NIST Technical Note 1337: Characterization of Clocks and Oscillators

528 LESAGE AND AUDOINMinrFrequent,requent,SDurce ~-(Xll-4-I SourcenOln02Fig. 15. Principle of the time difference method.(a precision of 10-100 ns is typical, depending onthe class of the counter), they are also used withc.w. frequency generators, as shown in Figure 15.This method is then well suited to the case in whichthe frequency sources under test have the samenominal frequency v o ' e.g., atomic frequency standards.An auxiliary frequency source, such as afrequency synthesizer, with frequency v ~ allowsone to obtain, at the output of the mixers, twobeat notes at the desired frequency VI = Ivo ­v~l. After amplification the zero crossing of oneof the beat notes starts the time interval counter,and the zero crossing of the other beat note stopsiL One has(14)where x I is the fractional phase fluctuation of thebeat note and X o and x~ that of the two frequencystandards. For instance, with V o = 5 MHz, VI =0.5 Hz, and a precision in the time interval measurementof 0.1 ~s a precision of 10 -14 S at the nominalfrequency Vo is achieved.6. CHARACTERIZATION OF FREQUENCY STABILITYIN THE TIME DOMAIN6.1. Significance of experimental dataIt is well established that measurement in thetime domain with an electronic counter samplesphase increments and gives A.'P(t k) dermed asA,.