section - Bitsavers

PHASE LOCKED LOOP APPLICATIONSPROBABILITY OF LOCK VERSUS INPUT CYCLES100%80%60%40%20%0%/VIIIf/v~NOTE:THE ABSCISSA WILL, IN GENERAL,BE DIFFERENT FOR EACH LOOPOPERATING CONDITION.10 15 20 25 30 35INPUT CYCLESFigure 8-13the lock-up time may be limited by the rate atwhich the low pass capacitor can charge withthe reduced phase detector output (see dabove).f.) Out-band signals and noise - Low levels ofextraneous signals and noise have little effecton the lock-up time, neither improving ordegrading it. However, large levels may overdrivethe loop input stage so that limitingoccurs, at which point the in-band signalstarts to be suppressed. The lower effectiveinput level can cause the lock-up time toincrease, as discussed in e above.g.) Center frequency - Since lock-up time can bedescribed in terms of the number of cycles tolock, fastest lock-up is achieved at higherfrequencies. Thus, whenever a system can beoperated at a higher frequency, lock willtypically take place faster. Also, in systemswhere different frequencies are being detected,the higher frequencies on the average will bedetected before the lower frequencies. However,because of the wide variation due toinitial phase, the reverse may be true for anysingle trial.c.) Loop damping - Loop damping for a simpletime constant low pass filter is:d.)~=2.~ 12 rKvDamping can be increased not only by reducingr, as discussed above, but also by reducing theloop gain Kv. By using the loop gain reductionto control bandwidth or capture and lockrange, we -achieve better damping for narrowbandwidth operation. The penalty for thisdamping is that more phase detector output isrequired for a given deviation so that phaseerrors are greater and noise immunity isreduced. Also, more input drive may berequired for a given deviation.I nput frequency deviation from free-runningfrequency - Naturally, the further an appliedinput signal is from the free-running frequencyof the loop, the longer it will take the loop toreach that frequency due to the charging timeof the low pass filter capacitor. Usually, however,the effect of this frequency deviation issmall compared to the variation .resulting fromthe initial phase uncertainty. Where loopdamping is very low, however, it may bepredominant.e.) In-band input amplitude - Since input amplitudeis one factor in the phase detector gainKd and since Kd is a factor in the loop gainKv, damping is also a function of inputamplitude. When the input amplitude is low,25