An electron‐paramagnetic‐resonance (EPR) reference‐arm microwave bridge is described that incorporates a FET microwave amplifier and a balanced quadrature mixer. An automatic frequency control (AFC) circuit is described that employs 70‐kHz frequency modulation of the oscillator. AFC feedback is based on formation of the sum of squares of theIandQoutputs of the mixer usinganalogdevices. The resulting AFC circuit is independent of the phase angle &agr; of the reference‐arm microwave voltage with respect to the signal‐arm microwave voltage at the mixer. AnL‐band bridge is described that incorporates the circuit. Good AFC lock was obtained from 1 &mgr;W to 100 mW of incident power independent of &agr; using a loop‐gap resonator ofQ=500.