[TMC] GPR-90 RXD Tuning

[Richard Knoppow]

----- Original Message ----- 
From: "John Poulton" <jp at cs.unc.edu>
To: <jvendely at cfl.rr.com>
Cc: <wb9tow at egr.msu.edu>; "tmc" <tmc at mailman.qth.net>
Sent: Thursday, March 13, 2014 12:06 PM
Subject: Re: [TMC] GPR-90 RXD Tuning


> Just for the hell of it, I modeled the LC filters in the 
> front end of
> the GPR-90 using HSPICE.  The filter comprising 
> L28,L29,L30 and
> C115,C117 appears to be there to attenuate the B/C band. 
> It's -6dB
> point is at almost exactly 2Mc and it has reasonably flat 
> response
> in both magnitude and phase above 3Mc.
>
> The other filter, L32,L33,L34 and C138,C139,C140,C141 
> appears to be a
> high-pass filter with a cutoff at about 4Mc, but with 
> significantly
> rougher mag and phase response.
>
> On higher bands, the two filters are cascaded, and the 
> higher frequency
> filter (of course) determines the input response.
>
> For both, I loaded the output of the filter with 250 ohms, 
> roughly 1/Gm
> for a 6AB4.
>
> John, does that square with what you expected from this 
> filter?
>
> What does the circuit comprising L35, C143, and C142 
> do..??
>
> 73, John K4OZY
>
     I was looking at the schematic and parts list in the 
GPR-90 hand book from your site. This is the standard 
resolution one with the 1962 errata sheet in it.  Curiously, 
all the caps and coils in the RF are described as being part 
of the "low-pass filter".  Of course with series caps and 
parallel coils it can't be. I also have an earlier version 
of the IB with the exact same error in it.  Very odd lapse 
of technical writing.


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