Hi Larry, Defender of the Multiple Conversion Faith charging in !
> In the meantime, here's a concept I have been toying with building. The > R-388, like most Collins gear, uses front-end converters to shift a > whole band segment so that the rest of the receiver becomes a tuneable > IF. This brings back some of the problems associated with multiple > conversion receivers that the K2 beats the pants off of... Glad that you said "associated" and not "occur"! The high IF receiver runs circles around the low IF receiver in spurious response / internal birdie performance IF the IFs are chosen properly and IF High Side mixing is used, all assuming that the right devices / circuits are used. The choice of IF and High Side mixing and devices /circuits applies to low IF receivers, but in their case there is a much smaller choice of IFs, even just for HF ham band coverage,and they must have good front end filters and internal shielding if one wants a "clean" receiver. I would suggest that the problem with "commerecial" High IF double conversion receivers for Amateur use, which has created the myth, has three parts 1) The technology used is not up to date in this fast moving field - it cannot be. 2) The production roofing filters are wide as barn doors, and the "retrofits" are not much better if you look down their skirts and their distortion contribution, who knows 3) Usually the choice of IFs appears to be driven rather than chosen. The main ham band only receiver here, homegrown, is double conversion first to low VHF where the roofing filter bandwidth can be selected - 1.5 kHz narrow.It is a "strong" receiver without any internal spurii above noise floor. Took much number crunching, but that occupied waiting time at airports. I think that tuneable IFs are attractive for general purpose use if really good intermod performance is not sought , although it can be if one stays away from diode ring mixers and uses modernish "strong" mixers and amplifiers. Also opens the option for low noise crystal oscillators for the first LO in those bands where one needs low phase noise from the LO. Collins also had an an eye on frequency readout, and further opted for the low values of IF / Signal Freq ratios, perhaps for VFO stability reasons, but because of the strong spurious responses on the high side, 2Fo - 2Fa = IF etc (where LO was high side), they needed good front end filters. So it might be worthwhile looking at say 24 or 28MHz as an IF. But beware of the N times signal frequency = IF monkeys, and friends. Good luck with whatever you do. 73, Geoff GM4ESD _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
