Frequency stabilized microwave source using an IQ mixer to detect amplitude modulation 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 fe 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 a reflected signal to produce a demodulation signal representative of the amplitude modulation of the reflected carrier at modulation frequency fm;
• and 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, said frequency error voltage controlling the tuning voltage to reduce the frequency error towards zero, wherein the AM detector comprises an IQ mixer comprising:
• an I mixer having an LO input port, an RF input port and an I output port;
• a Q mixer having an LO input port, an RF input port and a Q output port;
• a first microwave power splitter that receives a phase-shifted signal and produces first and second LO signals at the I and Q mixer input ports, respectively;
• a second microwave power splitter that receives the reflected signal and produces first and second RF signals at the I and Q mixer RF ports, respectively;
• wherein the first and second microwave power splitters are arranged to produce first and second relative phases between the first LO signal and first RF signal and second LO signal and second RF signal respectively, the difference between first and second relative phases being 900;
• wherein the I mixer produces the demodulation signal at its I output port;
• wherein the Q 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 the phase-shifted signal, wherein the phase-shifted signal is applied to the LO input port of the Q mixer such that the second LO signal and the second RF signal at the LO input port and RF input port of the Q mixer, respectively are maintained I quadrature such that the demodulation signal produced at the I output port is insensitive to phase modulation in the reflected signal.

An amplitude modulation (AM) detector for a Pound-stabilized microwave source in which a reference resonator having a center frequency fRes is configured to reflect a signal including a carrier at carrier frequency fc and upper and lower sidebands at fc+fm and fc−fm at modulation frequency fm, the AM detector comprising:

• an IQ mixer having LO and RF input ports and I and Q output ports;
• wherein the RF input port is configured to receive a reflected signal including a reflected carrier and reflected upper and lower sidebands, a portion of the reflected upper and lower sidebands being representative of AM of the reflected carrier at modulation frequency fm and an indicator of a frequency error fc−fRes;
• wherein the IQ mixer is configured to produce a demodulation signal representative of the amplitude modulation of the reflected carrier at modulation frequency fm at the I output port;
• wherein the IQ mixer is configured to produce 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.