Low-Frequency Quartz-Crystal Cuts Having Low Temperature Coefficients

Abstract

This paper discusses low-frequency, low-temperature-coefficient crystals which are suitable for use in filters and oscillators in the frequency range from 4 to 100 kilocycles. Two new cuts, the MT and NT, are described. These are related to the +5-degree X-cut crystal, which is the quartz crystal having the lowest temperature coefficient for any orientation of a bar cut from the natural crystal. When the width of the +5-degree X-cut crystal approaches in value the length, the motion has a shear component, and this introduces a negative temperature coefficient which causes the temperature coefficient of the crystal to become increasingly negative as the ratio of width to length increases. The MT crystal has its length along nearly the same axis as the +5-degree X-cut crystal, but its major-surface is rotated by 35 to 50 degrees around the length axis. This results in giving the shear component a zero or positive temperature coefficient and results in a crystal with a uniformly low temperature coefficient independent of the width-length ratio. A slightly higher rotation about the length axis results in a crystal which has a low temperature coefficient when vibrating in flexure and this crystal has been called the NT crystal. The NT crystal can be used in a frequency range from 4 to 50 kilocycles, while the MT is useful from 50 kilocycles to 500 kilocycles. A special oscillator circuit is described which can drive a high-impedance NT flexure crystal.