In a message dated 8/12/06 8:03:20 AM Eastern Daylight Time,
[EMAIL PROTECTED] writes:
> Hence, the resonator Q requirements for a 5 KHz filter at 40 MHz are not
> grossly different than for a 500 Hz filter at 4 MHz, which is quite
> achievable. But for a 400 Hz filter at 45 MHz, the required resonator
> Q's get into the million range.
>
Thanks, Jack - that's the answer I was looking for.
Most of us QRP/Elecraft types are going to want a rig with really good CW
selectivity. That means 500 Hz is the *wide* filter! Which means a
single-conversion VHF-IF rx isn't practical - yet.
Of course the situation may change at some point. For more than 20 years,
(1930s to mid 1950s?) "crystal filters" were pretty much limited to below about
500 kHz or so. Then in the late 1950s, high performance lattice filters for
the HF region appeared, and began to show up in amateur radio designs. ("What's
Wrong With Our Present Receivers?" QST, about 1957).
Those early HF xtal filters were rather expensive (the Hycon Eastern filters
in the above article were $44 each, which was a big pile of money in 1957!)
But of course prices (adjusted for inflation) came down over time.
73 de Jim, N2EY
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