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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