[TMC] GPR-90 RXD Tuning
Richard Knoppow
1oldlens1 at ix.netcom.com
Wed Mar 12 23:30:02 EDT 2014
----- Original Message -----
From: <jvendely at cfl.rr.com>
To: "tmc" <tmc at mailman.qth.net>; "Richard Knoppow"
<1oldlens1 at ix.netcom.com>
Sent: Wednesday, March 12, 2014 3:06 PM
Subject: Re: [TMC] GPR-90 RXD Tuning
It's interesting to compare the specified intermod
performance of the GPR-92, which has a tuned 1st RF stage
almost identical to the GPR-90RXD, and the GPR-92A, which
has the same grounded grid broadband RF stage as the GPR-90.
The GPR-92 intermod rejection is spec'd at 70 dB, whereas
the -92A is spec'd at just 35 dB. 35 dB intermod rejection
is surprisingly poor performance, and there seems no reason
to assume the GPR-90 would perform much differently than the
GPR-92A in this respect. Intermod performance is not even
spec'd for the GPR-90. I have a GPR-90RXD, and I must admit
it often has fairly severe intermod problems on 40M at
night. Of course, the broadcast stations are quite strong
here on the Atlantic coast of Florida.
But one can easily imagine the problems the big
point-to-point HF receiver sites might have had when these
receivers were used with large antenna systems. An
interesting example is the NASA HF network back in the days
of Project Mercury. Western Electric was awarded the
contract for the Mercury Ground Network and subcontracted
the HF portions of it to TMC, who supplied a number of
GPT-10K and 40K transmitters, and diversity receivers. The
receivers had a pair of GPR-90RXDs with a VMO, and two
SBC/AFC-2 ISB converters. Some receivers had the same setup
but with two GPR-92s and its accessory crystal oscillator
units instead of the GPR-90RXD. Someone who was "on the
scene" at the time told me that HF interference problems
occurred frequently, which were obviously receiver intermod
problems. A bad case of cross modulation occurred during
John Glenn's orbital flight, and Voice of America audio
could be heard. Official phone calls were quickly made
instructing VOA to cease transmitting on the problem
frequency, the only rapidly available solution.
NASA greatly expanded the network starting in 1965 to
support Project Gemini and the upcoming Project Apollo. New
remote-tuned TMC TSTE-10K transmitters were installed, and
the older receivers were replaced with the giant
remote-tuned DDR-506, which had much better front ends.
These systems were used at all ground stations and on board
the Apollo Tracking Ships until well into the 1980s.
It's remarkable that the GPR-90RXD and GPR-92, with their
obvious ham radio pedigree, were used as extensively as they
were in large government and military systems. Overall,
they apparently provided acceptable service.
73,
John K9WT
Thanks, John. It woud be interesting to see the IM
performance for the RCA AR-88 and CR-88 receivers.
Especially to see if the modifications to the AVC in the F
or diversity version made IM worse. I have an F and changed
the AVC back to the circuit used in the A and D versions.
Only a slight mod but the receiver seems to perform better.
I live about a mile from a MW transmitting site with
three local broadcast stations on it. One is a 50KW flame
thrower, one a 5K and one a 1K, all diplexed onto the same
sticks. The stations do not affect the AR-88 on HF but it
overloads if tuned to them with any sort of antenna. OTOH,
it will work find on the broadcast band with about a foot of
wire. I also have an R-388 which is just overwhelmed by
these stations with more than a foot or two of wire. The
50KW at 1580 blows through on all the odd bands above about
10 mhz. Not quite sure of where it starts. The 5K station
is the most recent. Moved to this site because their old one
got to be worth more as real estate than the station is
worth.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com
---- Richard Knoppow <1oldlens1 at ix.netcom.com> wrote:
>
> ----- Original Message -----
> From: <jvendely at cfl.rr.com>
> To: "tmc" <tmc at mailman.qth.net>; "Richard Knoppow"
> <1oldlens1 at ix.netcom.com>
> Sent: Wednesday, March 12, 2014 6:47 AM
> Subject: Re: [TMC] GPR-90 RXD Tuning
>
>
> > Richard,
> >
> > There is a 5 pole highpass filter at the input of the
> > GPR-90's grounded grid 1st RF stage, which I assume cuts
> > off just above the standard broadcast band, since this
> > stage is not operational in that range. Eliminating
> > strong BCB interferers probably improved HF performance
> > significantly. The GPR-90 front end has the advantage
> > of
> > providing a well controlled input impedance (75 or 300
> > ohms)over a wide frequency range. The input impedance
> > of
> > most HF receivers of the day varied all over the map,
> > even
> > at the tuned RF stage center freq. Return loss "off
> > channel" was usually very poor, though this really was
> > not
> > a problem in most applications. On the broadcast band,
> > the
> > tuned 2nd RF stage appears arranged to provide a
> > relatively high impedance antenna input which varies
> > considerably with frequency. This allows good
> > sensitivity
> > with short random wire antennas.
> >
> > Of course, the drawback is that the GPR-90 1st RF stage
> > gets bombarded by everything coming in the antenna,
> > hence
> > the receiver's reputation for intermod problems. As
> > broadband tube front ends go, it's actually pretty
> > linear,
> > though not nearly good enough for demanding
> > environments.
> > I suspect the main reason for the broadband RF stage was
> > cost reduction, as it eliminated an extra gang on the
> > tuning capacitor and a bank of bandswitched coils. The
> > combination of the 1st and 2nd RF stage plate tuning was
> > enough as-is to provide reasonably good image rejection.
> > But in the process, they traded off intermod
> > performance.
> >
> > I don't recall the original selling price, but it was at
> > the high end of the range for the amateur market.
> >
> > 73,
> >
> > John K9WT
>
> That pretty much confirms what I thought. I will have
> to draw out the filter and see if I can calculate what it
> does. The second tuned RF in the RXD would also reduce LO
> radiation, which might have been necessary for military or
> commercial multi-receiver applications.
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com
>
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