section - Bitsavers

PHASE LOCKED LOOP APPLICATIONSIn the transmit mode, PLL-1 is adjusted to free-run at theinversion frequency (105kHz), or Y2 the desired carrierfrequency. PLL-2 is phase locked to this oscillator andoperates on its second harmonic frequency (3kHz). Theaudio to be scrambled is applied to the input of themultiplier of PLL-2. This produces the sum and differenceproducts of the input audio and the encoding carrier. Thesum product is filtered out by the low pass filter (Lx-Cx),leaving only the difference product. This product consistsof the original audio signal with the frequency componentsinverted about Y2 the carrier frequency, or 1.5kHz. Thesquare wave output of the oscillator of PLL-1 is low passfiltered by network Ly-Cy to produce a sine wave syncsignal which is mixed with the inverted speech by OA-1to produce the final scrambled signal.In the receive mode, PLL-1 is phase locked to the 1.5kHzsynch and masking signal and acts as a signal conditionerfor this signal. PLL-2 doubles this frequency to producethe decoding carrier. The scrambled signal is notch-filteredby network Lz-Cz to remove the 1.5kHz .:synci and maskingsignal and the resultant inverted audio applied to themultiplier of PLL-2. Here it is re-inverted in a mannersimilar to that of the transmit mode. The resultantunscrambled audio is amplified in OA-1 for output tofurther system audio stages."[Note: It is suggested that the 1.5kHz "sync and masking"signal be changed to 1.0kHz, since PLL-2 (free-running at3kHz) will lock to the third harmonic of the 1 kHz squarewave from PLL-1 more readily than to the small secondharmonic of the 1.5kHz signal. It may then be desirableto disconnect the notch filter during transmission. Ofcourse, the filters may be implemented using active filtertechniques.]PRECISION PROGRAMMABLE TIME DELAYGENERATORSam Butt of Gaithersburg, Maryland, submitted the thirdplace entry. This circuit provides both a pulse and a highfrequency output at some time td after an input pulse isreceived. The interval td is programmable in 10 steps bymeans of a digital output switch.Figure 8-72 shows the circuit in simplified form. It worksas follows:The 562 VCO is set so that its center frequency is at a pointslightly above (say 10.5MHz) the window of the 10MHzband pass filter (BPF). Thus, no rf appears at the output.When a pulse is received at the input, the flip-flop is set soas to actuate one input A of the N8880A NAND gate.The other side of the NAND gate B receives a signal whichoccurs at the rate of frlM where M is set by the binaryoutputswitch. The output of the NAND gate begins todrive the N8291 A binary ripple counter which is connectedto the divide-by-N drcuit in the loop feedback path. When10 counts have been registered, the divide-by-N counter isputting out fo/10 or (in the assumed case) 1.05MHz.Since the phase comparator now sees two frequencies veryclose together, the ~oop locks up and fo = 10f r .Sincef r is1 MHz, the VCO operates at 10MHz rf which the band passfilter passes to the output. This rf is detected and used toreset the input flip- :Iop and the counter in preparation forthe next input pulse. The duration of the rf output isdetermined mainly by the detector time constant. Theduration of the output pulse, which begins when the rfdetector actuates the one-shot, is determined by theone-shot time const mt.The delay time iswhere M is settabL' between 1 and 10 using the digitalswitch.METAL DETECTOR USING PLL AS A FREQUENCYMETER (565)The metal detector shown in Figure 8-73 was submittedto the Signetics-EDN Phase Locked Loop Contest byJim Blecksmith of Irvine, California. It incorporates a 565as a frequency meter which indicates the frequency changein a Colpitts oscillator whose tank coil approaches a metalobject. The loop output voltage at pin 7 is compared withthe reference voltage at pin 6 and the difference amplifiedby meter amplifier 04, 05'To increase the loop output (pin 7) to about O.5V perpercent of frequency deviation, a current source (Q2, Q3) isused to supply most of the capacitor (2.5mA) charge anddischarge current at pin 8. The 20K resistor connected toO.5Vpin 8 changes the charge and discharge current by -- =20Kn.025mA or about 1 % per O.5V. Since the voltage at pins 8and 7 track, the loop output voltage is also O.5V perpercent deviation. (This technique of increasing loopoutput swing for small frequency deviations is discussedin the Expanding Loop Capability section of this chapter.)I ncreasing oscillator frequency, as indicated by a risingmeter indication, results when the search coil is broughtnear a non-ferrous metal object. Reduced oscillatorfrequency, as indicated by a dropping meter reading, resultsfrom the search coil being brought near a ferrous object.68