Design techniques and performance of digital IFM

The digital instantaneous-frequency-measuring receiver using delay-line discriminators was developed in the early 1960s specifically as a wideband and accurate analyser of pulsed radar signals. Various receiver configurations are incorporated in most of the more comprehensive EW systems currently in use. In the paper, the basic operating principles and design philosophy are reviewed, together with an indication of the performance presently achieveable in the field using modern MIC technology and digital signal-processing techniques. Component and operational factors affecting accuracy, sensitivity, dynamic range and band coverage are discussed. The instantaneous-frequency-following capability is assessed, including means of extracting or displaying information from frequency- or phase-modulation-on-pulse-type signals. Problems arising when multiple signals, pulse or CW, are present simultaneously can be overcome to a certain extent by using the large-signal capture effect in limiting RF preamplifiers. The degree of measurement integrity under these conditions is related to receiver design factors, and processing methods are described which can be used to identify ambiguous measurements when these occur. Finally, some practical implementations are given of digital instantaneous-frequency measurement in ESM and ECM equipments.