Diode Mixer Coefficients for Spurious Response Prediction

The precise prediction of spurious response levels in a superheterodyne receiver would greatly aid the receiver designer besides being necessary for the representation of a radio frequency interference environment. The power series is used to represent the nonlinear receiver mixer in order to obtain an expression for the receiver spurious response sensitivity. This sensitivity expression contains the power series coefficients and shows that the coefficient magnitudes must be known before the spurious response levels can be computed. Therefore, a method is developed to provide a solution for the power series coefficients of a typical mixer circuit configuration. In the analysis to determine the coefficients, the input voltage-output current loop equation is written for a crystal mixer equivalent circuit. This expression is expanded in a power series and then the reversion of a series technique is implemented to obtain the desired output-input expression in series form. The effects of the nonlinear element (diode) and the total circuit resistance are included in the results. Coefficient expressions are determined up to the ninth order in the manner described above and the level of each expression is computed versus the product α( IR + Iß) RT where α and IR are diode parameters, RT is the equivalent circuit resistance, and Iß, is the dc bias current. Also, the effect of diode bias upon spurious response sensitivity is considered and the results are plotted. The results show the mixer dc operating conditions which must exist in order to reduce spurious response sensitivity.