SSB GENERATION - THE PHASING METHOD
Contents
1. SSB GENERATION THE PHASING METHOD Po PREPARATION aaa ace A ah eka et aad at le tas hae aoe a 84 the filter method jc csateags gece wee A een nacre peo ee 84 the phasing methodi eiea e es 84 Weaver s methodssrccineniise ioie nanni a a E coieecuis 85 the SSB tte in fos cee e E E E eaten 85 theenvelope atn a dat ect A E ead eect ed Se tee 85 generator characteristics st jsusskaciaetalacnsistidaiasaiiia an vineilenaees 86 a phasing Senierator soninn n eek acai ce he en E 86 performance Criteria ee eeceecceseceseceeecseecseeeseeeeeeeeeeeeeeseeeseeneenseenseenaes 88 EXPERIMEN Meara cass e a a oar as A Eea 89 THEIOPS aiin cae nce E EE a a a a etaks 89 phasing generator MOdel cccsiee hee ical hue Steet Sashes 90 performance Measurements wy end elainise cea eds ecaees 91 degree of modulation PEP v 38is a Keaiisa edesieamiae 93 determining tated PEP sc5 503 sta vesesseececcteei E E 94 practical OBSETV ANON st cinecarnsteudaduseasdebe maaneoneansenseraeteel 94 TUTORIAL QUESTIONS x cccsostsedeoessCosaveieeceackes aces n eecaee 95 SSB generation the phasing method Vol Al ch 7 rev 1 1 83 SSB GENERATION THE PHASING METHOD ACHIEVEMENTS introduction to the QUADRATURE PHASE SPLITTER module QPS modelling the phasing method of SSB generation estimation of sideband suppression definition of PEP PREREQUISITES an acquaintance with DSBSC generation as in the experiment entitled DSBSC generation would be an ad2. A third method of generation and detection of single sideband signals Proc IRE Dec 1956 pp 1703 1705 SSB generation the phasing method Al 85 answer the message amplitude information is contained in the amplitude of the SSB and the message frequency information is contained in the frequency offset from of the SSB An SSB derived from a single tone message is a very simple example When the message contains more components the SSB envelope is no longer a straight line Here is an important finding An ideal SSB generator with a single tone message should have a straight line for an envelope Any deviation from this suggests extra components in the SSB itself If there is only one extra component say some leaking carrier or an unwanted sideband not completely suppressed then the amplitude and frequency of the envelope will identify the amplitude and frequency of the unwanted component generator characteristics A most important characteristic of any SSB generator is the amount of out of band energy it produces relative to the wanted output In most cases this is determined by the degree to which the unwanted sideband is suppressed 3 A ratio of wanted to unwanted output power of 40 dB was once considered acceptable commercial performance but current practice is likely to call for a suppression of 60 dB or more which is not a trivial result to achieve a phasing generator The phasing method
3. of SSB generation is based on the addition of two DSBSC signals so phased that their upper sidebands say are identical in phase and amplitude whilst their lower sidebands are of similar amplitude but opposite phase The two out of phase sidebands will cancel if added alternatively the in phase sidebands will cancel if subtracted The principle of the SSB phasing generator in illustrated in Figure 1 Notice that there are two 90 phase changers One operates at carrier frequency the other at message frequencies The carrier phase changer operates at a single fixed frequency rad s The message is shown as a single tone at frequency u rad s But this can lie anywhere within the frequency range of speech which covers several octaves A network providing a constant 90 phase shift over this frequency range is very difficult to design This would be a wideband phase shifter or Hilbert transformer 3 but this is not the case for Weaver s method 86 Al SSB generation the phasing method cos Lt message Figure 1 principle of the SSB Phasing Generator In practice a wideband phase splitter is used This is shown in the arrangement of Figure 2 cosut message Figure 2 practical realization of the SSB phasing generator The wideband phase splitter consists of two complementary networks say I inphase and Q quadrature When each network is fed from the same input signal the phase di
4. SB b Q indirectly by measuring P Q which is the peak to peak of the envelope As already stated the TIMS QPS is not a precision device and a sideband suppression of better than 26 dB is unlikely You will not achieve a perfectly flat envelope But its amplitude may be small or comparable with respect to the noise floor of the TIMS system The presence of a residual envelope can be due to any one or more of leakage of a component at carrier frequency a fault of one or other MULTIPLIER incomplete cancellation of the unwanted sideband due to imperfections of the QPS 6 distortion components generated by the MULTIPLIER modules other factors can you suggest any Any of the above will give an envelope ripple period comparable with the period of the message rather than that of the carrier Do you agree with this statement If the envelope shape is sinusoidal and the frequency is e twice that of the message then the largest unwanted component is due to incomplete cancellation of the unwanted sideband e the same as the message then the largest unwanted component is at carrier frequency carrier leak 92 Al 4 for the case ofa single tone message as you have 5 the TIMS user is not able to make adjustments to a MULTIPLIER balance 6 there is no provision for adjustments to the QPS SSB generation the phasing method If it is difficult to identify the shape of the envelope then it is proba
5. bly a combination of these two or just the inevitable system noise An engineering estimate must then be made of the wanted to unwanted power ratio which could be a statement of the form better than 45 dB and an attempt made to describe the nature of these residual signals T13 if not already done so use the FREQUENCY COUNTER to identify your sideband as either upper USSB or lower LSSB Record also the exact frequency of the message sine wave from the AUDIO OSCILLATOR From a knowledge of carrier and message frequencies confirm your sideband is on one or other of the expected frequencies To enable the sideband identification to be confirmed analytically see Question below you will need to make a careful note of the model configuration and in particular the sign and magnitude of the phase shift introduced by the PHASE SHIFTER and the sign of the phase difference between the I and Q outputs of the OPS Without these you cannot check results against theory degree of modulation PEP The SSB generator like a DSBSC generator has no depth of modulation as does for example an AM generator 7 Instead the output of an SSB transmitter may be increased until some part of the circuitry overloads giving rise to unwanted distortion components In a good practical design it is the output amplifier which should overload first 8 When operating just below the point of overload the transmitter output amplifier is said to be prod
6. ds of measurement One such method depends upon a measurement of the SSB envelope as already hinted Suppose that the output of an SSB generator when the message is a single tone of frequency u rad s consists only of the wanted sideband W and a small amount of the unwanted sideband U It may be shown that for U lt lt W the envelope is nearly sinusoidal and of a frequency equal to the frequency difference of the two components Thus the envelope frequency is 2u rad s Figure 3 measuring sideband suppression via the envelope It is a simple matter to measure the peak to peak and the trough to trough amplitudes giving twice P and twice Q respectively Then PHEW thipesisersreseet 6 Q W U o een 7 as seen from the phasor diagram This leads directly to P 201 dB ogol po sideband suppression If U is in fact the sum of several small components then an estimate of the wanted to unwanted power ratio can still be made Note that it would be greater better than for the case where U is a single component SSB generation the phasing method A third possibility the most likely in a good design is that the envelope becomes quite complex with little or no stationary component at either u or u 2 in this case the unwanted component s are most likely system noise Make a rough estimate of the envelope magnitude complex in shape though it may well be and from this can be estimated the wanted to unwanted
7. e balancing takes place Make sure you appreciate the convenience of this mode of triggering Separate DSBSC signals should already exist at the output of each MULTIPLIER These need to be of equal amplitudes at the output of the ADDER You will set this up at first approximately and independently then jointly and with precision to achieve the required output result T7 check that out of each MULTIPLIER there is a DSBSC signal T8 turn the ADDER gain G fully anti clockwise Adjust the magnitude of the other DSBSC g of Figure 5 viewed at the ADDER output on CH2 A to about 4 volts peak to peak Line it up to be coincident with two convenient horizontal lines on the oscilloscope graticule say 4 cm apart T9 remove the g input patch cord from the ADDER Adjust the G input to give approximately 4 volts peak to peak at the ADDER output using the same two graticule lines as for the previous adjustment T10 replace the g input patch cord to the ADDER The two DSBSC are now appearing simultaneously at the ADDER output Now use the same techniques as were used for balancing in the experiment entitled Modelling an equation in this Volume Choose one of the ADDER gain controls g or G for the amplitude adjustment and the PHASE SHIFTER for the carrier phase adjustment The aim of the balancing procedure is to produce an SSB at the ADDER output The amplitude and phase adjustments are non in
8. fference between the two outputs is maintained at 90 Note that the phase difference between the common input and either of the outputs is not specified itis not independent of frequency Study Figures 1 and 2 to ensure that you appreciate the difference At the single frequency u rad s the arrangements of Figure 1 and Figure 2 will generate two DSBSC These are of such relative phases as to achieve the cancellation of one sideband and the reinforcement of the other at the summing output You should be able to confirm this You could use graphical methods phasors or trigonometrical analysis The QPS may be realized as either an active or passive circuit and depends for its performance on the accuracy of the components used Over a wide band of audio frequencies and for a common input it maintains a phase difference between the SSB generation the phasing method Al 87 two outputs of 90 degrees with a small frequency dependant error typically equiripple performance Criteria 88 Al As stated earlier one of the most important measures of performance of an SSB generator is its ability to eliminate suppress the unwanted sideband To measure the ratio of wanted to unwanted sideband suppression directly requires a SPECTRUM ANALYSER In commercial practice these instruments are very expensive and their purchase cannot always be justified merely to measure an SSB generator performance As always there are indirect metho
9. ion the phasing method TUTORIAL QUESTIONS Q1 what simple modification s to your model would change the output from the current to the opposite sideband Q2 with a knowledge of the model configuration and the individual module properties determine analytically which sideband USSB or LSSB the model should generate Check this against the measured result Q3 why are mass produced and consequently affordable 100 kHz SSB filters not available in the 1990s Q4 what sort of phase error could the arrangement of Figure 4 detect Q5 is the OPS an approximation to the Hilbert transformer Explain Q6 suggest a simple test circuit for checking OPS modules on the production line Q7 the phasing generator adds two DSBSC signals so phased that one pair of sidebands adds and the other subtracts Show that if the only error is one of phasing due to the OPS the worst case ratio of wanted to unwanted sideband is given by SSR 20 logyo cot 5 4B where is the phase error of the OPS Typically the phase error would vary over the frequency range in an equi ripple manner so would be the peak phase error Evaluate the SSR for the case a 1 degree Q8 obtain an expression for the envelope of an SSB signal derived from a single tone message when the only imperfection is a small amount of carrier leaking through HINT refer to the definition of envelopes in the experiment entitled Envelopes in this Volume At what ratio
10. nnel For the input signal source use an AUDIO OSCILLATOR module For correct OPS operation the display should be an approximate circle We will not attempt to measure phase error from this display T2 vary the frequency of the AUDIO OSCILLATOR and check that the approximate circle is maintained over at least the speech range of frequencies phasing generator model When satisfied that the QPS is operating satisfactorily you are now ready to model the SSB generator Once patched up it will be necessary to adjust amplitudes and phases to achieve the desired result A hit and miss method can be used but a systematic method is recommended and will be described now CH1 B various Figure 5 the SSB phasing generator model T3 patch up a model of the phasing SSB generator following the arrangement illustrated in Figure 5 Remember to set the on board switch of the PHASE SHIFTER to the HI 100 kHz range before plugging it in T4 set the AUDIO OSCILLATOR to about 1 kHz T5 switch the oscilloscope sweep to auto mode and connect the ext trig to an output from the AUDIO OSCILLATOR It is now synchronized to the message 90 Al SSB generation the phasing method T6 display one or two periods of the message on the upper channel CH1 A of the oscilloscope for reference purposes Note that this signal is used for external triggering of the oscilloscope This will maintain a stationary envelope whil
11. of sideband to carrier leak would you say the envelope was roughly sinusoidal note expressions for the envelope of an SSB signal for the general message m t involve the Hilbert transform and the analytic signal Q9 sketch the output of an SSB transmitter as seen in the time domain when the message is two audio tones of equal amplitude Discuss Q10 devise an application for the OPS not connected with SSB SSB generation the phasing method Al 95 96 Al SSB generation the phasing method
12. st to the filter of the filter method Weaver s method In 1956 Weaver published a paper on what has become known either as the third method or Weaver s method of SSB generation 2 Weaver s method can be modelled with TIMS refer to the experiment entitled Weaver s SSB generator within Volume A2 Further amp Advanced Analog Experiments the SSB signal Recall that for a single tone message cosut a DSBSC signal is defined by DSBSC A cosut cosot tts 1 A 2 cos u t A 2 cos Mt te 2 lower sideband upper sideband otters 3 When say the lower sideband LSB is removed by what ever method then the upper sideband USB remains USB A 2 cos ut ttt 4 This is a single frequency component at frequency u 2 m Hz It is a co sine wave Viewed on an oscilloscope with the time base set to a few periods of it looks like any other sinewave What is its envelope the envelope The USB signal of eqn 4 can be written in the form introduced in the experiment on Envelopes in this Volume Thus USB a t cosf otwt 6O a The envelope has been defined as envelope Ja t Ln A 2 fromeqn 4 tts 7 Thus the envelope is a constant ie a straight line and the oscilloscope correctly set up will show a rectangular band of colour across the screen This result may seem at first confusing One tends to ask where is the message information 2 Weaver D K
13. suppression ratio using eqn 8 This should turn out to be better than 26 dB in TIMS in which case the system is working within specification The TIMS QPS module does not use precision components nor is it aligned during manufacture It gives only a moderate sideband suppression but it is ideal for demonstration purposes Within the working frequency range of the QPS the phase error from 90 between the two outputs will vary with frequency theoretically in an equi ripple manner EXPERIMENT the QPS Refer to the TIMS User Manual for information about the QUADRATURE PHASE SPLITTER the QPS Before patching up an SSB phasing generator system first examine the performance of the QUADRATURE PHASE SPLITTER module This can be done with the arrangement of Figure 4 I OSCILLOSCOPE Figure 4 arrangement to check QPS performance With the oscilloscope adjusted to give equal gain in each channel it should show a circle This will give a quick confirmation that there is a phase difference of approximately 90 degrees between the two output sinewaves at the measurement frequency Phase or amplitude errors should be too small for this to degenerate visibly into an ellipse The measurement will also show the bandwidth over which the QPS is likely to be useful SSB generation the phasing method Al 89 T1 set up the arrangement of Figure 4 The oscilloscope should be in X Y mode with equal sensitivity in each cha
14. teractive performance measurement Since the message is a sine wave the SSB will also be a sine wave when the system is correctly adjusted Make sure you agree with this statement before proceeding The oscilloscope sweep speed should be such as to display a few periods of the message across the full screen This is so that when looking at the SSB a stationary envelope will be displayed SSB generation the phasing method Al 91 Until the system is adjusted the display will look more like a DSBSC or even an AM than an SSB Remote from balance the envelope should be stationary but perhaps not sinusoidal As the balance condition is approached the envelope will become roughly sinusoidal and its amplitude will reduce Remember that the pure SSB is going to be a sinewave 4 As discussed earlier if viewed with an appropriate time scale which you have already set up this should have a constant flat envelope This is what the balancing procedure is aiming to achieve T11 balance the SSB generator so as to minimize the envelope amplitude During the process it may be necessary to increase the oscilloscope sensitivity as appropriate and to shift the display vertically so that the envelope remains on the screen T12 when the best balance has been achieved record results using Figure 3 as a guide Although you need the magnitudes P and Q it is more accurate to measure a 2P directly which is the peak to peak of the S
15. ucing its maximum peak output power commonly referred to as the PEP an abbreviation for peak envelope power Depending upon the nature of the message the amplifier may already have exceeded its maximum average power output capability This is generally so with tones or messages with low peak to average power waveform but not so with speech which has a relatively high peak to average power ratio of approximately 14 dB When setting up an SSB transmitter the message amplitude must be so adjusted that the rated PEP is not exceeded This is not a trivial exercise and is difficult to perform without the appropriate equipment 7 8 which has a fixed amplitude carrier term for reference why SSB generation the phasing method Al 93 determining rated PEP The setting up procedure for an SSB transmitter assumes a knowledge of the transmitter rated PEP But how is this determined in the first place This question is discussed further in the experiment Amplifier overload practical observation You might be interested to look at both an SSB and a DSBSC signal when derived from speech Use a SPEECH module You can view these signals simultaneously since the DSBSC is available within the SSB generator Q can you detect any difference in the time domain between an SSB and a DSBSC each derived from the same speech If so could you decide which was which if you could only see one of them 94 A1 SSB generat
16. vantage PREPARATION There are three well known methods of SSB generation using analog techniques namely the filter method the phasing method and Weaver s method This experiment will study the phasing method the filter method You have already modelled a DSBSC signal An SSB signal may be derived from this by the use of a suitable bandpass filter commonly called in this application an SSB sideband filter This the filter method is probably the most common method of SSB generation Mass production has given rise to low cost yet high performance filters But these filters are generally only available at standard frequencies for example 455 kHz 10 7 MHz and SSB generation by the filter method at other frequencies can be expensive For this reason TIMS no longer has a 100 kHz SSB filter module although a decade ago these were in mass production and relatively inexpensive the phasing method The phasing method of SSB generation which is the subject of this experiment does not require an expensive filter but instead an accurate phasing network or quadrature phase splitter QPS It is capable of acceptable performance in many applications l analog frequency division multiplex where these filters were used has been superseded by time division multiplex 84 Al SSB generation the phasing method The QPS operates at baseband no matter what the carrier frequency either intermediate or final in contra
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