5.2 - Infrared 10.6-micron heterodyne detection with gigahertz IF capability

Analyses and experiments have been carried out on heterodyne detection at 10.6 microns with the objective of obtaining IF bandwidth capability into the microwave region. UHF and microwave measurements on the quantum-noise-limited generation-recombination (G-R) noise spectrum of compensated copper-doped germanium photoconductive mixer elements, measured under operational conditions at 10.6 microns, have shown response to beyond 2 GHz. The experiments were carried out directly at 10.6 microns using a mixer geometry and circuit arrangement intended to yield large mixer conversion gain and IF bandwidth. Engineering design equations are given for noise equivalent power (NEP) and mixer conversion gain ( ) in terms of such parameters as IF noise factor, carrier transit time, carrier lifetime, mixer resistance, local oscillator power, dc bias power, etc. An expression for quantum-noise factor (QF) is defined. Graphs are also presented showing the effect on NEP, , and QF of various parameters, and the tradeoffs possible to achieve high-frequency IF capability. An alternative approach is presented in which mixer conversion gain is calculated directly from the mixer characteristic in a manner analogous to microwave mixers.