[TMC] GPR-90 RXD Tuning

[jvendely at cfl.rr.com]

Hi John,

Yep, that all makes good sense.  A BC highpass filter would be a necessity to prevent serious overload of the broadband RF stage.  The other HPF provides 3955 kc 1st IF rejection on the upper bands where double conversion was used.  Without that filter, antenna signals around 3955 kc would feed through the RF stage directly into the first IF.  L135/C142/C143 is evidently another switched filter section, but I'm not sure of its purpose.  Will have to look at it in more detail.  Perhaps someone else will know offhand...

73,

John K9WT  


---- John Poulton <jp at cs.unc.edu> wrote: 
> 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
> 
>