[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: Tuesday, March 11, 2014 8:09 AM
Subject: Re: [TMC] GPR-90 RXD Tuning


>
> Richard,
>
> The GPR-90 was noted for front-end overload and intermod 
> problems, as you pointed out.  I suspect the RXD version 
> was intended for use in larger, point to point stations 
> with big antennas (e.g. rhombics)for which the GPR-90 
> would have been poorly suited.  The tuned 1st RF stage of 
> the RXD was likely added to improve intermod and image 
> rejection.  I would guess they used a manually adjusted 
> 1st RF stage tuning capacitor to allow use of the same 
> ganged tuning capacitor as originally used in the GPR-90.
>
> It would be interesting to compare measured intermod and 
> image rejection performance of the two versions of the 
> receiver.  JP and I had discussed this, but neither of us 
> has gotten around to actually doing it.
>
> Regarding manually-tuned receiver preselectors, an example 
> of a military receiver using this method was the early 
> 1960s vintage AN/FRR-59 designed by The National Radio 
> Company.  No doubt many receiver designers (and especially 
> users) considered this arrangement inconvenient and a 
> detriment to "browsing", but it did have the advantage of 
> reducing front end tracking errors, and was cheaper.
>
> 73,
>
> John K9WT
>
     Thank you, this pretty much confirms what I guessed.  I 
suspect adding an additional section to the tuning cap would 
have required a very major re-design of the whole chassis. 
For essentially fixed frequency use the manual tuning would 
not have been any problem.   The RXD is obviously meant to 
use in diversity systems.   I have seen only one measurement 
on the web of intermod and that was of a receiver with 
unknown alignment etc.   TMC does give some image rejection 
numbers but not anything very detailed.   The image 
rejection of the GPR-90 is not bad.   This receiver looks so 
good one wants it to perform well too.



--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com