Noise in transistor mixers at low frequencies

The paper deals with the noise performance of transistor mixers at frequencies where reactive effects may be neglected (generally less than 0.1fT). The analysis involves the consideration of noise processes in a time-varying circuit, and the problem is treated by returning to the basic physical equations describing the transistor. The problem of noise in the transistor-base current is discussed in detail. The analysis allows a simple prediction of transistor-mixer noise figure at low frequencies, and the effects of variation in local-oscillator drive, source resistance, transistor parameters and operating current are readily predicted. The emphasis is on an engineering approach to the problem, and the results are presented in a form intended to facilitate the design of low-noise mixers.