The reflectometer-an indispensable instrument for eliminating frequency jumps and noise in precision crystal oscillators

An instrument that is able to identify a noisy quartz crystal or to cull out crystals that will produce frequency jumps before assembling into finished precision ovenized oscillators is continuously sought by industry. Noise and frequency jumps are phenomena exhibited in finished oscillators which can be caused by the quartz crystal; but can also be caused by popcorn noise in regulators, transistors, noisy components (especially capacitors), unstable oven(s), power supply noise, varactor diodes and other sources. To be able to characterize a quartz resonator as being acceptably low in noise and frequency jumps before assembling it in the oscillator circuitry results in significant savings in time and enhanced reliability. Typically in current manufacturing practice the entire precision oscillator is assembled and then the completed unit is tested for frequency stability. If the oscillator is found to have unacceptable frequency excursions, all the possible causes for this behavior, as delineated above, must be identified and remedied. The most difficult faulty component to replace, at the oscillator level, is the quartz crystal. The replacement process requires the complete disassembly of the oscillator, and at times collateral damage is done to the unit that may affect its long-term reliability. In this paper we present a precision instrument developed by Frequency Electronics, Inc., that pre-qualifies crystals before being assembled into oscillators. This unique instrument is the Reflectometer Model FE-6289A. It is capable of high resolution testing of quartz crystals as components, and detects noise and frequency jumps with a measuring resolution of 1xE-11. Furthermore, the FE-6289A embodies a very convenient characteristic for flexibility of measurement conditions- the crystal under test can be placed remotely from the test instrument-hence, this provides a convenient and useful methodology for measuring parameters such as radiation effects on quartz resonators.- - The critical design parameters of this instrument as well as test results are presented.