An analysis has been carried out of optical heterodyne conversion with an interdigitated‐electrode photomixer made from low‐temperature‐grown (LTG) GaAs and pumped by two continuous‐wave, frequency‐offset pump lasers. The analytic prediction is in excellent agreement with the experimental results obtained recently on a photomixer having 1.0‐&mgr;m‐wide electrodes and gaps. The analysis predicts that a superior photomixer having 0.2‐&mgr;m‐wide electrodes and gaps would have a temperature‐limited conversion efficiency of 2.0% at a low difference frequency, 1.6% at 94 GHz, and 0.5% at 300 GHz when connected to a broadband 100 &OHgr; load resistance and pumped ath&ngr;=2.0 eV by a total optical power of 50 mW. The predicted 3‐dB bandwidth (193 GHz) of this photomixer is limited by both the electron‐hole recombination time (0.6 ps) of the LTG‐GaAs material and the RC time constant (0.5 ps) of the photomixer circuit.