Broadband Fixed-Tuned Subharmonic Receivers to 640 GHz

This paper will describe the design and testing of solid-state subharmonically-pumped mixers from 380 GHz to 640 GHz. The main goal has been to develop robust, compact, solid-state room-temperature receivers with state-of-the-art sensitivity and broad IF bandwidths for applications such as airborne and space-based microwave sounding. Testing of a 380 GHz integrated mixer has yielded state-of-the-art performance, with a double-sideband (DSB) mixer noise temperature of 850 K and a mixer conversion loss of 8.5 dB (DSB) using 7 mW of local oscillator power. The success of this receiver is due to a combination of integrated diode technology and inherently broadband circuit designs achieved with modern high frequency design tools. The recently developed MASTER integrated diode technology allows for precise control of the circuit geometry and for the reduction of parasitic elements, thus allowing greater accuracy of computer simulations and therefore better high frequency performance and bandwidth. The split block geometry used for these mixers is relatively simple to machine, and yet allows for broad fixed-tuned RF and IF bandwidths, and is also compatible with molded and micromachined blocks. Finally, design techniques have been developed for these mixers that give excellent agreement with measurements, thus allowing for rapid prototyping. The major impact of this research is to demonstrate that excellent harmonically pumped mixers using integrated diodes and modern design tools are now a commercial possibility to at least 640 GHz. Mixer Layout The mixer block, similar to that described in [1], is split in the E-plane of the RF and LO waveguides, thus simplifying mixer assembly and reducing the losses in the waveguides. The planar diode and mixer circuitry are fabricated on a 35 μm thick fused-quartz substrate. The circuits are then placed in a shielded microstrip channel which runs perpendicular to the RF and LO waveguides. A schematic of the mixer block circuit configuration is shown in Fig. 1. The diodes are located in the microstrip channel. Waveguide-to-microstrip transitions are used to couple both the RF and LO into the channel. The microstrip metallization bridges across each guide, necessitating the use of reduced height waveguide to achieve reasonable fixed-tuned bandwidths [2]. For this mixer, half height waveguide was used for the RF, and third height waveguide was used for the LO. A low-pass microstrip filter is used to prevent the RF signal from coupling to the LO guide, and a short-circuited half-wave stub is used to provide the LO termination. Fig. 1. Schematic of subharmonic mixer configuration. Fig. 2. SEM of an integrated anti-parallel mixer circuit. Integrated Diode Fabrication Diode integration has many benefits. First the circuit geometry near the Schottky anodes is nearly planar and defined photolithographically. This simplifies the analysis of the circuit and allows more precise control of the embedding impedances. Second, the elimination of the high dielectric GaAs substrate reduces capacitance. This improves coupling to the Schottky diodes and increases receiver bandwidth. Finally, the assembly of the mixer is much simpler and a higher level of repeatability is achieved. The fabrication process for the integrated diodes is described in [3], and a view of one of the circuits is shown in Fig. 2.