section - Bitsavers
section - Bitsavers
section - Bitsavers
- No tags were found...
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
PHASE LOCKED LOOP APPLICATIONSTwo events must occur before an output is given. First, theloop portion of the 567 must achieve lock. Second, theoutput capacitor C3 must charge sufficiently to activatethe output stage. For minimum response time, these eventsmust be as brief as possible.As previously discussed, the lock time of a loop can beminimized by reducing the response time of the low passfilter. Thus, C2 must be as small as possible. However, C2also controls the bandwidth. Therefore, the response timeis an inverse function of bandwidth as shown by Figure8-41, reprinted from the 567 data sheet. The upper curvedenotes the expected worst-case response time when thebandwidth is controlled solely by C2 and the inputamplitude is 200mV rms or greater. The response time isgiven in cycles of center frequency. For example, a 2%bandwidth at a center frequency of 1000 cycles can requireas long as 280 cycles (280ms) to lock when the initialphase relationship is at its worst. Figure 8-42 gives atypical distribution of response time versus input phase.Note that, assuming random initial input phase, only30/180 = 1/6 of the time will the lock-up time be longerthan half the worst case lock-up time. Figure 8-43 showssome actual measurements of lock-up time for a set-uphaving a worst case lock-up time of 27 cycles and a bestcaselock-up time of four input cycles.LOCK-UP TIME VARIATION DUETO RANDOM INITIAL PHASEGREATEST NUMBER OF CYCLES BEFORE OUTPUT100050040030020010050403020101.0\..'\.."\.. I\.'\ , ,,~1\ ,"""I\.'\..r- BANDWIDTH LlMIHD :>-\BANDWOOTH LIM IT'D BY C, _EXTERNAL RESISTOR2I TNrUjC ).'\.'"3 4 5 10 20 30 40 50BANDWIDTH (% of fO)Figure 8-41LOCK-UP TIME VS INITIAL PHASE100Figure 8-43The lower curve on the graph shows the worst-case lock-uptime when the loop gain is reduced as a means of reducingthe bandwidth (see data sheet, Alternate Method ofBandwidth Reduction). The value of C2 required for thisminimum response time isIt is important to note that noise immunity and rejection ofout-band tones suffer somewhat when this minimum value(C2) of C2 is used so that response time is gained at theirexpense. Except at very low input levels, input amplitudehas only a minor effect on the lock-up time-usuallynegligible in comparison to the variation caused by inputphase.J.1F.8.6.4.2oo~\\""!30'",!60 !90~--- ....INITIAL PHASE DIFFERENCE (tPj - 90)Figure 8-42 .'150-'180Lock-up transients can be displayed on a two-channel scopewith ease. Figure 8-44 shows the display which results.The top trace shows the square wave which either gates theinput generator signal off and on (or shifts the frequency inand out of the band if you have a generator which has afrequency control input only). The lower trace shows thevoltage at pin 2, the low pass filter voltage. The inputfrequency is offset slightly from the center frequency sothat the locked and unlocked voltage are different. It isapparent that, while the C2 decay during unlock is alwaysthe same, the lock transient is different each time. This isbecause the turn-on repetition rate is such that a differentinitial phase relationship occurs with each appearance ofthe in-band signal. It is tempting to adjust the repetitionrate so that a fast, constant lock-up transient is displayed.However, in doing a favorable initial phase is created that is45