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PHASE LOCKED LOOP APPLICATIONSIf n =F 0 in the first term, the loop can lock when wi =(2n + 1) w o ' giving the dc phase detector component2AdVi7T(2n + 1)cos ()ishowing that the loop can lock to odd harmonics of thecenter frequency. The (2n + 1) term in the denominatorshows that the phase detector output is lower for harmoniclock, which explains why the lock range decreases ashigher and higher odd harmonics are used to achieve lock.Note also that the phase detector output during lock is(assuming Ad is constant) also a function of the inputamplitude Vi. Thus, for a given dc phase detector outputVd, an input amplitude decrease must be accompanied bya phase change. Since the loop can remain locked only for0i between 0 and 180 0 , the lower Vi becomes, the morereduced is the lock range.Going to the second term, we note that during lock thelowest possible frequency is Wo + wi = 2wi. A sumfrequency component is always present at the phasedetector output. This component is usually greatlyattenuated by the low pass filter capacitor connected tothe phase detector output. However, when rapid trackingis required (as with high-speed FM detection or FSKfrequencyshift keying), the requirement for a relativelyhigh frequency cutoff in the low pass filter may leave thiscomponent unattenuated to the extent that it interfereswith detection. At the very least, additional filtering maybe required to remove this component. Componentscaused by n -=1= 0 in the second term are both attenuated andof much higher frequency, so they may be neglected.Suppose that we have other frequencies represented byVk present. What is their effect for Vk -=1= O?The third term shows that Vk introduces another differencefrequency component. Obviously, if wk is close to wi, itcan interfere with the locking process since it may forma beat frequency of the same magnitude as the desiredlocking beat frequency. Suppose lock has been achieved,however, so that Wo = wi. In order for lock to bemaintained, the average phase detector output must beconstant. If Wo = wk is relatively low in frequency, thephase ()i must change to compensate for this beatfrequency. Broadly speaking, any signal in addition to thesignal to which the loop is locked causes a phase variation.Usually this is negligible since wk is often far removedfrom wi. However, it has been stated that the phase() i can move on Iy between 0 and 180 0 . Su ppose the phaselimit has been reached and Vk appears. Since it cannot becompensated for, it will drive the loop out of lock. Thisexplains vvhy extraneous signals can result in a decrease inthe lock : ange. If Vk is assumed to be an instantaneousnoise c:· )onent, the same effect occurs. When the fullswing of the loop is being utilized, noise will decrease thelock or tracking range. We can reduce this effect bydecreasing the cutoff frequency of the low pass filter sothat the Wo - wk is attenuated to a greater extent, whichillustrates that noise immunity and out-band frequencyrejection is improved (at the expense of capture range sinceWo - wi is likewise attenuated) when the low pass filtercapacitor is large.The third term can have a dc component when wk is anodd harmonic of the locked frequency so that (2n + 1)(w o - wi) is zero and ()k makes its appearance. This willhave an effect on 0i which will change the 0i versusfrequency wi. This is most noticeable when the waveformof the incoming signal is, for example, a square wave. The()k term will combine with the ()i term so that the phase isa linear function of input frequency. Other waveforms willgive different phase versus frequency functions. When theinput amplitude Vi is large and the loop gain is large, thephase will be close to 90 0 throughout the range of veoswing, so this effect is often unnoticed.The fourth term is of little consequence except that if wkapproaches zero, the phase detector output will have acomponent at the locked frequency Wo at the output. Forexample, a dc offset at the input differential stage willappear as a square wave of fundamental Wo at the phasedetector output. This is usually small and well attenuatedby the low pass filter. Since many out-band signals or noisecomponents may be present, many V k terms may becombining to influence locking and phase during lock.Fortunately, we need only worry about those close to thelocked frequency.The quadrature phase detector action is exactly the sameexcept that its output is proportional to the sine of thephase angle. When the phase () i is 90 0 , the quadraturephase detector output is then at its maximum, whichexplains why it makes a useful lock or amplitude detector.The output of the quadrature phase detector is given by:where Vi is the constant or modulated AM signal and()i ~ 90 0 in most cases so that sine ()i = 1 and7TThis is the demodulation principle of the autodyne receiverand the basis for the 567 tone decoder operation.17
~PHASE LOCKED LOOP APPLICATIONSFUNCTIONAL APPLICATIONSLOW PASS FILTERThe simplest type of low pass filter for the second orderloop is a single pole RC type shown in Figure 8-5b. I n allSignetics' loops, the resistor is internal and the capacitor isexternal. The inside resistor greatly improves the centerfrequency stability of the loop with temperature variations.Fortunately, the capture range and loop damping arerelated to the square root of this internal resistor value, sovariations in its absolute value have little effect on loopperformance. The nominal value of the internal resistorfor each loop is given in the circuit diagrams of thedetailed circuit descriptions in this chapter. The typicaltolerance on these integrated resistors is ±20%.FM TelemetryThis application involves demodulation of a frequencymodulated subcarrier of the main channel. A popularexample here is the use of the PLL to recover the SCA(storecast music) signal from the combined signal of manycommercial FM broadcast stations. The SCA signal is a67kHz frequency modulated subcarrier which puts it abovethe frequency spectrum of the normal stereo or monauralFM program material. By connecting the circuit of Figure8-8 to a point between the FM discrimator and thede-emphasis filter of a commercial band (home) FMreceiver and tuning the receiver to a station which broadcastsan SCA signal, one can obtain hours of commercialfree background music.As a functional building block, the phase locked loop issuitable for a wide variety of frequency related applications.These applications generally fall into one or more of thefollowing categories:a.)b.)c.)d.)e.)FM DemodulationFrequency SynthesizingFrequency SynchronizationSignal ConditioningAM DemodulationFM DEMODULATIONIf the PLL is locked to a frequency modulated (FM) signal,the VCO tracks the instantaneous frequency of the inputsignal. The filtered error voltage, which forces the VCO tomaintain lock with the input signal then becomes thedemodulated FM output. The linearity of this demodulatedsignal depends solely on the linearity of the VCO controlvoltage-to-frequencytransfer characteristic.510 pF 510pF0-lh-OEMFM4.7K4.7K10K4.7K1.8K~5KI2 83 9tOOlNE565.00110 l74.001 5 tJlK.. . 018lK.047lK+10 VOLTS+24 VOLTSBACKGRO UNOMUSIC (S CAl1-::-It should be noted that since the PLL is in lock during theFM demodulation process, the response is linear and can bereadily predicted from a root locus plot.FM demodulation applications are numerous; however,some of the more popular are:Broadcast FM DetectionHere, the PLL can be used as a complete I F strip, limiterand FM detector which may be used for detecting eitherwide or narrow band FM signals with greater linearity thancan be obtained by other means. For frequencies withinthe range of the VCO, the PLL functions as a self containedreceiver since it combines the functions of frequencyselectivity and demodulation. One increasingly popular useof the PLL is in scanning-receivers where a number ofbroadcast channels may be sequentially monitored bysimply varying the VCO free-running frequency.Figure 8-8Frequency Shift Keying (FSK)This refers to what is essentially digital frequencymodulation. FSK is a means for transmitting digitalinformation by a carrier which is shifted between twodiscrete frequencies. In this case, the two discretefrequencies correspond to a digital "1" and a digital "0,"respectively. When the PLL is locked to a FSK signal, thedemodulated output (error voltage) shifts between twodiscrete voltage levels, corresponding to the demodulatedbinary output. FSK techniques are often used in modems(modulator-demodulators), intended for transmitting dataover telephone lines.18
Page 2 and 3: TABLE OF CONTENTSSECTION TITLE PAGE
Page 4: SECTIONINTRODUCTIONPHASE LOCKED LOO
Page 7 and 8: USER'S QUICK-LOOK GUIDE TO SIGNETIC
Page 10: SECTIONTHE PHASE LOCKED LOOP PRINCI
Page 13 and 14: PHASE LOCKED LOOP APPLICATIONSIt is
Page 15 and 16: PHASE LOCKED LOOP APPLICATIONSROOT
Page 17: PHASE LOCKED LOOP APPLICATIONSINTEG
Page 21 and 22: PHASE LOCKED LOOP APPLICATIONSFREQU
Page 24 and 25: PHASE LOCKED LOOP APPLICATIONSGENER
Page 26 and 27: PHASE LOCKED LOOP APPLICATIONSPROBA
Page 28 and 29: ~PHASE LOCKED LOOP APPLICATIONSCapt
Page 30 and 31: PHASE LOCKED LOOP APPLICATIONSTRANS
Page 32 and 33: PHASE LOCKED LOOP APPLICATIONSThe V
Page 34 and 35: PHASE LOCKED LOOP APPLICATIONSINTER
Page 36 and 37: PHASE LOCKED LOOP APPLICATIONSIt is
Page 38 and 39: PHASE LOCKED LOOP APPLICATIONSWhen
Page 40 and 41: PHASE LOCKED LOOP APPLICATIONSSIMPL
Page 42 and 43: PHASE LOCKED LOOP APPLICATIONSSCHEM
Page 44 and 45: PHASE LOCKED LOOP APPLICATIONSNARRO
Page 46 and 47: PHASE LOCKED LOOP APPLICATIONSTwo e
Page 48: PHASE LOCKED LOOP APPLICATIONSd.)e.
Page 52 and 53: PHASE LOCKED LOOP APPLICATIONSFM IF
Page 54 and 55: PHASE LOCKED LOOP APPLICATIONSPHASE
Page 56 and 57: PHASE LOCKED LOOP APPLICATIONSTRANS
Page 58 and 59: PHASE LOCKED LOOP APPLICATIONSbaud
Page 60 and 61: PHASE LOCKED LOOP APPLICATIONSFigur
Page 62 and 63: PHASE LOCKED LOOP APPLICATIONSinput
Page 64 and 65: PHASE LOCKED LOOP APPLICATIONfTRIAN
Page 66 and 67: PHASE LOCKED LOOP APPLICATIONSRF-FM
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PHASE LOCKED LOOP APPLICATIONSVHF O
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PHASE LOCKED LOOP APPLICATIONSPRECI
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PHASE LOCKED LOOP APPLICATIONSFSK D
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PHASE LOCKED LOOP APPLICATIONSThe c
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PHASE LOCKED LOOP APPLICATIONSFLUTT
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PHASE LOCKED LOOP APPLICATIONSSALES
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PHASE LOCKED LOOP APPLICATIONSINTER
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