C-band down converter realization with GaAsFET active balun and balanced mixing structures

A C-band Down Converter with balanced quad GaAs FET mixer and RF active balun has been realized. The Down Converter works from 3.7 to 4.3 GHz with high IM3 and IM2 suppressions. The mixer was constructed using GaAsFETs in quad structure. The GaAsFETs were originally four NEC 76038 FETs. The RF signal was fed through a single section of GaAsFET phase shifter utilizing microstrip to rotate signal phase and to isolate RF input with outputs. The IF combiner used ratrace balun structure, while 5 GHz LO signal was fed through a microstrip phase shifter. Relatively flat performance can be addressed to broadband phase-shifting performance of RF balun divider. The mixer FETs were working in resistive region with low V_DS bias at 0.1 V. This linear operating condition produced IM3 and IM2 suppressions better than-40 dBm at 3.9 GHz RF input. The IM3 reached 48 dBm, while IM2 was better than -52 dBm. High suppression was obtained at relatively high LO input of 7 dBm. This excellent performance is a result of cold resistive operation of the GaAsFETs in mixing quad. The RF-IF isolation was low with only -28 dB isolation, while LO-IF isolation was about -24 dB. This condition of leakage may be due to `fringing field´ of lines in proximity