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R.W. BURHANS

WWV is a source of highly accurate time- and frequency-signals. Here's a method that letsyou use those signals to calibrate your own frequency standards.AS ELECTRONIC CIRCUITS GET MORE COM-plex, the level of accuracy needed to testthose circuits also increases. While accu-rate measurement of voltage, current, andresistance present their own problems,often it's ev en harder to perform accuratefrequency measurements. That's becausethey can only be as ac curate as the crystaltimebase in your frequency counter.Getting a high degree of accuracy whencalibrating a crystal timebase has beenalmost impossible without expensivecalibration equipme nt. That's especiallytrue if you wa nt the calibration to be trace-able to the NBS (National Bureau of Stan-dards). The only inexpensive alternativeis to calibrate the timebase using the timesignals t ransmit ted by radio stat ionsWWV and WWVH. Those stations areoperated by the National Bureau of Stan-dards and they transmit highly-accuratecesium-clock derived time signals on2 . 5 , 5 - , lo -, 1 5-, and 20-MHz. (A'com-plete guide to the WWV and WWVHbroadcast services can be found in NBSSpecial Publication 432, NBS Frequencyand Time Dissemination Services. Thedocument is available for 60 cents fromthe Superintendent of Documents, U.S.

P r i n ti n g O f f i c e , W a s h i n g t o n , D C2040 2.) The commonly accepted methodfor calibrating an oscillator using WWVtransmissions is to zero-beat the oscillatoragainst the WWV carrier frequency.However, a small controversy has de -veloped as to whether or not crystal oscil-lators such as those found in frequencycounters can be accurately calibrated us-ing that metho d. (See "Equipment Re-por t s , " S a b t r o n i c s 861OA , Radio -Electronics, February 19 82, and John H.H e n n i n g s r e b u t t a l l e t t e r , Rad i o -Electronics , June 1982) . The c on-troversy arises from the fact that theionosphere is unstable (its conditionfluctuates cyclicly over a 24-hour per-iod), and affects the propagation of theWWV transmissions. The unstable na-ture of the ionosphere limits the accuracyof the WWV transmissions to 0.1 part-per-million (1 Hz in 1 0 MH z). How ever,by applying a technique used long befor ethe days of atomic clocks , it is possible toobtain an accuracy of 0.001 part-per-million (1 Hz in 10 GHZ) using WWVtransmissions, provided that the oscilla-tor you're calibrating is kept operatingcontinuously for long periods of time.

The calibration techniqueRather than calibrate the oscillatoragainst the WWV carrier frequency, thetechnique outlined here uses the audibleticks that are transmitted by WWV andWW VH to identify the seconds. Each ticktransmitted by those services consists of a1000-Hz, 5-ms tone. The actual tick,therefore is composed of five cycles of a1000-Hz sinewave.The technique is illustrated in the blockdiagram in Fig. 1 . The received time tickfrom WWV is displayed on the verticalaxis of an oscilloscope. Th e output of thec rys t a l osc i l l a t o r i s c onne c t e d t o avariable-phase countdown circuit that di-vides the oscillator frequency down toone pu l se pe r s e c ond . Thumbwhe e lswitches in the countdown circuit vary thephase of that 1-pulse-per-second wave-for m, which is used to trigger the oscillo-scope . The swee p rate is set to l-ms-p er-division and the thumbwheel switches onthe countdown circuit are adjusted until a

sinewave appears. Since the 1000-Hztone from W WV occurs only once eachsecond and lasts for only 5 m s, the thumb-wheel switches may have to be adjustedalmost through their entire range before

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7F R A C T I O N A L F R E Q U E N C Y E R R O R [Aflf O R Atl t )

2-THE FRACTIONALFREQUENCY ERROR of the local clock can be determined from this graph

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the sinewave can be seen. Remember, itlasts for only YZOOecond.After the sinewave does appear, thethumbwheel switches are again adjusteduntil the leading edg e of the first cycle ofthe sinewave aligns with the start of theoscilloscope trace. The drift of that tracedetermines the frequency error of thecrystal oscillator. If the sinewave driftsone full cycle in abo ut a minute, then thecrystal oscillator is set to within on ly k 10parts-per-million. If the sinewave driftsone cycle in 15 minutes then the fre-quency s tabi l i ty i s about 1 part-per-million. T he rate of change of th at drift iscalled the aging rate of the crystal.To obtain higher accur acies, it is neces-sary to measure the drift over severaldays. The ionosphere will indeed affectthe accuracy of the results, but its effectwill be minimized if the measurementsare always made at the same time of day.Th e change in the settings of the thumb-whee l switches required to align the sine-wave with the beginning of the trace isrecorded every day at the same time eachday. The drift can be estimated to withinperhaps 0.1 millisecond by observingwhere the leading edg e of the first cycle ofthe sinewave is with respect to the startingpoint of the trace. The drift (in mil-liseconds) over a 24 -hour period of timecan be used to determine the fractionalfrequency error of the clock by using thegraph shown in Fig. 2. Although that maybe a lot more trouble than the averageobserver is willing to take to determinehis clock err or, the poin t is that if accura-cy is necessary, it can be obtained usingWWV time signals.The a g i ng ra t e o f a c rys t a l i t s e l fchanges with time, so it is necessary toreset the fine trim on the local clock-oscillator abou t once a month to maintainaccuracy. An additional problem witht e mpe ra t u re -c ompe nsa t e d a nd ove n-controlled crystal oscillators is that whenthey are shut off and then turned back onagain, the aging rate starts at a new point.However, that problem is eliminated ift h e o s c i l l a t o r i s k e p t r u n n i n g c o n -tinuously, as in the frequency counters. now sold that have standby operation, inthose, an oven-stabilized crystal oscilla-tor is always on.The countdown circuit

The variable-phase countdown circuitis shown in Fig. 3 . Thi s circuit is intendedto be driven from a 100-kHz or 60-kHzsource, but it can easily be driven from aI-MH z or 10 -MHz source with the addi-tion of a couple of decade dividers usingSchottky TT L or even some CM OS IC 's.The thum bwheels can be adjusted to pro-vide any phase of a I-second tick from 0

IABLE PHASE COUNTDOWN CIRCUIT. The thumbwheels are adjusted until the sinewave 999 inbeginning of the tick trace. increments. R-E63