Yet it was at that exact time that amateur designs were moving away from LC
filtering (such as the 50-60 kc. second IFs of receivers like the Hallicrafters
SX-88/S-76/SX-96/SX-100/SX-101) to HF crystal filters. The move to
transceivers and matched receiver/transmitter pairs (Heath SB line) may have had an
effect, too.
It's interesting that Collins added the mechanical filter to the R390
(thus producing the R390A) as part of a 1955 cost reduction study. The
study report is floating around the Internet and I took a look at it
again tonight to refresh my recollection. The mechanical filter
substitution is discussed almost completely in terms of improved skirt
selectivity with little mention of cost savings. It's also interesting
to note that the mechanical filters made the receiver unusable for
direction finding as there was too much phase shift change with
frequency when compared with the R390's tuned transformer design.
Drake's first R4 used 50 KHz IF for selectivity but later receivers went
to crystal filters.
I imagine the move to crystal filters was driven by the desire to reduce
receiver size and to remove the need for skilled technicians to align
the IF stages. In the VHF and UHF world, Motorola stuck with potted LC
IF filters at 455 KHz (the famous "Permakay") well past the 1970's,
although GE went to crystal filters with the Master Pro (and maybe
before that; I'm a bit hazy on GE's gear between the ProgLine and
MasterPro.)
I'm not sure of the exact dates when it was first done, but by the mid 1960s
the use of computers to do circuit simulation and calculation was mainstream
in electronic design. Such design tools probably had an effect in that many
"paper designs" could be tried out in a short time, particularly for things like
filters..
I received my EE undergrad degree in 1968 and I recall using a
transmission line simulation program. Batch mode, submit your card deck
and get the answer back the next day. Zverev's classic book on filter
design was published in 1967 and its data tables are derived from
computer programs, but the book itself is silent on using computers to
design filters.
Agreed. The main filter in the K2 uses selected microprocessor crystals and
some varicaps, yet gives very good CW performance and multiple bandwidths. By
comparison, one can easily spend half the cost of a basic K2 (or more) on a
couple of packaged CW filters for an HF IF. For VHF? I don't want to go there.
I've been quite pleased with the filters as narrow as 200 Hz that I've
built using 8 MHz micro processor crystals. I have to grade them for
minimum Q and frequency matching, but the resulting filters are textbook
matches to the Gaussian prototype designs.
One of the two articles I've submitted to the ARRL for consideration
for QEX covers some of my crystal filter work. Have yet to hear if it's
accepted, however.
Jack
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