Intermodulation and Cross Modulation as a Function of Mixer Pinch-Off Viktage in an FET FM Receiver

The superheterodyne receiver was first described in 1919,1 and in a discussion of choice of intermediate frequency which acompanied that description the possibility of multiple responses was first suggested.2 Thus, the receiver spurious response has reached its golden anniversary, and like many an old timer it provokes disinterest if not outright boredom. In spite of the variety of transistors and other devices newly available to designers in the past decade, no documented improvement in receiver spurious response performance has been achieved. Apparently it is assumed that there is an adequate knowledge of the mechanisms of spurious responses, methods for the measurement of same and design techniques for their reduction. The results presented here indicate that much design improvement is possible, for it was found that the cross modulation performance of a production FM tuner could be improved by 50 dB simply by replacing the mixer transistor with another having different characteristics.