Fall 2005 Issue

Real-life Dynamic Range
of Modern Amateur Transceivers

Reprinted from DUBUS magazine, this article deals with the correct way to measure transceiver quality.

By Leif Åsbrink, SM5BSZ
 

Following the recent general advances in receiver design, the receiver part of a typical amateur transceiver now has quite good ability to handle strong, unwanted signals—but only if those signals are free from unwanted spurious sidebands (notably keyclicks, splatter, and other transients). In contrast, the transmitters have been almost completely neglected. This article gives measured data for several different transceivers from different manufacturers, and it shows that the transmitters are becoming the most important source of inter-station interference. A major contribution to unwanted sidebands comes from ill-designed ALC circuits. The article also discusses what we can do to avoid generating interference to one another, by controlling the output power by means other than the internal ALC.

Introduction

Inter-station interference can occur when a receiver is trying to listen on a clear frequency, but there is a very strong transmitter using a frequency close by. All transmitters have unwanted sideband emissions (keyclicks, splatter, and other transients—the type depends on the transmission mode). If the suppression of these sidebands is worse than the dynamic range of the receiver, then the transmitter will be mostly responsible for the interference experienced by the receiver. Modern amateur receivers have quite good dynamic range, on the order of 90 to 100 dB with the usual definitions (500-Hz bandwidth, and at frequency separations beyond a few kHz). To avoid causing interference to such receivers, the unwanted sidebands from our transmitters must be suppressed to better than –120 dBc/Hz on the frequency to which the receiver is attempting to listen.
 

Figure 1. Simplified ALC schematic.

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