[Elecraft] Down-conversion
Geoffrey Mackenzie-Kennedy
gm4esd at btinternet.com
Thu Nov 18 07:49:05 EST 2010
Bruce,
I believe that your suspicion is correct. The sticking point in the design
of a high performance up-conversion receiver at this time is the first LO,
whose phase noise must be suitably "low" and whose cost is acceptable. There
can be problems with VHF "Roofing" filters, but there are ways to overcome
these.
Regarding the use of very narrow roofing filters (crystal) in a
down-conversion receiver, there is an underlying filter generated IMD
problem which might or might not affect the IMD performance of the overall
receiver - depending upon the "IMD performance" of those parts of the
receiver ahead of and behind the filter. The problem is that for any given
quality of quartz used in the crystals, the IMD performance of a crystal
filter can be shown to worsen as the filter's bandwidth narrows. I suspect
that the 6 kHz roofing filter used in Kenwood's TS-590S ahead of the IF
crystal filters is there to give some protection to the narrower IF filters.
73,
Geoff
GM4ESD
Bruce McLaughlin wrote on Wednesday, November 17, 2010, at 11:02 PM:
>I suspect it is going to become the standard for many if not most of the
>new
> radios. As soon as it became apparent that effective roofing filters can
> really improve the close in IMD performance, it seems as if a low first IF
> frequency has become almost mandatory. I note that the Orion and the new
> Yaesu FT 5000 both use down conversion for the first IF and, therefore,
> have
> effective narrow roofing filters. I wonder when, or if, ICOM and the
> others
> will follow suit.
<snip>
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