FDTD simulation and measurement of a 90 GHz quasi-optical annular slot receiver

The authors describe the full design, modelling and measurement of a quasi-optical annular slot lens receiver at 90 GHz. The annular slot antenna mounted on an elliptical lens with a planar Schottky diode acts simultaneously as an antenna and a mixer. Measurement of input impedance, radiation pattern and conversion loss of the annular slot lens receiver is performed and compared with in-house finite-difference time-domain simulations at 90 GHz. A thru-reflect-line (TRL) manufactured on the same wafer is used as the calibration standard for de-embedding in the impedance measurements. In mixer designs, an important feature is to match the antenna with the diode for the mixer to have optimum performance. Two configurations are considered whereby an extra matching stub is added to one of the designs to conjugate match the diode. The optimum conversion loss for the design without matching stub is 17.4 dB with bias at 0.65 mA and LO power of -7.3 dBm, while the optimum conversion loss for the design with the matching stub is 16.7 dB with bias at 0.36 mA and available LO power of -7.5 dBm. This antenna is well suited for use in millimetre-wave imaging arrays.