Abstract
A 290–310 GHz Schottky diode based subharmonic mixer with integrated, low loss, impedance matching waveguide cavity filters is presented in this paper. This mixer was designed for use in a 300 GHz communication system with a 20 GHz intermediate frequency band centred at 15 GHz. Image rejection of the 260–280 GHz lower sideband, as well as impedance matching, was achieved using an integrated third-order filter in the RF waveguide. The conventional coupling matrix was used to design the filter even though the impedance presented to the RF port was complex and frequency dependent. The mixer was measured to have: 1) single sideband conversion loss of 9–10 dB across the upper sideband, with a mixer noise temperature of 2000–2600 K; 2) a return loss at the RF port better than 12 dB, with three filter reflection zeroes (poles) distinguishable; and 3) a sideband rejection ratio from 13 to 25 dB, demonstrating the RF filter's excellent performance in terms of impedance matching and filtering.
Highlights
T HERE is a need for Schottky diode based mixers, operating at submillimeter-wave frequencies for diverse applications
The design approach presented in this paper differs from the conventional approach where the diodes are coupled to the output waveguide, and followed by an external filter
We provide a new approach where the diode chip is directly coupled to the resonators of waveguide filters via E-plane probes and impedance matched using the RF/LO waveguide filters
Summary
T HERE is a need for Schottky diode based mixers, operating at submillimeter-wave frequencies for diverse applications. For a conventional DSB mixer, the Schottky diodes are impedance matched on the microstrip circuit, and a microstrip to waveguide transition is used. Moving the filter toward the diodes eliminates one of the microstrip matching stages, shortens waveguides thereby reducing loss, and makes the structure much more compact. Image rejection over the 260–280 GHz LSB and impedance matching are achieved simultaneously using a third-order waveguide cavity filter. The sideband rejection approach proposed by us gives a comparable performance in terms of sideband rejection ratio (SBR) and conversion losses, it has much simpler design and offers easier fabrication
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