Broadband Telecommunication services and wireless local-area networks (WLANs) increasingly depend on millimeter-wave operation. Millimeter-wave components based on galliumarsenide (GaAs) and indium-phosphide (InP) monolithic microwave integrated circuits (MMICs) are at the heart of such wideband systems. At Australia's CSIRO ICT Centre, John W. Archer and Juan Tello took part in the design and testing of a mixer that was made by United Monolithic Semiconductors using a GaAs-based Schottky-diode process. The mixer, which is pumped at the fourth local-oscillator (LO) subharmonic, boasts low conversion loss and high image suppression.
The nonlinear element used by this mixer circuit is a 5-mm-diameter GaAs Schottky diode. The subharmonically pumped (fourth LO harmonic) mixers (SHPMs) utilize a pair of these diodes connected in anti-parallel. The LO signal across the diode pair provides nonlinear modulation of the overall diode current and voltage. The result is a conductance variation at the common terminal at twice the LO frequency. Frequency conversion then occurs only for RFs close to the harmonics of the doubled LO frequency.
A simulation of the single-ended mixer's conversion performance shows conversion loss and RF input return loss as the radio frequency is swept from 170 to 220 GHz for a fixed LO of 47 GHz. The RF input power level was -20 dBm while the LO drive level was +13 dBm. Mixer-conformance tests also verified the hybrid design's accuracy. Although the mixer was designed as a block downconverter with a fixed LO of 47 GHz, it could be used over a range of LO frequencies. See "A 180-196 GHz Image-Reject Schottky-Diode MMIC Mixer," Microwave and Optical Technology Letters, Oct. 2007, p. 2319.
