The Type 1232-PI RF Mixer is a sensitive crystal mixer intended primarily as an accessory to the Type 1232-A Tuned Amplifier and Null Detector. As such, it extends the frequency range of the detector to 10 Me,* thus permitting its use with General Radio rf impedance bridges. In this application, separate rf and local-oscillator sources are required. The instrument combination is also useful as a wide-range heterodyne receiver with excellent harmonic rejection. When the frequency of the local oscillator is swept, the combination can be used as a spectrum analyzer,with an oscilloscope display. It can also be used as a sensitive level indicator in other rf measurements, including those of attenuation, leakage, and filter stopband response.