A study of a low noise millimeter wave mixer

A breadboard model of a broadband 30/11 GHz mixer with low conversion loss was designed, measured, and delivered. Schottky-barrier mixer diodes of different makes were characterized and one type was selected for the mixer. Filter circuits for the mixer ports were specified and designed. The mixer section was optimized with regard to diode loading at the mixer frequencies. Special attention was paid to image enhancement in proper phase and to trapping of local oscillator harmonics. Due mainly to the inherent high Q of the packaged mixer diodes available, the instantaneous bandwidth was restricted to 2 GHz with mechanical tuning over the full 3.5 GHz originally specified. The conversion loss in this bandwidth can be kept below 5.6 dB with a maximum input of 3.1:1 over the whole range of operating temperature and IF load mismatch. A low local oscillator power (2 to 3 mW) was chosen to minimize the gain variation. At delivery the mixer was tuned to a bandwidth of 1.4 GHz with conversion loss below 4.6 dB and input VSWR below 2:1 (room temperature and load VSWR = 1. 0:1).