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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
PHASE LOCKED LOOP APPLICATIONSPLL MEASUREMENT TECHNIQUESThis <strong>section</strong> deals with user measurements of PLLoperation. The techniques suggested are meant to help theuser in evaluating the performance of his PLL during theinitial setup period as well as to point out some pitfallsthat may obscure loop evaluation. Recognizing that theuser's test equipment may be limited, we have stressedthe techniques which require a minimum of standard testitems.CENTER FREQUENCYCenter frequency measurements are easily made byconnecting a frequency counter or oscilloscope to theVCO output of the loop. The loop should be connected inits final configuration with the chosen values of input,bypass and low pass filter capacitors. No input signalshould be present. As the center frequency is read out, itcan be adjusted to the desired value by the adjustmentmeans selected for the particular loop. It is important notto make the frequency measurement directly at the timingcapacitor unless the capacity added by the measurementprobe is much less than the timing capacitor value sincethe probe capacity will then cause a frequency error.When the frequency measurement is to be converted to adc voltage for production readout or automated testing,a calibrated phase locked loop can be used as a frequencymeter (see Applications Section).CAPTURE AND LOCK RANGEFigure 8-14a shows a typical measurement setup forcapture and lock range measurements. The signal inputfrom a variable frequency oscillator is swept linearlythrough the frequency range of interest and the loop FMoutput is displayed on a scope or (at low frequencies)X-Y recorder. The sweep voltage is applied to the X axis.CAPTURE AND LOCK RANGEMEASUREMENT SETUPFigure 8-14b shows the type of trace which results. Thelock range (also called hold-in or tracking range) is given bythe outer lines on the trace, which are formed as theincoming frequency sweeps away from the center frequency.The inner trace, formed as the frequency sweepstoward the center frequency, designates the capture range.Linearity of the VCO is revealed by the straightness of thetrace portion within the lock range. The slope (l1f/l1 V) isthe gain or conversion factor for the VCO.CAPTURE AND LOCK RANGE OSCILLOGRAMRgure 8-14bBy using the sweep technique, the effect on centerfrequency, capture range and lock range of the inputamplitude, supply voltage, low pass filter and temperaturecan be examined.Because of the lock-up time duration and variation, thesweep frequency must be very much lower than the centerfrequency, especially when the capture range is below 10%of center frequency. Otherwise, the apparent capture andlock range will be a function of sweep frequency. It is bestto start sweeping as slow as possible and, if desired, increasethe rate until capture range begins to show an apparentreduction-indicating that the sweep is too fast. Typicalsweep frequencies are in the range of 1/1000 to 1/100,000of the center frequency. I n the case of the 561 and 567,the quadrature detector output may be similarly displayedon the Y axis, as shown in Figure 8-15, showing theoutput level versus frequency for one value of inputamplitude.~--------------lPHASE DETECTOR OUTPUTS VS FREQUENCYFigure 8-14aFigure 8-1526
Page 2 and 3: TABLE OF CONTENTSSECTION TITLE PAGE
Page 4: SECTIONINTRODUCTIONPHASE LOCKED LOO
Page 7 and 8: USER'S QUICK-LOOK GUIDE TO SIGNETIC
Page 10: SECTIONTHE PHASE LOCKED LOOP PRINCI
Page 13 and 14: PHASE LOCKED LOOP APPLICATIONSIt is
Page 15 and 16: PHASE LOCKED LOOP APPLICATIONSROOT
Page 17 and 18: PHASE LOCKED LOOP APPLICATIONSINTEG
Page 19 and 20: ~PHASE LOCKED LOOP APPLICATIONSFUNC
Page 21 and 22: PHASE LOCKED LOOP APPLICATIONSFREQU
Page 24 and 25: PHASE LOCKED LOOP APPLICATIONSGENER
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Page 30 and 31: PHASE LOCKED LOOP APPLICATIONSTRANS
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