Intermodulation analysis of crystal mixer

An expression for the amplitude of the intermodulation products and harmonics produced in a crystal mixer is derived using the coefficients of the power series expansion of the device. Using this expression and an exponential approximation of the current through a diode [i = i0(ε^αv- 1)], the amplitude of intermodulation produced in a crystal mixer is found to be 2i0ε^αV0R0Is(αV1)Ib(αV2). In(x) is an nth order modified Bessel function of the first kind. The quantities s and b are the signal harmonic and the oscillator harmonic, R0is the output resistance, and V0, V1, and V2are the bias, signal, and oscillator voltages, respectively. The quantities i0, α, R0, and V2are found from the dc E-I diode characteristics, the mixer bias current, and the loss in the desired signal. Experimental tests on a mixer operating from 450 Mc to 850 Mc show that the signal input power necessary to produce a given intermodulation output power can be predicted within 6 db.