Performance of Precision Quartz-Crystal Controlled Frequency Generators

Abstract

The present nearly universally accepted frequency standard is the atomic cesium resonance at 9.192631770 Gc. However, for many reasons it is not practical to use a cesium device for continuous frequency generation. Secondary frequency standards such as quartz-crystal controlled oscillators are commonly used for this purpose. Recent improvements in oscillator, oven and crystal unit design have produced crystal oscillators which require infrequent adjustment to maintain their frequency constant to a few parts in 1011 (pp 1011). This paper will present the performance of several of these oscillators and the state of the art in crystal unit fabrication and temperature control. The average drift rate and cyclic variations of 2.5 and 5 Mc oscillators will be given. The megacycle oscillators appear to have an average drift rate of less than 1 pp 1010/day while that of the 2.5-Mc oscillators is less than 1 pp 1011/day. The short-time stability (down to 10-msec period) of both types of units when measured in the kilomegacycle range appears to be of the order of a few parts in 1010. The effect of oven cycling and oscillator shutoff on the average drift rate has been measured. The period required for the frequency to stabilize after oven shutdown depends on the oven off-time. The oscillator frequency will return to a normal drift rate within 96 hours after a 24-hour shutdown, while it requires 5 to 6 days to return to a normal drift rate after a week shutdown.