section - Bitsavers

PHASE LOCKED LOOP APPLICATIONSFigure 8-56 is an oscillogram of the input and output ofthe Figure 8-55a circuit. The output can easily be filteredto remove the sum frequency component.PHASE MODULATION USING THE PLLWAVEFORMS OF LIGHT-COUPLEDANALOG ISOLATOR (CIRCUIT A)~~~¢j 5 6 2~:- PHASEIT CONTROL OR-::=.1 MODULATION';'" VOLTAGEFigure 8-57bPHASE MODULATORSIf a phase locked loop is locked onto a signal at the centerfrequency, the phase of the VCO will be 90 0 with respectto the input signal. If a current is injected into the VCOterminal (the low pass filter output), the phase will shiftsufficiently to develop an opposing average current out ofthe phase detector so that the VCO voltage is constant andlock is maintained. When the input signal amplitude is lowenough so that the loop frequency swing is limited by thephase detector output rather than the VCO swing, thephase can be modulated over the full range of 0 to 180 0 .If the input signal is a square wave, the phase will be alinear function of the injected current.A block diagram of the phase modulator is given in Figure8-57a. The conversion factor K is a function of which loopis used, as well as the input square wave amplitude. Figure8-57b shows an implementation of this circuit using the567.PHASE MODULATION USING THE PLL¢j+ KII------------------~IIveo IOUTPUT IIIIII L __________________ J IFigure 8-57aPHASEMOOULATIONINPUTDUAL TONE DECODERS (567)Two integrated tone decoders can be connected (as shownin Figure 8-58a) to permit decoding of simultaneous orsequential tones. Both units must be on before an output isgiven. R,C1 and R,C, are chosen, respectively, for tones1 and 2. If sequential tones (1 followed by 2) are to bedecoded, then C3 is made very large to delay turn off ofunit 1 until unit 2 has turned on and the NOR gate isactivated. Note that the wrong sequence (2 followed by 1)will not provide an output since unit 2 will turn off beforeunit 1 comes on. Figure 8-58b shows a circuit variationwhich eliminates the NOR gate. The output is taken fromunit 2, but the unit 2 output stage is biased off by R2 andCR 1 until activated by tone 1. A further variation is givenin Figure 8-58c. Here, unit 2 is turned on by the unit 1~utput when tone 1 appears, reducing the standby powerto half. Thus, when unit 2 is on, tone 1 is or was present.If tone 2 is now present, unit 2 comes on also and anoutput is given. Since a transient output pulse may appearduring unit 1 turn-on, even if tone 2 is not present, theload must be slow in response to avoid a false output dueto tone 1 alone.HIGH SPEED, NARROW BAND TONE DECODER (567)The circuit of Figure 8-58a may be used to obtain a fast,narrow band tone decoder. The detection bandwidth isachieved by overlapping the detection bands of the twotone decoders. Thus, only a tone within the overlap portionwill result in an output. The input amplitude should begreater than 70mV rms at all times to prevent detectionband shrinkage and C2 should be between 130lf o and1300/f o mfd wher~ fo is the nominal detection frequency.The small value of C2 allows operation at the maximumspeed so that worst-case output delay is only about 14cycles.59