monitor the output of the detuned resonant circuit block at TP40 together with the audio signal at TP1 triggering the scope from TP 1 Trim transformer T4 until the DC level at TP40 is at its most position and the amplitude of the audio frequency component is minimized Then turn transformers T4 slowly counter clockwise from its present core position until a position is found where the AC signal at TP40 is an audio frequency sine wave and has maximum amplitude Finally remove both connections Quadrature detector tuned circuit transformer T5 Turn the audio oscillator block s amplitude potentiometer to its fully clockwise position Turn the mixer amplifier block s amplitude potentiometer to its fully clockwise position and monitor the FM output signal at TP34 Note the position of the reactance varactor switch and adjust the selected m
34. ncreases the voltage VD1 and VD2 will try to increase and these in turn will try to increase the voltages across both R1 and R2 However since C6 is large the overall voltage across R1 and R2 will not respond to the fast change in input amplitude The result is that the demodulated audio output is unaffected by fast changes in the amplitude of the incoming FM signal R3 and R4 are current limiting resistors to prevent momentary high levels of current through the diodes which would cause a brief fluctuation in the output voltage ST2203 Scientech Technologies Pvt Ltd 56 Experiment 7 Objective Study of Operation of Ratio Detector Procedure This experiment investigates how the ratio detector on the ST2203 module performs frequency demodulation The operation of this detector circuit will be described in detail and its sensitivity to noise on the incoming FM signal will be investigated The on board amplitude limiter will then be used to remove any amplitude modulations due to noise before they reach the detector This allows the student to draw conclusions as the whether is necessary to precede this type of detector with and amplitude limiter stage in a practical FM receiver 1 Ensure that the following initial conditions exist on the ST2203 module a All Switched Faults in Off condition b Audio amplifier block s amplitude potentiometer in fully clockwise maximum position c Audio amplifier block s frequency potentiomet
35. nerate our FM signal since this is the more linear of the two frequency modulators 5 To select the varactor modulator put the reactance varactor switch in the varactor position Ensure that the varactor modulator s carrier frequency potentiometer is in the midway position arrowhead pointing towards top of PCB 6 The audio oscillator s output signal which appears at TP1 is now being used by the varactor modulator to frequency modulate a 455 KHz carrier sine wave As we saw earlier this FM waveform appears at the FM output socket from the mixer amplifier block You may like to examine this FM waveform at TP34 However with the varactor modulator s carrier frequency potentiometer in its present midway position the frequency deviation is quite small To be able to notice such a ST2203 Scientech Technologies Pvt Ltd 30 small frequency deviation you will probably need to have a control on your Oscilloscope If you have such a control display 20 25 cycles of the waveform on the Oscilloscope and then use the X expansion control to expand up the right most cycles of the display There should be a slight ambiguity in the positions of these cycles indicating that the sine wave at TP34 is being frequency modulated 7 Now monitor the audio input signal to the varactor modulator block at TP14 together with the output from the detuned resonant circuit block at TP40 triggering the Oscilloscope on TP14 The signal
36. nnect the amplitude limiter block between the mixer amplifier block and the quadrature detector block as shown in figure 30 15 Monitor the amplitude limiter s output at TP68 triggering the Oscilloscope from TP5 the noise input from the signal generator Note that the amplitude modulations due to the noise input have been removed Remove the Oscilloscope probe from TP68 and put it on TP73 the output form the low pass filter amplifier block Note that the amplitude of any remaining noise component at TP73 is now minimal 16 By using the optional audio input module and audio output module the human voice can be used as the audio modulating signal instead of using ST2203 s audio oscillator block 17 Throughout this experiment frequency modulation has been performed by ST2203 s varactor modulator block Using the reactance modulator block we may perform frequency modulation ST2203 Scientech Technologies Pvt Ltd 40 Figure 29 ST2203 Scientech Technologies Pvt Ltd 41 Figure 30 ST2203 Scientech Technologies Pvt Ltd 42 Phase Locked Loop Detector This is another demodulator that employs a phase comparator circuit It is a very good demodulator and has the advantage that it is available as a self contained integrated circuit so there is no set up required You plug it in and in works For these reasons it is often used in commercial broadcast receivers It has very low levels of dist
37. nvestigate the effect of transmission path noise would have on the final demodulated audio signal 12 Monitor the audio modulating signal at TP14 and the output of the low pass filter amplifier block at TP73 triggering the Oscilloscope from TP14 Note that there is now an additional component at TP73a sine wave at the frequency of the noise input To see this clearly it may be necessary to slightly adjust the frequency of the signal generator s output until the superimposed noise sine wave can be clearly seen 13 Remove the Oscilloscope probe form TP73 and place it on TP40 the output form the detuned resonant circuit detector Note that the noise component is still present illustrating that this type of detector is very susceptive to amplitude variations in the incoming FM signal Put the Oscilloscope probe on TP39 the collector of the detuned resonant circuit s transistor to ensure that you fully understand why this type of detector is so sensitive to amplitude variations 14 Turn the audio oscillator block s amplitude potentiometer to its minimum position so that no frequency modulation takes place Then monitor the noise input at TP5 and the output from the low pass filter amplifier block at TP73 triggering the Oscilloscope from TP5 The signal at TP73 in now purely composed of the noise output resulting from amplitude variations occurring at the input to the detuned resonant circuit Measure and record t
38. o amplitude 9 The amplitude of the FM carrier at TP34 is adjustable by means of the mixer amplifier block s amplitude potentiometer from zero to its present level Try turning this potentiometer slowly anticlockwise and note that the amplitude of the FM signal can be reduced to zero Return the amplitude potentiometer to its fully clockwise position 10 The frequency of the FM carrier signal at TP34 should be approximately 455 KHz at the moment This carrier frequency can be varied from 453 KHz to 460 KHz approximately by adjusting the carrier frequency potentiometer in the reactance modulator block Turn this potentiometer over its range of adjustment and note that the frequency of the monitored signal can be seen to vary slightly Note also that the carrier frequency is maximum when the potentiometer is in fully clockwise position 11 Try varying the amplitude amp frequency potentiometer in audio oscillators block and also sees the effect of varying the carrier frequency potentiometer in the mixer amplifiers block 12 Monitor the audio input at TP6 and the FM output at TP34 triggering the Oscilloscope on the audio input signal Turn the audio oscillator s amplitude potentiometer throughout its range of adjustment and note that the amplitude of the FM output signal does not change This is because the audio information is contained entirely in the signal s frequency and not in its amplitude 13 The complete circuit
39. odulator s carrier frequency potentiometer until the monitored sine wave s frequency is 455 KHz 0 5 KHz ST2203 Scientech Technologies Pvt Ltd 61 Make the following connections 1 Output of audio oscillator block to audio input of modulator circuits block 2 FM output of mixer amplifier block to input of de tuned resonant circuit block Monitor the output of the quadrature detector block at TP46 together with the signal at TP1 triggering on TP1 Trim transformer T5 so that the audio frequency sine wave at TP46 has maximum amplitude Finally remove both connections Foster Seeley ratio detector tuned circuit transformer T6 Turn the audio oscillator block s amplitude potentiometer to its fully clockwise position Turn the mixer amplifier block s amplitude potentiometer to its fully clockwise position and monitor the FM output signal at TP34 Note The position of the reactance varactor switch and adjust the selected modulator s carrier frequency potentiometer until the monitored sine wave s frequency is 455 KHz 0 5 KHz Make the following connections 1 Output of audio oscillator block to audio input of modulator circuits block 2 FM output of mixer amplifier block to input of foster Seeley ratio detector block Put the Foster Seeley ratio switch in the Foster Seeley position Monitor the foster Seeley output at TP52 together with the signal at TP1 triggering the Oscilloscope on TP1 T
40. onant circuit detector by examining the circuit diagram for the detector at the end of this manual and monitoring Test Points within the circuit ST2203 Scientech Technologies Pvt Ltd 33 Figure 25 ST2203 Scientech Technologies Pvt Ltd 34 Figure 26 ST2203 Scientech Technologies Pvt Ltd 35 Quadrature Detector This is another demodulator again fairly simple but is an improvement over the previous design It causes less distortion and is also better though not perfect when it comes to removing any superimposed noise The incoming signal is passed through a phase shifting circuit The degree of phase shift that occurs is determined by the exact frequency of the signal at any particular instant The rules for the degree of phase shift are 1 If the carrier is un modulated the phase shift is 90 2 If the carrier increases in frequency the phase shift is less than 90 3 If the carrier decreases in frequency the phase shift is greater than 90 We now only require a circuit which is capable detect the changes in the phase of the signal A phase comparator circuit as shown in figure 27 Quadrature Detector Figure 27 This circuit compares the phase of original input signal with the output of the phase the comparison according to the following rules 1 It provides no change in output voltage if the signal phase has been shifted to 90 2 Phase over 90 result in an decreased DC
41. ons due to noise before they reach the detector This allows the student to draw conclusions as to whether it is necessary to precede this type of detector with an amplitude limiter stage in a practical FM receiver 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 ST2203 module a All Switched Faults in Off condition b Audio amplifier block s amplitude potentiometer in fully clockwise maximum position c Audio amplifier block s frequency potentiometer in fully counter clockwise Ensure that the following initial conditions exist on the ST2203 clockwise minimum position d Amplitude potentiometer in the mixer amplifier block in fully clockwise position e VCO switch in phase locked loop detector block in On position 2 Make the connections shown in figure 35 3 Turn on power to the ST2203 module 4 Now monitor the audio input signal to the varactor modulator block at TP14 together with the output from the phase locked loop detector block at TP60 triggering the Oscilloscope in TP14 The signal at TP68 should contain three components A positive DC offset voltage A sine wave at the same frequency as the audio signal at TP14 A high frequency ripple component 5 The low pass filter amplifier block strongly attenuates the high frequency ripple
42. ortion and is almost immune from external noise signals and provides very low levels of distortion Altogether a very nice circuit Phase Lock Loop Detector Figure 31 The overall action of the circuit may at first seem rather pointless As we can see in Figure 31 there is a Voltage Controlled Oscillator VCO The DC output voltage from the output of the low pass filters controls the frequency of this oscillator Now this DC voltage keeps the oscillator running at the same frequency as the original input signal and 90 out of phase And if we did then why not just add a phase shifting circuit at the input to give the 90 phase shift The answer can be seen by imagining what happens when the input frequency changes as it would with a FM signal If the input frequency increases and decreases the VCO frequency is made to follow it To do this the input control voltage must increase and decrease These change of DC voltage level that forms the demodulated signal The AM signal then passes through a signal buffer to prevent any loading effects from disturbing the VCO and then through an audio amplifier if necessary The frequency response is highly linear as shown in figure 32 Figure 32 ST2203 Scientech Technologies Pvt Ltd 43 Controlling the VCO To see how the VCO is actually controlled let us assume that it is running at the same frequency as an un modulated input signal The waveforms are given in figure 33 Figure 33
43. rim transformer T6 so that average level of the signal at TP52 is 0 volts Finally remove both connections ST2203 Scientech Technologies Pvt Ltd 62 Switched Faults This chapter lists the switched faults on the ST2203 module There are eight fault switches on the module The component references given below refer to the circuit diagrams at the end of this manual Open circuit faults 1 Open circuits the 68mH choke from TP19 transistor TR4 s collector in the varactor modulator block preventing any reverse bias from being applied across the BB329 varactor diode This causes the varactor modulator s output at TP24 to be an un modulated sine wave whose output frequency is fixed at approximately 450 KHz irrespective of the position of the block s carrier frequency potentiometer 2 Fault disables the output from the detuned resonant circuit at TP39 by disconnection the grounded end of T4 s secondary winding from 0 Volts 3 Fault disconnects the quadrature detector s input socket and TP41 from the 10nf Capacitor C44 which drives the carrier input pin 10 of IC A 1496 This prevents the non phase shifted FM signal from reaching the 1496 so that phase comparison with the phase shifted signal at TP43 cannot take place The result is a vast reduction in the amplitude of the output signal at TP46 4 Fault removes the base bias voltage of all three transistors TR9 10 amp 11 in the amplitude limiter block
44. rlier In reality it also includes the loading effect caused by the other winding which acts as a transmitter secondly the signal at the collector of the transistor includes an amplitude modulated component which is passed to the diode detector In the figure 23 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 ST2203 Scientech Technologies Pvt Ltd 28 Figure 23 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 the input signal is able to switch it on again The output is passed to the low Pass Filter Amplifier block The unwanted DC component is removed and the low pass filter removes the ripple at the IF frequency One disadvantage is that any noise spikes included in the incoming signal will also be passed through the diode detector and appears at the output if we want to avoid this problem we must remove the AM noise before the input to the demodulator We do this with an Amplitude Limiter circuit Amplitude Limiter An Amplitude limiter circuit is able to place an upper and lower limit on the size of a signal In figure 24 the potentiometer limits are shown by dotted lines Any signal which exceeds these levels is simply chopped off This makes it very easy to remove any unwanted amplitude modulation due to noise or interference Figure 24
45. stment and note that the amplitude of the FM output signal does not change This is because the audio information is contained entirely in the signals frequency and not in its amplitude 14 By using the optional audio input module ST2108 the human voice can be used as the audio modulating signal instead of using ST2203 s audio oscillator block If you have an audio input module connect the module s output to the audio input socket in the modulator circuit s block The input signal to the audio input module may be taken from an external microphone be supplied with the module or from a cassette recorder by choosing the appropriate switch setting on the module Consult the user manual for the audio input module for further details ST2203 Scientech Technologies Pvt Ltd 19 Figure 16 ST2203 Scientech Technologies Pvt Ltd 20 Reactance Modulator Figure 17 shows a complete reactance modulator Figure 17 In figure 17 the left hand half is the previous varactor modulator simply an oscillator and a tuned circuit which generates the un modulated carrier The capacitor C and the resistor R are the two components used for the phase shifting and together with the transistor form the voltage controlled capacitor This voltage controlled capacitor is actually in parallel with the tuned circuit This is not easy to see but figure 18 may be helpful In the first part of the figure the capacitor and associated components have
46. t Ltd 26 Figure 21 Any design of circuit that has a linear voltage frequency characteristic would be acceptable and we are point to consider the five most popular types In each case the main points to look are How do they convert FM signals into AM signals How linear is their response this determines the amount of distortion in the final output How good are they at rejecting noise signals ST2203 Scientech Technologies Pvt Ltd 27 Detuned Resonant Circuit Detector This is the simplest form of demodulator It works but it does have a few drawbacks A parallel tuned circuit is deliberately detuned so that the incoming carrier occurs approximately halfway up the left hand slope of the response Figure 22 In figure 22 above we can see that the amplitude of the output signal will increase and decrease as the input frequency changes For example if the frequency of the incoming signal were to increase the operating point would move towards the right on the diagram This would cause an increase in the amplitude of the output signal A FM signal will therefore result in an amplitude modulated signal at the output it is really that simple Figure 23 below shows the circuit diagram of the detuned resonant circuit detector If we break it down the operation becomes very clear The FM input is applied to the base of the transistor and in the collector there is the detuned resonant circuit that we have met ea
47. tance varactor switch in varactor position Observe the output of mixer amplifier circuit Keep the Oscilloscope in X10 position now observe the full waveform by shifting the X position It is as shown in figure Mark the resemblance between the output of VCO and the Varactor modulator They are same The Frequency modulation in VCO was more because the Frequency difference between the carrier and the modulating signal was very less But in real life applications reactance and varactor modulation techniques are used which utilizes high frequency carrier and you will not observe signal as shown in figure 14 above but you will see as shown in figure 16 ST2203 Scientech Technologies Pvt Ltd 16 Figure 15 Mind you both are frequency modulation and there should be no ambiguity about this The above is purposely included to make the students clearer in mind that the varactor and reactance modulators used in this trainer are frequency modulators only We shall now start experiment with varactor modulator ST2203 Scientech Technologies Pvt Ltd 17 Experiment 1 Objective Study of Frequency Modulation using Varactor modulator Procedure This experiment investigates how ST2203 s varactor modulator circuit performs frequency modulation This circuit modulates the frequency of a carrier sine wave according to the audio signal applied to its modulating input 1 Ensure that the following initial conditions exist on the
48. ter Note also that the amplitude of this audio modulating signal can be reduced to zero by turning the audio oscillator s amplitude potentiometer to its fully counter clockwise position 5 Connect the output socket of the audio oscillator block to the audio input socket of the modulator circuit s block as shown in figure 19 6 Put the reactance varactor switch in the reactance position This switches the output of the reactance modulator through to the input of the mixer amplifier block and also switches off the varactor modulator block to avoid interference between the two modulators 7 The output signal from the reactance modulator block appears at TP13 before being buffered and amplified by the mixer amplifier block Although the output from the reactance modulator block can be monitored directly at TP13 any capacitive loading affect this point e g due to an Oscilloscope probe may slightly affect the modulator s output frequency In order to avoid this problem we will monitor the buffered FM output signal from the mixer amplifier block at TP34 8 Put the reactance modulator s potentiometer in its midway position arrow pointing towards top of PCB then examine TP34 ST2203 Scientech Technologies Pvt Ltd 23 Note that the monitored signal is a sine wave of approximately 1 2Vpp centered on 0 volts DC This is our FM carrier and it is presently un modulated since the reactance modulator s audio input signal has zer
49. through the varactor but blocks the RF signals The operation of the varactor modulator 1 The information signal is applied to the base of the input transistor and appears amplified and inverted at the collector 2 This low frequency signal passes through the RF choke and is applied across the varactor diode 3 The varactor diode changes its capacitance in according to the information signal and therefore changes the total value of the capacitance in the tuned circuit 4 The changing value of capacitance causes the oscillator frequency to increase and decrease under the control of the information signal The output is therefore a FM signal Before we start the study of varactor reactance modulation techniques we shall study a simple VCO circuit Simply connect the audio output to the socket labeled VCO modulation in and observe the FM modulated waveform on the Oscilloscope at the VCO modulation out terminal Keep the amplitude of audio output to approximately 4 V pp and frequency 2 KHz approximately Observe a stable FM modulated waveform on CRO This should look like as under Similar waveforms are shown in Kennedy s book ST2203 Scientech Technologies Pvt Ltd 15 Figure 13 Now turn the time base speed of CRO little higher and you will observe the same waveforms as under like Bessel function Figure 14 Now disconnect the audio amplifier s output from modulation In and connect it to audio In keep the reac
50. tion on a radio without interference from other stations ST2203 Scientech Technologies Pvt Ltd 10 Disadvantages of FM This requires the wide bandwidth of the transmission The medium frequency broadcast band extends from about 550 KHz to 1 600 KHz and is therefore only a little over 1MHz in width If we tried to use FM using a bandwidth of 250 KHz for each station it would mean that no more than four stations could be accommodated This wide bandwidth forces us to use higher carrier frequencies usually in the VHF band which extends from about 85 MHz to 110MHz This is a width of 25MHz and would hold many more stations Bandwidth of an FM Signal The frequency modulation process generates a large number of side frequencies Theoretically the sidebands are infinitely wide with the power levels becoming lower and lower as we move away from the carrier frequency The bandwidth of 250 KHz was chosen as a convenient value to ensure a low value of distortion in the received signal whilst allowing many stations to be accommodated in the VHF broadcast band Communication signals which do not require the high quality associated with broadcast stations can adopt a narrower bandwidth to enable more transmissions within their allotted frequency band Marine communications for ship to ship communications for example use a bandwidth of only 25 KHz but this is only for speech and the quality is not important These bandwidth figure bear no easy
51. ulated output the circuit is susceptible to noise interference and should be preceded by a noise limiter circuit ST2203 Scientech Technologies Pvt Ltd 51 Experiment 6 Objective Study of Operation of Foster Seeley Detector Procedure This experiment investigates how the Foster Seeley detector on the ST2203 module performs frequency demodulation The operation of this detector circuit will be described in detail and its sensitivity to noise on the incoming FM signal will be investigated The onboard amplitude limiter will then be used to remove any amplitude modulations due to noise before they reach the detector This allows the student to draw conclusions as to whether it is necessary to precede this type of detector with an amplitude limiter state In a practical FM receiver 1 Ensure that the following initial conditions exist on the ST2203 module a All Switched Faults in Off condition b Audio amplifier block s amplitude potentiometer in fully clockwise maximum position c Audio amplifier block s frequency potentiometer in fully counter clockwise minimum position d Amplitude potentiometer in the mixer amplifier block in fully clockwise position e VCO switch in phase locked loop detector block in Off position 2 Make connection as shown in figure 42 3 Turn on power to the ST2203 module 4 We will now investigate the operation of the Foster Seeley detector on the ST2203 mo
52. ve DC offset voltage b A sine wave at the same frequency as the audio signal at TP14 c A high frequency ripple component of small amplitude Check that the audio frequency component is a reasonable sine wave It is likely that the entire frequency of the varactor modulator s FM output needs right adjustment To do this trim transformer T2 in the varactor modulator block in accordance with the instructions given in chapter coil adjustments 7 The low pass filter amplifier block strongly attenuates the high frequency ripple component at the detector s output and also blocks the DC offset voltage Consequently the signal at the output of the low pass filter amplifier block at TP73 should very closely resemble the original audio modulating signal 8 Monitor the audio input to the varactor modulator at TP14 and the output of the low pass filter amplifier block at TP73 and adjust the gain potentiometer in the low pass filter amplifier block until the amplitudes of the monitored audio waveforms are the same 9 Adjust the audio oscillator block s amplitude and frequency potentiometer and compare the original audio signal with the final demodulated signal 10 We will now investigate the effect of noise on the system Adjust the signal generator for a sinusoidal output of amplitude 100m Vpp and frequency 2 KHz this will be our noise input Connect the output of the signal generator to the noise input socket in ST2203 s
53. wer Consumption 3VA approximately Interconnections 4 mm Banana sockets Dimensions mm W420 x H100 x D255 Weight 3 Kgs approximately ST2203 Scientech Technologies Pvt Ltd 6 Frequency Components of the Human Voice When we speak we generate a sound that is very complex and changes continuously so that at a particular instant of time the waveform may appear as shown in figure 1 below 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 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 Figure 2 The third way is to display all the information on a diagram Such a figure shows the frequency spectrum It is graph with amplitude plotted against frequency A single vertical line the length of which represents each separate frequency Represents the amplitude of the sine wave Such a diagram is shown in figure 3 Note that nearly all speech information is contained within the frequency range of 300Hz to 304 KHz ST2203 Scientech Technologies Pvt Ltd 7 Figure 3 Although an Oscilloscope will only show the original complex waveform it is
54. y potentiometer to its midway position arrowhead pointing towards top of PCB Monitor TP34 in the modulator circuit s block And adjust transformer T1 until the frequency of the monitored sine wave is 455 KHz 0 5 KHz Varactor modulator tuned circuit Transformer T2 The procedure is same as that of reactance modulator Mixer amplifier tuned circuit transformer T3 Turn the mixer amplifier block s amplitude potentiometer to its fully clockwise position and monitor the FM output signal at TP34 Note The position of the reactance varactor switch and adjust the selected modulator s carrier frequency potentiometer until the monitored frequency is 455 KHz 0 5 KHz Finally adjust transformer T3 until the amplitude of the monitored sine wave is a maximum Amplitude limiter tuned circuit transformer T7 Turn the mixer amplifier block s amplitude potentiometer to its fully clockwise position and monitor the FM output signal at TP34 Note The position of the reactance varactor switch and adjust the selected modulator s carrier frequency potentiometer until the monitored sine wave s frequency is 455 KHz 0 5 KHz Link the FM output from the mixer amplifier block to the input socket of the amplitude limiter block Monitor the output from the amplitude limiter block at TP68 and adjust transformer T7 until the monitored sine wave has maximum amplitude Finally remove the mixer amplifier to amplitude limiter connectio
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