Considerations for Homodyne Detection Systems

The effects of modulation frequency, RF reference power, and external bias upon the sensitivity and dynamic range of microwave homodyne detection systems was measured for point contact diodes and low l/f noise Schottky and backward diodes. The measurements were made at 4.89 GHz using a signal to noise ratio of 3 dB and a detection system bandwidth of 10 Hz. Maximum sensitivities of -135, -150, and -145 dBm, and dynamic ranges of 92, 110, and 124 dB were measured for the point contact, Schottky, and backward diodes at modulation frequencies of 30, 30, and 3 kHz, respectively. It was found that the level of RF reference signal needed to obtain the maximum sensitivity was equal to or somewhat above the point where the diode changes from square law to linear detection. The results are significant in that previously reported homodyne sensitivities (not necessarily maximum) were on the order of -90 to -130 dBm for point contact diodes and no data are available for Schottky and backward diodes. Significantly improved stability, sensitivity, and dynamic range can be achieved using these low 1/f noise devices, the correct external bias, and the optimum RF reference power.