Comparative adjacent-channel selectivity performance of phase-locked pilot-tone SSB mobile radio receivers with particular reference to the long-, short- and split-loop configurations

There has been considerable interest, both in the USA and in the UK, in the use of 5 kHz pilot-tone single-sideband modulation for the VHF and UHF civil land mobile radio service. However, if single sideband (SSB) is going to be a commercially viable proposition, it will be necessary to use some form of automatic frequency control to reduce the specification and hence the cost of reference oscillators. The use of such techniques has the added advantage that system frequency drifts will be tracked out. To date, `long-loop¿¿ phase-locked receiver configurations have been employed to achieve this aim. Unfortunately, this long-loop technique has several drawbacks, including a possible large degradation in adjacent-channel immunity. This problem was alluded to in a recent publication by McGeehan and Lymer, and these authors have indicated several ways of reducing the problem. In the paper, a technique is described which overcomes many of the drawbacks associated with long-loop receivers, notably the problem of poor adjacent-channel performance. To this end, the results of laboratory tests comparing the adjacent-channel performance of conventional long-loop, short-loop (equivalent to a fixed oscillator receiver) and the new split-loop technique are presented, together with an analysis of the receiver mechanisms which limit adjacent-channel selectivity. It is shown that satisfactory control of the first local oscillator may be obtained with the split-loop configuration.