Method and Apparatus Used at the Bureau of Standards in Testing Piezo Oscillators for Broadcast Stations

A piezo oscillator may be used either to check the frequency of the station or to control its frequency. Most of the piezo oscillators tested by the Bureau are for use in checking the frequency of the station. Piezo oscillators are capable of high precision in the measurement of their frequency if a beat note is produced by adjusting another generator to a similar frequency. This principle is employed in the methods and apparatus described in the paper. After preliminary tests to determine the suitability of the quartz plate and its fundamental frequency, the piezo oscillator is kept in a temperature-controlled room for not less than two days, during which frequency measurements are made. The method consists in measuring the frequency of the piezo oscillator under test in terms of a 200-kc temperature-controlled piezo oscillator. This is accomplished by adjusting a radio-frequency generator to the frequency which the piezo oscillator under test should have. This adjustment is made using harmonics from a 10-kc generator which is kept accurately set in terms of the 200-kc standard by observing a special form of beat indicator. The frequency difference between the test piezo oscillator and the generator set in terms of the standard is measured by comparison with an audio-frequency generator. A frequency meter of special design is used to check the frequency difference and determine the sign of the correction to be applied. The method described is also useful in the calibration of frequency meters and the measurement of station frequencies.