A fixed tuned subharmonic 340 GHz GaAs Schottky diode mixer design is presented. The mixer consists of a 3 mil quartz substrate that is suspended in a LO and RF reduced height waveguide splitblock, and uses a coaxial connector for the IF interface. It has been designed for the VDI SC1T2-D20 antiparallell diode chip that is flip-chip mounted on the circuit. The performance of three types of RF stop filter topologies have been evaluated and will be presented