Bob W7AVK wrote:
Jack - Have been reading the mail and enjoyed the discussion. Might
also point out a reason for the mechanical filter in the 390A was
Collins had the patent and Art was pushing their use. I worked for
Art a few years later and he was still activity pushing the filters
for use in a data modems and other application. It can be said work
we did at that time [early 1960s] became the basis for the Rockwell
Collins modem chip that was an industry standard in the 80-90s.
I'm shocked to think that Collins would suggest something like that ...
pushing the government to use their proprietary technology to increase
profits to Collins ... who would ever think that could happen.
Also you might check the things about the phase shift across the
mechanical filters pass band. I'm sure its bad, but its been
questioned that say that was only a CIA [remember them] cover story
for a very black commo system of the day, parts of which are only now
being discussed on the net.
I have a genuine Collins manufactured R390A here, and the equipment to
do group delay and phase shift testing, but I don't like the though of
moving the receiver and steel cabinet from the second floor to the
basement shop. Ugh. Too much like work. I need to do it anyway, as the
receiver has been gradually developing a problem that causes the carrier
strength meter (my receiver has the original "glow in the dark" meters)
to show nearly full scale regardless of AGC setting or signal strength.
Probably a leaky cap in the AGC bus, but working on that receiver is not
my favorite activity.
I seem to recall reading that there might have been some early CDMA
systems on HF that were used by the CIA and that would require a wide
bandwidth receiver. However, I remember that as being well over 20 KHz
wide and maybe closer to 200 KHz. The idea is that they were low
probability of intercept signals as well as having spreading gain.
Jack
Not sure, just my $0.02. But maybe do some enjoyable surfing.
73 Bob W7AVK
Jack Smith wrote:
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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