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PHASE LOCKED LOOP APPLICATIONSNARROW BANDWIDTH FM DEMODULATOR--1~~-+---4INPUT600600Figure 8-37DEMODULATEDOUTPUTDiode 01 is a Zener diode, used to allow a larger voltagedrop across RA than would otherwise be available. 04 is adiode which should be matched to the emitter-basejunction of 01 for temperature stability. In addition, 01and 02 should have the same breakdown voltages and 03and 04 should be similar so that the voltage seen acrossR Band Re is the same as that seen across pins 10 and 1 ofthe phase locked loop. This causes the frequency of theloop to be insensitive to power supply variations. Thecenter frequencY'can be found by:HzEXPANDED LOCK RANGE (565)When the 565 is connected normally, feedback to the veofrom the phase detector is internal. That is, an amplifiermakes the pin 8 voltage track the pin 7 (phase detectoroutput) voltage. Since the capacitor e1 charge current isdetermined by the current through resistance R 1, thefrequency is a function of the voltage at pin 8. It ispossible, however, to bypass and swamp the internal loopamplifier so that the current into pin 8 is no longer afunction of the pin 8 voltage but only of the pin 7 voltage.This makes a greater charge-discharge current variationpossible, allowing a greater lock range. Figure 8-38 showssuch a circuit in which the 5741 operational amplifier is setfor a differential gain of 5, feeding current to pin 8 throughthe 33K resistor (simulating a current source). Not only isthe tracking range greatly expanded, but the output voltageas a function of frequency is five times greater than normal.In setting up such a circuit, the user should keep in mindthat for best frequency stability, the charge-dischargecurrent should be in the range of 50 to 1500JlA which alsospecifies the pin 8 input current range, showing that aratio of upper to lower lock extremes of about 30 can beachieved.EXPANDED LOCK RANGECONFIGURATION FOR 565+10Vand the total lock range is given by:8.2K22K (19 TO 1556 Hz LOCK)33K (57 TO 1910 Hz LOCK)~----~~------~~--.where: forward biased diode voltage ~ 0.7VZener diode breakdown voltagepositive supply voltagenegative supply voltagefree-running veo center frequencyWhen the output excursion at pin 7 need be only a volt orso, diodes ° 1, 02 and 03 may be replaced by shortcircuits.The value of R 1 can be selected to give a prescribed outputvoltage for a given frequency deviation.where fo is the center frequency and .6.f is the desiredfrequency deviation per volt of output.In most instances, RB and RA are chosen to be equal sothat the voltage drop across them is about 200m V. For besttemperature stability, diode ° 1 should be a base-collectorshorted transistor of the same type as 01.Figure 8-38BREAKING THE INTERNAL FEEDBACK LOOP (565)Many times it would be advantageous to be able to breakthe feedback connection between the output (pin 7) andthe control voltage terminal (01) of the veo. This can beeasily done o'nce it is seen that it is the current into pin 8which controls the veo frequency. If the external resistorR 1 is replaced with a current source, such as in Figure8-39, we have effectively broken the internal voltagefeedback connection. The current flowing into pin 8 isnow independent of .the voltage on pin 8. The outputvoltage (on pin 7) can now be amplified or filtered and usedto drive the current source by a scheme such as that shownin Figure 8-39. This scheme allows the addition of enough43
PHASE LOCKED LOOP APPLICATIONSgain for the loop to stay in lock over a 100: 1 frequencyrange, or conversely, to stay in lock with a precise phasedifference (between input and VCO signals) which is almostindependent of frequency variation. Adjustment of thevoltage to the non-inverting input of the op amp, togetherwith a large enough loop gain allows the phase difference tobe set at a constant value between 0 0 and 180 0 • Inaddition, it is now possible to do special filtering toimprove the performance in certain applications. Forinstance, in frequency multiplication applications it may bedesirable to include a notch filter tuned to the sumfrequency component to minimize incidental FM withoutexcessive reduction of capture range.INCREASED LOOP GAIN ANDLOCK RANGE FOR 565BREAKING THE INTERNAL FEEDBACK LOOP(560,561,562)The internal control voltage feedback loop can also beeasily broken on the 560, 561 and 562. The key in thiscase is to bias the range control terminal (pin 7) to +2Vwhich turns off the controlled current source. Now thephase comparator output voltage will have no effect onthe charging current which sets the VCO frequency. Nowan external feedback loop can be built with the desiredtransfer function. Figure 8-40 shows a practical applicationof this principle. The control voltage is taken fromacross the low pass filter terminals, amplified, and used toadd or subtract current into the timing capacitor nodes.MINIMIZING TONE DECODER RESPONSE TIME (567)Figure 8-39The 567 Tone Decoder is a specialized loop which can beset up to respond to a given tone (constant frequency)within its bandwidth. The center frequency is set by aresistor R 1 and capacitor C1 which determine the freerunningfrequency. The bandwidth is controlled by thelow pass filter capacitor C2' A third capacitor C3 integratesthe output of the quadrature phase detector (QPD) so thatthe dc lock-indicating component can switch the poweroutput stage on when lock is present. The 567 is optimizedfor stability and predictability of center frequency andbandwidth.+Vcc n....---------------;p---~-t____,1312INPUTSFigure 8-4044
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
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Page 15 and 16: PHASE LOCKED LOOP APPLICATIONSROOT
Page 17 and 18: PHASE LOCKED LOOP APPLICATIONSINTEG
Page 19 and 20: ~PHASE LOCKED LOOP APPLICATIONSFUNC
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Page 24 and 25: PHASE LOCKED LOOP APPLICATIONSGENER
Page 26 and 27: PHASE LOCKED LOOP APPLICATIONSPROBA
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