Very Long Period Random Frequency Fluctuations in SAW (Surface Acoustic Wave) Oscillators,

The long-term frequency stability of SAW oscillators is of considerable importance to many applications. Therefore, extensive work has been done to understand and reduce the observed changes in frequency of SAW oscillators which occur over periods of time ranging from days to years. With improvements in the fabrication and packaging of SAW devices, the systematic long-term frequency drift has been reduced to a level where 19 out of 23 oscillators have drifted less than 2 ppm after 1 year. However, with the reduced systematic drift, it has become apparent that there is a significant level of random frequency fluctuations occuring with periods ranging up to months and years. A typical delay line oscillator may show a systematic drift of 1 ppm in a year and an rms deviation of 0.3 ppm. The value of the systematic drift might lead one to expect a week-to-week stability of 2 x 10 to the minus 8th power, but the random fluctuations make week-to-week changes of 1 to 2 x 10 to the minus 7th power a common occurrence.