Response Coefficients of a Double-Balanced Diode Mixer

When receivers are to be deployed in a closely-spaced environment with one or more transmitters, the degree to which the receivers respond to spurious signals becomes important. A major determiner of the receiver's spurious/intermodulation response levels is the first mixer. In recent years, the use of balanced mixers has become widespread. The degree to which the components of these circuits are balanced can have a great effect on the levels of many of these undesired responses. This paper presents the results of a computer-assisted analysis of a commonly used double-balanced diode mixer circuit. It is shown that the magnitudes of some of the responses to one or more input radio frequency signals are sharply dependent on the degree to which the diodes comprising the mixer are balanced. The average rejections imparted to a particular undesired intermodulation (or spurious) response by a random sample of such mixers are plotted as a function of the tightness of the control of diode characteristics of the population from which the four diodes comprising a single mixer are selected. A computer program based on a Fourier series expansion of the time-dependent coefficients of a Taylor series representation of the four diode currents was employed to compute the response coefficients of the mixer. The response coefficients are defined as the magnitudes of the intermodulations of sinusoidal input radio frequency signals and the local oscillator frequency signal. The method treats the switching action characteristic of the large local oscillator power levels rather than relying on the more usual and impractical assumption of "mild" nonlinearity. Orders of nonlinearity up to tenth are treated.