section - Bitsavers

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