section - Bitsavers

PHASE LOCKED LOOP APPLICATIONSThe display gives the time to the nearest minute and isupdated every minute. Seconds are added simply by ablinker, or else a divide-by-60 counter and decoder drivermay be used. This counter may be reset every minute bythe sync pulse; thus, after a power failure, the clock wouldcorrect its seconds reading within a minute of the powerbeing reapplied. If desired, the day of the year may besimilarly displayed and if local time is preferred to GMT, asuitable BCD adder or subtractor may be placed betweenthe shift register and the latches.Figure 8-79 shows the PLL detector circuit. It was foundthat either a roOftop hula hoop or a ferrite rod and a local10dB gain amplifier was sufficient to drive the PLL if asingle transistor gain stage was added between the two. Theamplified 60kHz signal is applied directly to the R F inputand applied to the AM Multiplier input via a 90 0 phaseshift network. The output of the multiplier is triply RCf'iltered to give a good rejection of 60Hz hum, while stillallowing the 0.2 second minimum-width pulses to passwith good fidelity."PLL PORTION OF SELF·RESETTING DIGITAL CLOCK+18V3KFINETUNING+18V16 15 14 13 12 11 10 9NE561B+18V (TOP VIEW)4:JK5K-=-60 KHzWWVB 8.2K 8.2KTIME CODEFROM PREAMP~+OUTPUTI11lF~F ±F TO-=- COMPARATOR-=- -=-Figure 8-79TAPE RECORDER FLUTTER METERUsing the 561 as a flutter meter for tape recorders wassuggested by Ronald Blair of Houston, Texas. His circuit isgiven in Figure 8-80."The Signetics PLL 561 B is used to detect the frequencyvariations of the playback 3kHz tone. The VCO frequencyis set to a nominal 3kHz by Co and fine tuning trimmer.The demodulated output is ac coupled to a high inputimpedance amplifier. An oscilloscope can be used tomeasure peak deviations and a true R MS voltmeter is usedto make RMS flutter readings. Note: Waveform is complexand averaging or peak reading meters will not give truereadings.The output may be calibrated by feeding in a 3kHz tonefrom an oscillator and offsetting the frequency by 1 % andmeasuring the output level shift. Good recorders have RMSflutter of less than 0.1 %. The output can be filtered tostudy selected frequency bands.Speed variations in the movement of tape across theheads in a 4 tape recorder cause the playback frequency tovary from the original signal being recorded. These speedvariations are caused by mechanical problems associatedwith the tape drive and tape guidance mechanisms. Thevariation in frequency of the playback signal is calledflutter and is generally measured over a frequency range of0.5Hz + 0.200Hz.Test tapes with low recorded flutter variations areavailable to test playback mechanisms. These tapes arestandardized at 3kHz. With systems equipped with recordheads, a 3kHz tone can be recorded for analysis."[Note: A 565 may be used in place of the 561 B since thefrequency is quite low.]74