A VHF, quartz crystal oscillator exhibiting exceptional vibration immunity

Abstract

This paper describes the design and performance of a 100 MHz quartz crystal oscillator employing a combination of technologies to achieve exceptionally good spectral performance under vibration. The oscillator is intended for use as the exciter master oscillator in a multi-function radar. The oscillator design utilizes two, series-connected, third overtone, SC-cut crystals oriented to provide partial cancellation of vibration occurring in the plane of the crystal blanks. The oscillator design incorporates a "building block" approach, including the use of a self-limiting, modular amplifier sustaining stage. In order to minimize oscillator signal spectral degradation encountered as a result of enclosure and component mechanical resonances, the following design was implemented: (1) Except for the crystals and a voltage regulator IC, the oscillator circuit employs leadless, surface mount components. (2) The use of a rigid chassis employing closely-spaced stiffening ribs which trisect both the PWA and steel covers. (3) Use of a thermally isolated but rigidly mounted crystal heater block. In addition, the oscillator assembly is designed with its center of gravity (C.G.) co-incident with the isolation system center of mass and is mounted via an elastomeric isolation system developed specifically for attenuating dynamic disturbances in small assemblies. The prototype unit produced resonant frequencies between 70 and 85 Hz with an acceleration spectral density roll-off rate up to 12 dB/octave. Oscillator at-rest phase noise is characterized by L(100 Hz)=-135 dBc/Hz and a phase noise floor level of -175 dBc/Hz. Phase noise performance under vibration measurements indicate very little degradation due to mechanical resonances. The performance that has been achieved in the region of unity transmissibility in the isolators (fm<75 Hz) corresponds to a net fractional frequency sensitivity to vibration of less than 2 to 3/spl times/10/sup -10/ per g. The corresponding sensitivity achieved for vibration occurring in the direction of the crystal blanks is on the order of 1/spl times/10/sup -10/ per g.