Good Morning - You are correct, RCA made some mechanical filters before
Collins. But the Collins patent and improvement was not the filter but
the unique disk shape and material that made them self temperature
compensating so they were mil spec. It was the first that opened the door.
73 Bob W7AVK
[EMAIL PROTECTED] wrote:
In a message dated 8/13/06 7:56:46 AM Eastern Daylight Time,
[EMAIL PROTECTED] writes:
If you don't have it, I'll send you a PDF of the "Cost Reduction Study"
report Collins did that lead to the R390A.
I'd really like a copy - thanks! I think others would too.
The story goes like this:
Back during the Cold War, No Such Agency
....
It is my understanding that, long after the F9C system was in operation,
an
electronically steered RDF system using the receivers *was* developed!
All ancient history now.
I worked for the FCC as an engineer for several years after EE graduate
school and had an opportunity to visit a monitoring station. As I
recall, they used 51J4's for general listening and Racal 17 (US model
numbers RA117 and RA6117) for DF use with the Wullenwebber array.
I don't know if the R-390/R-725 were used with the Wullenwebber system or
something else. Either way, it makes sense that the phase characteristics of the
mechanical filters were a problem.
The
RA17 series receiver used LC filters for wider bandwidths and a crystal
lattice filter for the narrower bandwidths. The military used the same
Wullenwebber antenna system (elephant cages) and I had heard (but never
seen one in person) that they used R390's.
Or R-725s, which would look very similar.
Drake's first R4 used 50 KHz IF for selectivity but later receivers went
to crystal filters.
Actually the R4, R4A, and R4B all used LC filtering at 50 kHz for
selectivity
and passband tuning. Only the R4C used all-crystal filtering.
My first Drake was a R4C.
A family of HF ham receivers that bridged the change from low-frequency LC
selectivity to HF crystal filters. R-4B and the Drake 2-B/2-C family may have
been the last manufactured ham rxs to use LC IF filtering for multiple-bandwidth
selectivity.
Could it be that obtaining a suitable FM-bandwidth filter was easier/less
expensive (for Motorola, anyway) with LC circuits?
The design problem with FM filters is both bandwidth and ensuring group
delay specs are met, as otherwise audio distortion is an issue. My
junkbox also has a wideband 70 MHz center frequency IF filter from a
Western Electric analog microwave system. It has a bandwidth of 10 or 20
MHz and excellent group delay characteristics, which, of course, were
essential for a microwave system carrying hundreds of multiplexed analog
SSB channels.
By contrast, IIRC, RCA made some mechanical filters for receivers they
provided to the military in the 1950s. From what I read, they were just different
enough from Collins design that they avoided patent infringement. They were also
(allegedly) quite fragile.
> > Well, I graduated EE school in 1976, and by then programs like SPICE
were
part of the curriculum. Still batch processing, though, but the whole idea
was
neither new nor revolutionary then.
In my senior year (1968) the engineering department was bringing on line
a remote access time share system using IBM-selectwriters (IBM golf-ball
typewriter type terminals) but it was highly experimental and was down
more than it was up. My course in computer programming (Numerical
Methods) was oriented towards solving physics problems.
Perhaps my timeline was a bit too soon. By the time I was in EE school (fall
of 1972), SPICE and other circuit-simulation-by-computer stuff was common at
the school I went to. Perhaps it was a lot more recent than I realized at the
time.
73 de Jim, N2EY
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