[Synchrodyne]

Certainly that is a pair of interesting and thought-provoking questions.

“Reciprocal mixing” I imagine reared its head once synthesizers came into use for professional HF receivers, early versions suffering from relatively high phase noise as compared to their conventional oscillator predecessors, and also as compared with the Wadley loop. Given that at the same time, receiver performance was improving, or at least improvements were being sought in other directions, particularly dynamic range, the phase noise problem arrived at an inconvenient time, and could have been a significant limiting factor to realizing the desired overall improvements.

Based upon limited data on hand, nothing was said about phase noise or reciprocal mixing in respect of the Plessey PR155, GEC RC410, Marconi Hydrus or Redifon R550/551, all early examples with partial or full frequency synthesis. Racal may have been mute on the issue until the release of the RA1772, whose good reciprocal mixing performance was strongly featured. But then Racal wouldn’t have wanted to get into a dynamic range debate involving its RA17 and particularly its RA217 models, although it probably could have claimed low phase noise.

So maybe Sosin and his team were the first to “flesh out” the reciprocal mixing concept, and perhaps they even coined the term. Its lack of mention in respect of the Hydrus suggests that it came out of the H2900 development programme.

“Roofing filters” in a functional sense have been a round for a long time. I had a vague notion that the actual term came into use when double conversion, with say 10.7 MHz 1st IF and 455 kHz 2nd IF came into more widespread use for consumer-type HF receivers, and a standard block filter, crystal or ceramic, was used at 10.7 MHz. Possibly though it traces back to dual-conversion VHF RT receivers of the kind where the main selectivity was done at 455 kHz, with a wider filter at 10.7 MHz.

In the HF realm, the examples preciously quoted, Plessey PR155, GEC RC410, and Redifon R550/R551 all had what were roofing filters in their respective 1st IF sections, but I don’t think that any of these makers used the term “roofing filter”. So perhaps Sosin’s team also coined this term.

So, the net answer is that I am not aware of any earlier use than by Sosin of the “reciprocal mixing” and “roofing filter” terms; it does not turn up in a relatively superficial search amongst some of the usual suspects.


Cheers,