section - Bitsavers

PHASE LOCKED LOOP APPLICATIONSA considerable quantity of detailed specifications andpublications information for these products is included inthe Linear Spec. Handbook. Because many readers arelikely to be unfamiliar with the terminology and operatingcharacteristics of phase locked loops, a glossary of termsand a general explanation of PLL principles are includedhere with a detailed discussion of the action of theindividual loop elements.The tradeoff and setup section wi" assist the reader insome of the considerations involved in selecting andapplying the loop products to meet system requirements.A brief summary of measurement techniques has beenpresented to aid the user in achieving his performance goals.Detailed descriptions have been provided for each of theloop products. The user can supplement the suggested, connection diagrams with his own schemes.>;c ••Perhaps the best way to become familiar with the manyuses of phase locked loops is to actually study the variousapplication circuits provided. These circuits have beendrawn from many sources - textbooks, users, Signetics'applications engineers and the 1970 Signetics-EDN PhaseLocked Loop contest. Every effort has been made toprovide usable, workable circuits which may be copieddirectly or used as jumping-off points for other imaginativeappl ications.The section on interfacing wi" aid the user in drivingdifferent forms of logic from PLL outputs and the sectionon expanding loop capabilities wi" show how to achieveimproved performance in certain difficult applications.PHASE LOCKED LOOP TERMINOLOGYThe following is a brief glossary of terms encountered inPLL literature.CAPTURE RANGE (2 wC) - Although the loop wi"remain in lock throughout its lock range, it may not be ableto acquire lock at the tracking range extremes. The rangeover which the loo~. can acquire lock is termed capturerange. The capture range is sometimes called the LOCK-I NRANGE. (The latter refers to how close a signal must beto the center frequency before acquisition can occur. It isthus one-half the capture range or wC.)CURRENT CONTROLLED OSCILLATOR (CCO) - Anoscillator similar to a VCO in which the frequency isdetermined by an applied current.DAMPING FACTOR (n - The standard damping constantof a second order feedback system. In the case of the PLL,it refers to the ability of the loop to respond quickly to aninput frequency step without excessive overshoot.FREE-RUNNING FREQUENCY (fo' w o ) - Also called theCENTER FREQUENCY, this is the frequency at which theloop VCO operates when not locked to an input signal. Thesame symbols (f o ' w o ) used for the free-running frequencyare commonly used for the general oscillator frequency. Itis usually clear which is meant from the context.LOCK RANGE (2 wL) - The range of input frequenciesover which the loop will remain in lock. It is also called theTRACKING RANGE or HOLD-IN RANGE. (The latterrefers to how far the loop frequency can be deviated fromthe center frequency and is one-half the lock range or wL.)LOOP GAIN (K v ) - The product of the dc gains of all theloop elements, in units of (sec)-l.LOOP NOISE BANDWIDTH (BL) - A loop propertyrelated to damping and natural frequency which describesthe effective bandwidth of the received signal. Noise andsignal components outside this band are greatly attenuated.LOW PASS FILTER (LPF) - A low pass filter in the loopwhich permits only dc and low frequency voltages to travelaround the loop. It controls the capture range and the noiseand out-band signal rejection characteristics.NATURAL FREQUENCY (w n ) - The characteristic frequencyof the loop, determined mathematically by the finalpole positions in the complex plane. May be determinedexperimentally as the modulation frequency for which anunderdamped loop gives the maximum output and at whichphase error swing is the greatest.PHASE DETECTOR GAIN FACTOR (Kd) - The conversionfactor between the phase detector output voltage andthe phase difference between input and VCO signals involts/radian. At low input signal amplitudes, the gain is alsoa function of input level.PHASE DETECTOR (PO) - A circuit which compares theinput and VCO signals and produces an error voltage whichis dependent upon their relative phase difference. Thiserror signal corrects the VCO frequency during tracking.Also called PHASE COMPARATOR. A MULTIPLIER orMIXER is often used as a phase detector.QUADRATURE PHASE DETECTOR (QPD) - A phasedetector operated in quadrature (90 0 out of phase) withthe loop phase detector. It is used primarily for AMdemodulation and lock detection.VCO CONVERSION GAIN (Ko) - The conversion factorbetween VCO frequency and control voltage in radians/second/volt.VOLTAGE CONTROLLED OSCILLATOR (VCO) - Anoscillator whose frequency is determined by an appliedcontrol voltage.7