It wasn't just crystal filters that improved. The older methods of
computing component values (Image Parameter) was replaced by modern
network theory in the 1950's and '60's and LC filters became a lot
better performing as a result.
I don't see the widespread cheap availability of crystals with Q > 1
million as likely to emerge without a major demand and some considerable
advances in the materials available, as the present devices seem to do
the job required.
There's an interesting article "The Design and Performance of
Ultraprecise 2.5-mc Quartz Crystal Units" by A. W. Warner from the
September 1960 B.S.T.J. discussing crystals with a Q of 5 to 6 million.
You can read it at http://www.ieee-uffc.org/freqcontrol/warner.pdf
Now, if we could obtain a few dozen of those at 45 MHz, we would have
something indeed.
Jack
[EMAIL PROTECTED] wrote:
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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