Book by Carlson Pages 176 and 177 - I76 CHAPTER4 o Lineaer...

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Unformatted text preview: I76 CHAPTER4 o Lineaer Mixtuiation All of the foregoing expressions come from simple trigonometric expansions. Clearly. in both DSB and 353. a frequency drift that’s not small compared to W will substantially alter the detected tone. The effect is more severe in DSB since a pair of tones. f... + f’ and flu - f’. is produced. lff' << I... this sounds like war- bling or the heat note heard when two musical instruments play in unison but slightly out of tune. While only one tone is produced with 383. this too can be dis- turbing. particularly for music transmission. To illustrate. the major triad chord con- sists of three notes whose frequencies are related as the integers 4, 5. and 6. Fre- quency error in detection shifts each note by the same absolute amount. destroying the harmonic relationship and giving the music an East Asian flavor. [Note that the effect is not like playing recorded music at the wrong speed. which preserves the frequency ratios} For voice transmission. subjective listener tests have shown lhal frequency drifts of less than 3: [0 H: are tolerable. otherwise. everyone sounds rather like Donald Duck. As to phase drift. again DSB is more sensitive, for iftb' = :90" (L0 and car- rier in quadrature). the detected signal vanishes entirely. With slowly varying (31'. We get an apparent fading effect. Phase drift in 533 appears as delay distortion, the extreme case being when d)’ = i90° and the demodulated signal becomes Ht). However. as was remarked before, the human ear can tolerate sioeable delay distor- tion. so phase drift is not so serious in voice-signal SSB systems. To summarize. Ml WWMWWFMWWWWSSE automatons. enctvtduowwimppmadm M ' 'lly, Mtén‘ '__ _l§lfflpbfiim + CWMWMVSS. Envelope Detection Very little was said earlier in Sect. 4.5 about synchronous demodulation ofAM for the simple reason that it’s almost never used. True. synchronous detectors work for AM. but so does an envelope detector. which is much simpler. Because the enve~ lope of an AM wave has the same shape as the message. independent of carrier fre~ quency and phase. demodulation can be accomplished by extracting the envelope with no worries about synchronization. A simplified envelope detector and its waveforms are shown in Fig. 4.5«6, where the diode is assumed to be piecewise—linear. In absence of further circuitry, the voltage U would bejust the half—rectified version of the input 0,... But RIC. acts as a Iowpass filter. responding only to variations in the peaks of tr... provided that l w << it—' «1;. [a] ICI 4.5 Frequency Conversion and Demodulation v in lb] Figure 4.5-6 Envelope detection {0} Circuit; [13} waveforms. Thus. as noted earlier. we need 1;. >> W so the envelope is clearly defined. Under these conditions. C1 discharges only slightly between carrier peaks. and o approxi- mates the envelope oi" oi“. More sophisticated filtering produces further improve— ment if needed. Finally, REC; acts as a dc block to remove the bias of the unmodu~ lated carrier component. Since the dc block distorts low-frequency message components. conventional envelope detectors are inadequate for signals with impor- tant low-frequency content. The voltage U may also be filtered to remove the envelope variations and pro- duce a dc voltage proportional to the carrier amplitude. This voltage in turn is fed back to earlier stages of the receiver for automatic volume control (AVC J to com- pensate for fading. Despite the nonlinear element. Fig. 4.5-6 is termed a linear envelope detector; the output is linearly proportional to the input envelope. Power- law diodes can also be used, but then :2 will include terms ofthe form of", of... and so on, and there may be appreciable second-harmonic distortion unless p. << I. Some BBB and SSH demodulators employ the method of envelope recon— struction diagrammed in Fig. 4.5—7. The addition of a large, locally generated car- rier to the incoming signal reconstructs the envelope For recovery by an envelope detector. This method eliminates signal multiplication but does not get around the synchronization problem, for the local carrier must be as well synchronized as the L0 in a product demodulator. 177 ...
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