Waveguide Mixer Sum frequency Termination Considerations in an FM Communication System

In a 6-GHz FM communications system using a mixer with its diode mounted in waveguide, group delay and linearity anomalies were observed which resulted in serious degradation of intermodulation signal-to-noise performance. This paper describes the problem, presents the reasoning and the laboratory tests by which improper termination of the mixergenerated sum frequency was isolated as its source, and describes a waveguide sum frequency absorber located at the mixer to solve the problem. The necessity of giving attention to all the signals generated in a nonlinear device in waveguide is discussed in this paper and has application to all such components in FM systems.