Homodyne frequency demodulator for phase noise measurement systems

Abstract

Every phase noise measurement system which is based on frequency demodulator transforms frequency noise (FM) into proportional noise voltage waveform.After spectral analysis of the output noise voltage, the spectral density of frequency fluctuations S,,, [HzQHz] or other quantities are calculated. At the output of frequency demodulator, the near carrier phase noise is transformed into a close to DC noise component. Therefore the noise iloor of such a system at low Fourier frequencies is quite high due to the flicker noise (Vf noise) of semiconductor devices: detector or mixer diodes. Here homodyne phase modulation is proposed. The whole noise spectrum appears at a modulation frequency (about 30 MHz). The noise signal at a modulation frequency is amplified and then downconverted to DC by RF mixer. The homodyne modulation gives about 20 dB improvement at 100 Hz from carrier. Common Schottky mixer diodes with relatively high comer frequencies and large flicker noise were used.