[TMC] GPR-90 RXD Tuning

[Richard Knoppow]

----- 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