Inband IMD cannot occur here.įirst, the author performed measurements on an older, homebrew "Standard" HF double superhet, with a 40 MHz 1st IF with 12 kHz bandwidth, and a 2nd IF with switchable KVG 9 MHz filters for FM (12 kHz), AM (6 kHz) and SSB (2.4 kHz).Īt offsets ranging from 100 kHz down to 20 kHz, IP 3 is +28 dBm on all bands (Fig. 2: Block diagram of a single-conversion superhet with complete selectivity filtering in a single IF. If this mixer has not been designed to the same IMD specifications as the first mixer, the receiver's strong-signal handling will inevitably be degraded (see Fig. Unfortunately, it does - especially in receivers employing multiple conversions (as almost all receivers do.) When the spacing between the received signals is less than the bandwidth of the crystal filter immediately following the first mixer - also termed the "roofing filter" - the IMD immunity falls off sharply in most cases, as now the second mixer is responsible for the IMD immunity of the receiver as a whole. Thus the question arises as to whether the offset has any influence on the receiver's IMD immunity. The results are usually stated in product data sheets. IMD dynamic range measurements are usually performed with two unmodulated RF signals, offset from one another by 20 to 100 kHz. With such receivers, optimised for large-signal handling and dynamic range, IMD interference should be a thing of the past - even when a highly-efficient antenna is connected.
Modern HF receivers have a 3rd-order intercept point (IP 3) of +40 dBm (10W), and are thus practically immune from interference by strong adjacent-channel signals. The author explains the background, and provides examples. In order to determine this parameter correctly via measurements, one should observe some basic rules and have suitable test equipment at one's disposal. Werner Schnorrenberg, DC4KU:Born 1951, radio amateur since 1973.Telecommunications engineering degree from Cologne worked many years at Tektronix GmbH, Cologne, and Rohde & Schwarz,Munich.Other hobbies: Windsurfing, classic cars, PC's.Īddress: Habichtweg 30,51429 third-order intercept point significantly defines the characteristics of a receiver. Inband IMD Immunity Testing by Werner Schnorrenberg DC4KU Translated from the original German by A.
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