An Active FET Receiver Front End Mixer

In many recent receiver designs, the front-end mixer determines the receiver\u27s sensitivity and its susceptibility to distortion from large input signals. Two balanced FET mixers are developed for such an application; a common source configuration and a common gate configuration. Each exhibits a range of 110 dB between the available input power necessary for 10 dB((S+N))/N sensitivity and the available input power of a two-tone signal which causes intermodulation products down 40 dB from the desired signals. An expression for the conversion transconductance of a junction FET used as a mixer is first derived from the device transfer characteristic equation. The advantages of a balanced mixer con figuration are then discussed. A noise equivalent circuit which describes the sources of noise in each of the proposed mixers is developed. This leads to the calculation of noise factor and hence, sensitivity of the common source mixer as a function of RS, the generator resistance. The dependence of distortion on R for the common source mixer is then found and plotted. A comparison of this plot and the sensitivity vs. RS plot shows that the maximum dynamic range results from a choice of RS equal to 200 ohms. The dynamic range of the common gate mixer is investigated and found to be equal to that of the common source mixer. The sensitivity, however, is found to be 6 dB worse. The effects of frequency dependent terminations on the mixer ports are examined and found to be quite important to mixer performance. The bias problem is solved with provision made for the variation in parameters from device to device. The methods of testing the mixer are then discussed and test results are shown to agree excellently with the results predicted by the theoretical analysis. A brief description of FET operation and an observation on the calculation of noise factor are presented in the appendices