Measurement of FM Noise Spectra of Low-Noise VHF Crystal Controlled Oscillators

The design and characterization of stable, low-noise crystal controlled VHF oscillators are described. Operating in the 60 MHz to 80 MHz frequency range, these oscillators have shown total rms frequency deviations of less than 4 parts per 1010 in a 5 kHz band. Noise characteristics have been determined from power spectra measurements, FM noise measurements, and phase noise measurements. The equivalent noise sideband power spectra corresponding to each method of measurement were calculated and compared to the spectra predicted by a very simple model of the oscillator circuit. Agreement within a few dB was found for the region within about 500 Hz of carrier frequency, but measured noise power was found to be considerably higher than predicted for higher baseband frequencies in the range from 800 Hz to 5 kHz. Measurement techniques and evaluation of data are presented.