NIST Technical Note 1337: Characterization of Clocks and Oscillators

12 FREQUENCY AND TIME MEASUREMENT219occurs to N/v lO , where N is the divisor. Such measurement systems are usedat many standards laboratories for the long-term measurement of atomicclocks, whose output is usually divided down to I pulse/sec. Since timeintervalcounters with resolution better than 0.1 nsec are available, thismeasurement scheme is suitable for long-term performance monitoring,yielding frequency-measurement precision of 10- 14 for 1-day averages.12.2.2 Frequency 1\;leasurementsA'/erage frequency is measured most directly using a frequency counter.Used this way, the counter determines the number of whole cycles Moccurring during a time interval T given by the counter's time base. Thusv(O; T) = (M + 6..\-1)/T ~ M/T, (12-46)where V(tl: t 1 ) denotes the average frequency over the interval from t 1 to t 2and 6.""f, the fractional cycle, is not measured by the counter. The startingtime is arbitrarily called t = O. Thus, the quantization error is given by~VQ!