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PHASE LOCKED LOOP APPLICATIONSgain for the loop to stay in lock over a 100: 1 frequencyrange, or conversely, to stay in lock with a precise phasedifference (between input and VCO signals) which is almostindependent of frequency variation. Adjustment of thevoltage to the non-inverting input of the op amp, togetherwith a large enough loop gain allows the phase difference tobe set at a constant value between 0 0 and 180 0 • Inaddition, it is now possible to do special filtering toimprove the performance in certain applications. Forinstance, in frequency multiplication applications it may bedesirable to include a notch filter tuned to the sumfrequency component to minimize incidental FM withoutexcessive reduction of capture range.INCREASED LOOP GAIN ANDLOCK RANGE FOR 565BREAKING THE INTERNAL FEEDBACK LOOP(560,561,562)The internal control voltage feedback loop can also beeasily broken on the 560, 561 and 562. The key in thiscase is to bias the range control terminal (pin 7) to +2Vwhich turns off the controlled current source. Now thephase comparator output voltage will have no effect onthe charging current which sets the VCO frequency. Nowan external feedback loop can be built with the desiredtransfer function. Figure 8-40 shows a practical applicationof this principle. The control voltage is taken fromacross the low pass filter terminals, amplified, and used toadd or subtract current into the timing capacitor nodes.MINIMIZING TONE DECODER RESPONSE TIME (567)Figure 8-39The 567 Tone Decoder is a specialized loop which can beset up to respond to a given tone (constant frequency)within its bandwidth. The center frequency is set by aresistor R 1 and capacitor C1 which determine the freerunningfrequency. The bandwidth is controlled by thelow pass filter capacitor C2' A third capacitor C3 integratesthe output of the quadrature phase detector (QPD) so thatthe dc lock-indicating component can switch the poweroutput stage on when lock is present. The 567 is optimizedfor stability and predictability of center frequency andbandwidth.+Vcc n....---------------;p---~-t____,1312INPUTSFigure 8-4044