Spectral Degradation in VHF Crystal Controlled Oscillators Due to Short-Term Instability in the Quartz Resonator

Abstract

Significant degradation and variation in the output signal phase sideband spectra of VHF crystal controlled oscillators has been observed. The spectral degradation. which occurs for carrier offset frequencies extending to several hundred kilohertz, has been traced to short-term stability in the fifth overtone AT-cut and the third overtone SC-cut quartz resonators used in the oscillators. The spectral degradation is characterized by flicker-of-frequency for f, 5 10 kHz and peaks at the location of prominent resonator anharmonic spurious responses. Up to 25 dB AM and PH spectral variation has been measured for identically fabricated crystals. The degradation is independent of oscillator type or circuit configuration. For high noise crystals the variation is drive-dependent. No correlation has been observed between resonator short- term stability and either drive, stress, or temperature coefficients. Oscillator signal phase levels appear influenced by crystal instability even when the lowest crystals are used. Details of the resonator stability measurement apparatus are described, and examples of correlation observed between short-term instability in the oscillator output signal and oscillator resonators are given for both single and dual crystal oscillator designs. Simple screening tests, that do not require sophisticated or expensive instrumentation, are now available for incorporation into crystal purchase part drawings associated with low oscillator designs.