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~PHASE LOCKED LOOP APPLICATIONSCapture and lock range measurements may also be madeby sweeping the generator manually through the band ofinterest. Sweeping must be done very slowly as the edgesof the capture range are approached (sweeping towardcenter frequency) or the lock-up transient delay will causean error in reading the band edge. Frequency should beread from the generator rather than the loop VCO becausethe VCO frequency gyrates wildly around the centerfrequency just before and after lock. Lock and unlock canbe readily detected by simultaneously monitoring the inputand VCO signals, the dc voltage at the low pass filter or theac beat frequency components at the low pass filter. Thelatter are greatly reduced during lock as opposed tofrequencies just outside of lock.FM AND AM DEMODULATION DISTORTIONThese measurements are quite straightforward. The loop issimply setup for FM or AM (561 or 567) detection and thetest signal is applied to the input. A spectrum analyzer ordistortion analyzer (HP 333 A) can be used to measuredistortion at the FM or AM output.For FM demodulation, the input signal amplitude must belarge enough so that lock is not lost at the frequencyextremes. The data sheets give the lock (or tracking) rangeas a function of input signal and the optional range controladjustments. Due to the inherent linearity of the VCOs, itmakes little difference whether the FM carrier is at thefree-running frequency or offset slightly as long as thetracking range limits are not exceeded.The faster the FM modulation in relation to the centerfrequency, the lower the value of the capacitor in thelow pass filter must be for satisfactory tracking. As thisvalue decreases, however, it attenuates the sum frequencycomponent of the phase detector output less. The demodulatedsignal will appear to have greater distortion unlessthis component is filtered out before the distortion ismeasured. The same comment applies to the measurementof AM distortion on the 561.When AM distortion is being measured, the carrierfrequency offset becomes more important. The. lowestabsolute value of carrier voltage at the modulation valleysmust be high enough to maintain lock at the frequencydeviation present. Otherwise, lock will periodically be lostand the distortion will be unreasonable. For example, thetypical tracking range as a function of input signal graph inthe 561 data sheet gives a total 3% tracking range at O.3mVrms input. Thus, for a carrier deviation of 1.5%, the carriermust not drop below O.3V rms in the modulation valleys.Naturally, the AM amplitude must not be too high or theAM information will be suppressed.NATURAL FREQUENCY (w n)Expressions for the natural frequency in terms of the loopgains and filter parameters are given in Table 8-2.EXPRESSIONS FOR wn AND ~ IN SECOND ORDER LOOPLOOP FILTER TYPE NATURAL FREQUENCY wn DAMPING ~R1"171 = R1C R21/2 wn (72+ --)72 = R2e71 +72 KoKd0ICotf£R1::T JKoKdWno I 0---71 2 KoDdTable 8-227