Quote:
Originally Posted by turretslug
Thanks- there was an interesting and evolving post-war middle-ground in comms receivers between those that followed the very traditional and relatively straightforward route set by the trail-blazing HRO and cemented by the AR88 et al (e.g Murphy B40, Eddystone 680/730, GEC BRT400) and the sophisticated and very expensive 1MHz-per-span pace-setters like the Racal RA17 and Collins R390. Some of these middle-ground sets took advantage of dual-conversion to provide incremental tuning, and it looks as though the Airmec was a good example of this trend with a particularly wideband 800kHz IF.
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I think so. Also, meeting the GPO marine requirements probably pretty much meant using an “intermediate” layout during the valve era. The frequency coverage requirements essentially outruled single-conversion with a single IF, given that it would need to be placed somewhere south of 100 kHz. Single conversion with a dual-frequency IF strip, as in the IMR 54, was possible, but perhaps unwieldy in that all selectivity options had to be provided at both frequencies. Double conversion, with single conversion on some of the lower frequency bands seems to have been the favoured landing place. The front end selectivity requirements would have made upconversion difficult in respect of front end tuning tracking in the mechanical tuning era.
Even into the early solid-state era the need for unusual layout seemed to persist. The Eddystone EC958 was designed inter alia to meet the GPO requirements, and was variously triple-, double- and single-conversion; with low IFs (i.e. not upconversion) having been a deliberate choice. Normally the RF input was bandpass above 1.6 MHz, single-tuned below, but the marine version (EC958/5, aka Marconi Nebula) required bandpass down to 54 Hz to meet the GPO specifications. The Marconi Apollo (MWT N2050) marine receiver was double-and single-conversion with low IFs, even though Marconi had used upconversion for the preceding solid-state H2001 Hydrus compact ISB/SSB point-to-point receiver. So one assumes that the Apollo layout was a deliberate “horses-for-courses” choice, and not simply a carryover of valve-era practice. The Marconi N2020 not-quite-all-solid-state naval receiver of the same era was also triple or double conversion with low IFs.
Cheers,