section - Bitsavers

PHASE LOCKED LOOP APPLICATIONSwhere F(jw c ) is the low pass filter amplitude response atw = wL. Note that at all times the capture range is smallerthan the lock range. If the simple lag filter of Figure 8-5bis used, the capture range equation can be approximated asjfv2wcAFL~ 2 - = 2 -71 71Thus, the capture range decreases as the low pass filter timeconstant is decreased, whereas the lock range is unaffectedby the filter and is determined solely by the loop gain.Figure 8-6 shows the typical frequency-to-voltage transfercharacteristics of the PLL. The input is assumed to be a sinewave whose frequency is swept slowly over a broadfrequency range. The vertical scale is the correspondingloop error voltage. In Figure 8-6a, the input frequency isbeing gradually increased. The loop does not respond to thesignal until it reaches a frequency w1, corresponding to thelower edge of the capture range. Then, the loop suddenlylocks on the input and causes a negative jump of the looperror voltage. Next, V d varies with frequency with a slopeequal to the reciprocal of veo gain (1 /Ko) and goesthrough zero as wi = wOo The loop tracks the input untilthe input frequency reaches w2, corresponding to theupper edge of the lock range. The PLL then loses lock andthe error voltage drops to zero. If the input frequency isswept slowly back now, the cycle repeats itself, but it isinverted, as shown in Figure 8-6b. The loop recaptures thesignal at w3 and tracks it down to w4. The total captureand lock ranges of the system are:2wc = w3 - w1 and 2wL = w2 - w4Note that, as indicated by the transfer characteristics ofFigure 8-6, the PLL system has an inherent selectivityabout the center frequency set by the veo free-runningfrequency wo; it will respond only to the input signalfrequencies that are separated from Wo by less than Wc orw L, depending on whether the loop starts with or withoutan initial lock condition. The linearity of the frequency-tovoltageconversion characteristics for the PLL is determinedsolely by the veo conversion gain. Therefore, in most PLLapplications, the veo is required to have a highly linearvoltage-to-frequency transfer characteristic.PHASE LOCKED LOOP BUILDING BLOCKSVOLTAGE CONTROLLED OSCI LLATORSince three different forms of veo have been used in theSignetics PLL series, the veo details will not be discusseduntil the individual loops are described. However, a fewgeneral comments about veos are in order.When the PLL is locked to a signal, the veo voltage is afunction of the frequency of the input signal. Since theveo control voltage is the demodulated output during FMdemodulation, it is important that the veo voltage-tofrequencycharacteristic be linear so that the output is notdistorted. Over the linear range of the veo, the conversiongain is given by Ko (in radian/volt-sec).TYPICAL PLL FREQUENCY-TO-VOL TAGETRANSFER CHARACTERISTICS FORINPUT FREQUENCY INCREASINGAND DECREASINGVd:~ DIREC~SWEEP1-01----1I1 I-- 2WC --IaINCREASINGFREQUENCY+ : INCREASINGP.SWEEPbFREQUENCYVd 0 DIREi~CT-'ON"'OF"':''''r-4 -----*:......-~------ -Since the loop output voltage is the veo voltage, we canget the loop output voltage asThe gain Ko can be found from the data sheet by taking thechange in veo control voltage for a given percentagefrequency deviation and multiplying by the center frequency.When the veo voltage is changed, the frequencychange is virtually instantaneous.PHASE DETECTORFigure 8-6All Signetics phase locked loops use the same form of phasedetector-often called the doubly-balanced multiplier ormixer. Such a circuit is shown in Figure 8-7.15