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Two Crystals and Frequency Pulling in Precision 100 MHz OCXO.

This paper considers a method of OCXO development which allows one to use two coupled crystal resonators in an oscillator feedback. A coupling coefficient provided by internal capacitive binding was optimized in order to get maximal attainable quality factor of a resulting network. In this case a close coupling causes a frequency pulling effect (two oscillation frequencies and two values of a real part of the feedback impedance correspond to each value of an imaginary part of the feedback impedance). The presented mathematical model of the oscillator shows that the second branch of oscillation frequencies may be eliminated from operating range while the quality factor of the resulting network remains high. The 100 MHz OCXO containing two identical AT-cut crystals was built in accordance with obtained recommendations. The phenomenon of frequency pulling was not observed throughout the entire electrical tuning range. The phase noise level -151 dBc/Hz at 1 kHz frequency offset is by 11 dB lower than that for the same type of the oscillator with single crystal in the feedback