A new optical receiving system is described, which can improve the SNR when a very wide range of high-frequency signals is to be detected. It uses an avalanche photodiode both as an optical detector and as an electronic mixer. A theory is presented whereby mixer conversion losses and noise performance can be calculated. The results of a number of calculations for typical diodes are presented. Measurements of conversion losses and SNR have been made at 70 MHz. The results are in broad agreement with theory. The advantages of the system are shown to lie in that frequency down conversion in the diode permits more advantageous matching to the following stage, and that the optimum value of current gain required is much reduced. A simple numerical example is quoted that illustrates that an SNR improvement of 13 dB is obtainable in a particular case.
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