NIST Technical Note 1337: Characterization of Clocks and Oscillators

(5) All oscillators in the range of 5 MHz ± 5H~ may be compared. Other carrier frequen­Cles such as 1 MHz, 5.115 MHz, 10 MHz and10.23 101Hz are also usable. However, differentcarrier frequencies may not be mixedon the same system. The sys tem has beensuccessfully tested with an oscillator offset4.6 Hz from nominal 5MHz. Measurementswere made at intervals of 2 hours betweenwhich the sys~m had to accumulate approximately2 x 10 n. The system has also b~§ntested with an oscillator offset 4 x 10 ,and no errors were detected during a periodof 40 days.(6) All sampling times in the range of 1 secondto 16 days with a resolution of 0.1 secondare possible. Measurements may be made oncommand or in a preprogrammed sequence.(7) Measurements are synchronized precisely,i.e. at the picosecond level, with thereference clock. They may therefore besynchronized with important user systemevents, such as the switching times of aFSK or PSK system.(B)All oscillators are compared synchronouslyand all measurements are performed within amaximum interval of 0.1 second. As a result,the phase of any oscillator needs tobe i nterpo1ated to the chosen measurementtime for an interval of 0.1 second maximum.This capability, which is not present ineither si ngl e heterodyne measurement systemsor switched measurement systemseliminates a source of "measurement" errorwhich is generally much larger than thenoise induced errors. For example, interpe1ationof the PD!!e Rf a high performanceCs clock (0 - 10 II~) over a period of 3hours would' produce approximately 1.5 nsphase uncertainty. To maintain 4 ps accuracyrequ i res measurements simultaneous toO.ls.(9) There are no phase errors due to the switchingof rf signals since there is noswitching anywhere in the analog measurementsystem.(10) No appreciable phase errors are introducedwhen it is necessary to change the referenceclock since, as shown in Figure 6, thepeak error due to changes in synthesizerphase is 20 ps.(11) The measurement system is capable of measuringits own phase noise when the samesignal is app1 ied to two input ports.Figure 7 shews the phase deviations betweentwo such channels over a period of 75,000seconds and Fi gure B is the correspondi ngAllan variance plot. ~igure 9 shows thephase deviations between 2 input channelsover a period of 40 days.(12) Since the IEEE-533 (CAMAC) interface standardhas been followed for all tne customhardware, the system may be easily interfacedto almost any instrument controller.NBS has already tes ted the sys tern us i ng alarge minicomputer, a small minicomputerand a desk top calculator. Interfacesbetween IEEE-583 and IEEE-48B controllersare available and have been used successfully.(13) The system is capable of camparing a verylarge number of oscillators at a reasonablecost per device.There are also disadvantages to this measurementsystem. The most important are:(l) The camp 1exity of the hardware is greaterthan for some systems. It is possiblethat this will reduce reliability.(2) A high level of redundancy is difficult toachieve. The system design stresses size,power, convenience and cost, resulting inan increase in the number of possiblesingle point failure mechanisms compared tosome other techniques. For example, aCAMAC power supply failure will result in aloss of data for all devices being measured.(3) A SUbstantial committment is required inboth specialized hardware and software.(4) If an oscillator under test experiences aphase jump which exceeds 1 cycle, themeasurement system records a jump withincorrect absolute magnitude. As a reSUlt,it may not be applicable to signals whichare frequency modulated with discontinuousphase steps larger than 2n.ConclusionsWe have demonstrated a new phase measurementsystem with very desirable properties: Alloscll 1ators in the range of 5MHI ± 5Hz may bemeasured directly. The sampling times are onlyrestricted by the requirement that they exceedone jiecond. The noise floor is a (2,t) = 3 x10- 1 It in short term and the tin?e deviationsare less than 100 ps. All circuitry is designedas modules which allows expansion at modestcost. Compatibility with a variety of computersis insured through the use of the lEEE-5B3interface and adapters are available to permituse with an IEEE-4BB controller. The systemmakes it feasible to make completely automatedphase measurements at predetermined times onlarge numbers of atomic clocks. It's own noiseis one-hundred times less than the state-of-theartin clock performance. It will be used inthe near future to make all measurement neededto compute NBS atomic time, but it will also bevery valuable for any laboratory which usesthree or more atomic clocks.References1. O. W. Allan, "The measurement of frequency316