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PHASE LOCKED LOOP APPLICATIONSGENERAL LOOP SETUP AND TRADEOFFSIn a given application, maximum PLL effectiveness can beachieved if the user understands the tradeoffs which can bemade. Generally speaking, the user is free to select thefrequency, tracking or lock range, capture range and inputamplitude.CENTER FREQUENCY SELECTIONSetting the center frequency is accomplished by selectingone or two external components. The center frequency isusually set in the center of the expected input frequencyrange. Since the loop's ability to capture is a function ofthe difference between the incoming and free-runningfrequencies, the band edges of the capture range are alwaysan equal distance (in Hz) from the center frequency.Typically, the lock range is also centered about the freerunningfrequency. Occasionally, the center frequency ischosen to be offset from the incoming so that detection ortracking range is limited on one side. This permits rejectionof an adjacent higher or lower frequency signal withoutpaying the penalty for narrow band operation (reducedtracking speed).All of Signetics' loops use a multiplier in which the inputsignal is multiplied by a unity square wave at the VCOfrequency. The odd harmonics present in the square wavepermit the loop to lock to input signals at these oddharmonics. Thus, the center frequency may be set to, say,1/3 or 1/5 of the input signal. The tracking range however,will be considerably reduced as the higher harmonics areutilized.The foregoing phase detector discussion would suggestthat the PLL cannot lock to subharmonics because thephase detector cannot produce a dc component if wi isless than WOoThe loop can lock to both odd harmonic and subharmonicsignals in practice because such signals often containharmonic components at f o . For example, a squarewave of fundamental fo/3 will have a substantial componentat fo to which the loop can lock. Even a puresine wave input signal can be used for harmonic locking ifthe PLL input stage is overdriven (the resultant internallimiting generates harmonic frequencies). Locking to evenharmonics or subharmonics is the least satisfactory sincethe input or VCO signal must contain second harmonicdistortion. If locking to even harmonics is desired, the dutycycle of the input and VCO signals must be shifted awayfrom the symmetrical to generate substantial even harmoniccontent.In evaluating the loop for a potential application, it is bestto actually compute the magnitude of the expected signalcomponent nearest f o . This magnitude can be used toestimate the capture and lock range.All of Signetics' loops are stabilized against centerfrequency drift due to power supply variations. Both the565 and the 567 are temperature compensated over theentire military temperature range (-55 to +125 0 C). Tobenefit from this inherent stability, however, the user mustprovide equally stable (or better) external components.For maximum cost effectiveness in some noncritical applications,the user may wish to trade some stability for lowercost external components.TRACKING OR LOCK RANGE CONTROLTwo things limit the lock or tracking range. First, anyVCO can only swing so far; if the input signal frequencygoes beyond this limit, lock will be lost. Second, thevoltage developed by the phase detector is proportional tothe product of both the phase and the amplitude of thein-band component to which the loop is locked. If thesignal amplitude decreases, the phase difference betweenthe signal and the VCO must increase in order tomaintain the same output voltage and, hence, the samefrequency deviation. It often happens with low inputamplitudes that even the full ±90 0 phase range of thephase detector cannot generate enough voltage to allowtracking wide deviations. When this occurs, the effectivelock range is reduced. We must, therefore, give up sometracking capability and accept greater phase errors if theinput signal is weak. Conversely, a strong input signal willallow us to use the entire VCO swing capability and keepthe VCO phase (referred to the input signal) very close to90 0 throughout the range. Note that tracking range doesnot depend on the low pass filter. However, if a lowpass filter is in the loop, it will have the effect oflimiting the maximum rate at which tracking can occur.Obviously, the LPF capacitor voltage cannot changeinstantly, so lock may be lost when large enough stepchanges occur. Between the constant frequency input andthe step-change frequency input is some limiting frequencyslew rate at which lock is just barely maintained. Whentracking at this rate, the phase difference is at its limit ofo or 180 0 . It can be seen that if the LPF cutoff frequencyis low, the loop will be unable to track as fast as if the LPFcutoff frequency is higher. Thus, when maximum trackingrate is needed, the LPF should have a high cutofffrequency. However, a high cutoff frequency LPF willattenuate the sum frequencies to a lesser extent so that ouroutput contains a significant and often bothersome signal23
PHASE LOCKED LOOP APPLICATIONSat twice the input frequency. (Remember that themultiplier forms both the sum and difference frequencies.During lock, the difference frequency is zero, but the sumfrequency of twice the locked frequency is still present.)This sum frequency component can then be filtered outwith an external low pass filter.demodulate an FM signal. Although the following discussiondwells largely on lock-up time, the same commentsapply to tracking speed.No simple expression is ayailable which adequatelydescribes the acquisition or lock-up time. This may beappreciated when we review the following factors whichinfluence lock-up time.CAPTURE RANGE CONTROLThere are two main reasons for making the low pass filtertime constant large. First, a large time constant provides anincreased memory effect in the loop so that it remains at ornear the operating frequency during momentary fading orloss of signal. Second, the large time constant integratesthe phase detector output so that increased immunity tonoise and out·band signals is obtained.a.)b.)c.)d.)e.)f.)g.)I nput phaseLow pass filter characteristicLoop dampingDeviation of input frequency from centerfrequencyI n-band input amplitudeOut-band signals and noiseCenter frequencyBesides the lower tracking rates attendant to large loopfilters, other penalties must be paid for the benefits gained.The capture range is reduced and the capture transientbecomes longer. Reduction of capture range occurs becausethe loop must utilize the magnitude of the differencefrequency component at the phase detector to drive theVCO towards the input frequency. If the LPF cutofffrequency is low, the difference component amplitude isreduced and the loop cannot swfng as far. Thus, the capturerange is reduced.CHOICE OF INPUT LEVELWhenever amplitude limiting of the in-band signal occurs,whether in the loop input stages or prior to the input, thetracking (lock) and capture range becomes independent ofsignal amplitude.Better noise and out-band signal immunity is achieved whenthe input levels are below the limiting threshold since theinput stage is in its linear region and the creation of crossmodulationcomponents is reduced. Higher input levels willallow somewhat faster operation due to greater phasedetector gain and will result in a lock range whichbecomes constant with amplitude as the phase detectorgain becomes constant. Also, high input levels will resultin a linear phase versus frequency characteristic.LOCK-UP TIME AND TRACKING SPEED CONTROLIn tracking applications, lock-up time is normally of littleconsequence, but occasions do arise when it is desirable tokeep lock-up time short to minimize data loss when noiseor extraneous signals drive the loop out of lock. Lock-uptime is of great importance in tone decoder type applications.Tracking speed is important if the loop is used toFortunately, it is usually sufficient to know how we canimprove the lock-up time and what we must tradeoff toget faster lock-up. Suppose we have set up a loop or tonedecoder and find that occasionally the lock-up transient istoo long. What can be done to improve the situationkeepingin mind the factors that influence lock?a.)Initial phase relationship between incomingsignal and VCO - This is the greatest singlefactor influencing the lock time. If the initialphase is wrong, it first drives the VCO frequencyaway from the input frequency so thatthe VCO frequency must walk back on thebeat notes. Figure 8-13 gives a typical distributionof lock-up times with the input pulseinitiated at random phase. The only way toovercome this variation is to send phase informationall the time so that a favorable phaserelationship is guaranteed at t = O. For example,a number of PLLs or tone decoders may beweakly locked to low amplitude harmonics ofpulse train and the transmitted tone phaserelatedto the same pulse train. Usually, however,the incoming phase cannot be controlled.b.) Low pass filter - The larger the low pass fjJtertime constant, the longer will be the lOCK-UPtime. We can reduce lock-up time by decreasingthe filter time constant, but in doing so, wesacrifice some of the noise immunity andout-band signal rejection which caused us touse a large filter in the first place. We must alsoaccept a sum frequency (twice the VCOfrequency) component at the low pass filterand greater phase jitter resulting from out-bandsignals and noise. I n the case of the tonedecoder (where control of the capture range isrequired since it specifies the device bandwidth)a lower value of low pass capacitorautomatically increases the bandwidth. We gainspeed only at the expense of added bandwidth.24
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 and 18: PHASE LOCKED LOOP APPLICATIONSINTEG
Page 19 and 20: ~PHASE LOCKED LOOP APPLICATIONSFUNC
Page 21 and 22: PHASE LOCKED LOOP APPLICATIONSFREQU
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
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Page 48: PHASE LOCKED LOOP APPLICATIONSd.)e.
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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
Page 68 and 69: PHASE LOCKED LOOP APPLICATIONSVHF O
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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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