Phase noise in microwave bridge oscillators

This paper deals with phase noise in low noise microwave oscillators utilizing bridge (differential) networks to increase their effective Q-factor. As such oscillators do not apply an automatic frequency control they are free from auxiliary circuitry self noise influence that, in turn, permits to incorporate successfully any stabilizing resonators including cryogenically cooled ones. Comparing of the microwave bridge oscillator (MBO) phase noise with that one of the classic Leeson´s model reveals at least one drastic distinction that demonstrates itself as deep oscillator phase noise decrease at output of a sustaining amplifier beyond a resonant network bandwidth. The effect is explained by fundamental properties of the bridge network where growing detuning leads to a transmission phase shift approaching plusmn180 degree that changes a sign of a feedback for distortions from positive to negative. This feature distinguishes MBO from frequency stabilization schemes using an automatic frequency control. The method permits to improve oscillator close-in phase noise by 10...15 dB. As expected, a room temperature X-band MBO arranged with a SiGe HBT sustaining amplifier and a whispering gallery sapphire resonator could have # as low as -140 dBc/Hz at 1 kHz offset