A crystal mixer for c.w. radars

A crystal valve which is passing current generates both shot and flicker noise, the spectral density of the latter increasing approximately inversely with frequency. Pulse radars are not affected by flicker noise lower in frequency than the pass-band of the i.f. amplifier and are thus subject to only a relatively small degradation in sensitivity. With Doppler radars, however, noise at audio frquencies is most important, and as the flicker contribution can be extremely large here, serious curtailment of system sensitivity may result. An investigation into the mechanism whereby crystal noise reaches the receiver output is described, as is also a study of the relationship between the magnitude of flicker noise and various crystal parameters. This led to attempts to reduce the flicker noise with existing types of crystal contacts and eventually to the development of a new silicon mixer suitable for frequencies up to X-band. In addition to improved flicker-noise properties, this crystal has excellent mixing performance over a wide range of local-oscillator drive power and is available in both polarities.