135-165GHz Sub-harmonic Mixer Based on Schottky-diode Circuit Model

A low-cost fix-tuned 135-165 GHz sub-harmonic mixer (SHM) with low conversion loss (CL) is presented. The SHM is designed adopting GaAs Schottky dual-diode chip DBES105a, delivered by UMS, Inc. In consideration of extremely short signal wavelength and small circuit size at terahertz band, complicated parasitic effect, brought by packaging of the adopted Schottky chip, can lead to a limitation on optimum CL and operation bandwidth. To reduce the parasitic elements, one of the diodes in series is shorted and single-junction operation is enabled in the chip. The corresponding equivalent circuit model is developed for impedance matching and reliable circuit design. Passive circuits, including the waveguide-to-microstrip-line (WG-MSL) transition, the matching networks and the diplexer, are optimized to minimize the CL based on the equivalent circuit model. The experimental results show that the measured single-side-band CL of the SHM is 11-17 dB over 135-165 GHz, and is 12±1 dB over 140-160 GHz. Wide IF frequency bandwidth is attained, covering 0-20 GHz. The measured results agree well with the simulated ones, demonstrating the accuracy of the equivalent circuit model and the effectiveness of the design methodology.