[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
>
    Very interesting. The second bank of filters a bit below 
the second IF frequency. Perhaps to act as a sort of roofing 
filter to keep signals below the second LO from getting into 
the mixer.


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