RF Intermodulation Characteristics of VHF Telemetry Preamplifiers and Receivers

Abstract

Tests have been conducted on ten preamps and six receivers to quantitatively determine their linear operating range. The range of linear operation was determined by the measurement of, or the effects of, intermodulation (IM) products that were produced by the simultaneous application of two input signals. Measurements were made at a frequency equal to twice the frequency of one signal source minus the frequency of the other signal source. Tests of the ten preamps, which included eight solid state units, indicated that their dynamic range varies from approximately 46 to 70 db. Since the IM products of interest vary as the cube of the input signal, the output IM products may vary over a 72 db range, between the preamps tested, for a given input condition. It was found, under the best conditions with some receivers, that their proper operation was prevented by the presence of two signals that were less than 30 db larger than the desired signal. The two interfering signals were located well outside the rated - 60 db bandwidth of the receiver. The results of the tests should provide receiving system designers and operators with a better insight into the problem of RF intermodulation interference. Since no known military interference specifications consider intermodulation as investigated, it is believed that some standard test procedures should be developed by the telemetering community for preamp and receiver testing.