As I think more about it Geoff, I would need a pair of mixers in order
to get the tuning right-side up again. A first conversion to, say 45
MHz... and a second back to 28 MHz. Injection would be from a xtal osc
for the first conversion and a VCO running in the 46-73 MHz range. High
side injection on both would get the tuning back to right-side up. With
two mixers, a strong mixer amp would be required between stages. I have
a couple designs I have used with good success that are easy to
implement there.
I can see this will take some thought to get the right set of compromises.
Larry N8LP
Geoffrey Mackenzie-Kennedy wrote:
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
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