DESIGN OF A MICROSTRIP BALANCED MIXER FOR SATELLITE COMMUNICATION

The design and measured results of a compact, low cost, low conversion loss microstrip single balanced Schottky diodes mixer is proposed. This mixer is designed for Ka-band satellite transponder simulator to convert the 30 GHz radio-frequency (RF) signal down to the 20 GHz intermediate-frequency (IF) signal with 9.8 GHz local oscillator (LO) frequency. This design takes full advantage of the frequency relationship of the RF, IF and LO, which is 3 : 2 : 1. A microstrip rat-race ring is designed at the LO frequency, which also functions as a 180-degree hybrid coupler at the RF frequency by its intrinsic multi-band characteristic. The amplitude and phase balance at both LO and RF frequency are analyzed, which guarantee the state-of-art performance of this single balanced mixer. The multi-function open/short stubs and a lowpass filter (LPF) with bonding wires across the rat-race ring are optimized to realize this low conversion loss mixer. The measured results show that the conversion loss is less than 9 dB at the IF frequency from 20.0 to 21.6 GHz, and the power of the second harmonic of LO is -45 dBm with +6.5 dBm LO drive power. The 3rd order inter-modulation products (IMD3) could be lower than -50 dBc with LO power higher than +7.8 dBm at the input RF power of -15 dBm.