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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
PHASE LOCKED LOOP APPLICATIONSThe natural frequency (w n ) of a loop in its final circuitconfiguration can be measured by applying a frequencymodulated signal of the desired amplitude to the loop(Table 8-2 shows that the natural frequency is a functionof Kd, in turn a function of input amplitude). As themodulation frequency (w m ) is increased, the phaserelationship between the modulation and recovered sinewave will go through 90 0 at Sm = wn and the outputamplitude will peak.UNDERDAMPED ~ ~ 0.3DAMPING (nAs shown in Table 8-2 in the discussion on low pass filter,damping is a function of Ko, Kd and the low pass filter.Since Ko and Kd are functions of center frequency andinput amplitude, respectively, damping is highly dependenton the particular operating condition of the loop. Dampingestimates for the desired operating condition can be madeby applying an input signal which is frequency modulatedwithin the lock range by a square wave. The low pass filtervoltage is then monitored on an oscilloscope which issynchronized to the modulating waveform, as shown inFigure 8-16. Figure 8-17 shows typical waveformsdisplayed. The loop damping can be estimated by comparingthe number and magnitude of the overshoots withthe graph of Figure 8-18, which gives the transient phaseerror due to a step in input frequency.Figure 8-17a [C ext > Ccrit- R ext = 0]CRITICALLY DAMPED ~ ~ 1Figure 8-17bOVERDAMPED ~ ~ 10MEASUREMENT SETUP FOR DISPLAYOF LOOP TRANSIENT RESPONSEFigure 8-17e [C < c crit' R ext > 0]HIGHLY OVERDAMPED ~ > 10Figure 8-16Figure 8-17d [C « c crit' R ext= 0)28
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
Page 26 and 27: PHASE LOCKED LOOP APPLICATIONSPROBA
Page 30 and 31: PHASE LOCKED LOOP APPLICATIONSTRANS
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Page 38 and 39: PHASE LOCKED LOOP APPLICATIONSWhen
Page 40 and 41: PHASE LOCKED LOOP APPLICATIONSSIMPL
Page 42 and 43: PHASE LOCKED LOOP APPLICATIONSSCHEM
Page 44 and 45: PHASE LOCKED LOOP APPLICATIONSNARRO
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PHASE LOCKED LOOP APPLICATIONSSALES
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PHASE LOCKED LOOP APPLICATIONSINTER
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