The noise characteristics of radar receivers

摘要

The various quantities which determine the performance of radar receivers are discussed. Most of the effort towards improvement has been concentrated on securing the lowest possible noise-factor in amplifiers and mixers and this is the main theme of the paper. The theoretical design of amplifiers for minimum noise factor is discussed and it is concluded that simple equivalent circuits such as those used by Herold and Makers are of value subject to certain limitations, more exact treatments being often difficult to apply to practical cases. The relation between band-width and noise factor is discussed with the conclusion that in general these quantities are independent up to about 4 Mc/s band-width. Important developments in amplifiers during the last few years have included the VR136 pentode in which screen current is reduced by alignment of grids, the grounded-grid triode, and the recent neutralized triode circuit. These are reviewed in the light of theory and practical experience; it is concluded that the two best methods of obtaining low noise factor are by using either a low intermediate frequency or a neutralized triode. The grounded-grid triode is at present the best method of r.f. amplification in use for the region of 90 to 600 Mc/s. In the case of a low intermediate frequency, local oscillator noise may be important and methods of reducing it are summarized and discussed. In regard to crystal mixers it is concluded that realization of the best possible performance requires an overall performance measurement, or its equivalent, to be included in the factory selection of crystal valves. Methods which make use only of low-frequency or high-level measurements and which take no account of noise can leave a spread of several decibels in the performance of crystals which are passed as equally good. It is shown that an improvement of at least 2 db in average performance over prevailing standards should be realizable.