Frequency stabilized and phase noise suppressed microwave source using an IQ mixer to detect amplitude modulation and phase perturbation of the reflected signal

A frequency-stabilized microwave source, comprising:

• an oscillator configured to produce a signal including a carrier at a microwave carrier frequency fc and having a tuning port responsive to a tuning voltage to control the frequency fc;
• a Pound-stabilization loop comprising:
• a modulation source configured to generate a modulation signal at modulation frequency fm, said modulation signal configured to add upper and lower sidebands at fc+fm and fc-fm to the signal;
• a reference resonator having a center frequency fRes configured to receive at least a portion of the signal and to produce a reflected carrier and reflected upper and lower sidebands, a portion of the reflected upper and lower sidebands being representative of amplitude modulation (AM) of the reflected carrier at modulation frequency fm and an indicator of a frequency error fc-fRes;
• an AM detector responsive to the reflected signal to produce a demodulation signal representative of the amplitude modulation of the reflected carrier at modulation frequency fm;
• a synchronous detector that mixes the demodulation signal with the modulation signal to produce a frequency error voltage indicative of the frequency error fc-fRes;
• and
• an integrator that integrates the frequency error voltage to produce a Pound-servo voltage that controls the tuning voltage to reduce the frequency error towards zero,
• wherein the AM detector comprises an IQ mixer having LO and RF input ports and I and Q output ports, wherein the RF input port receives the reflected signal and the demodulation signal is produced at the I output port,
• wherein the IQ mixer produces a phase error voltage indicative of a phase difference between the carrier and the reflected carrier at the Q output port,
• an LO phase shifter responsive to the phase error voltage, that samples the signal and produces a phase-shifted signal,
• wherein the phase-shifted signal is applied to the LO input port of the IQ mixer such that the demodulation signal produced at the I output port is insensitive to phase modulation in the reflected signal and suppresses close-in phase noise near carrier frequency fc;
• and
• a bandpass filter (BPF) coupled to the IQ mixer's Q output to filter the phase error voltage to produce a broadband phase correction voltage that is added to the Pound-servo voltage to suppress wideband phase noise.