FET Mixers for Communication Satellite Transponders

Two different types of FET mixer circuits have been developed for 6/4 GHz frequency translation for communications sateIIite transponder. Gate mixer uses non-linear I _d - V _g characteristic, with LO being injected into the gate circuit, while in the drain mixer the LO is injected into the drain circuit and non-linear I _d - V _d characteristic is utilized. Both circuits yielded conversion gains up to 3 dB over 500 MHz bandwidth. However, the noise figure of the drain mixer as low as 4 dB was measured compared with 5.7 dB for the gate mixer.