[TMC] GPR-90 RXD Tuning

[John Poulton]

I was a bit more curious about the two high-pass filters (it IS odd that
TMC got the sense of the filter wrong in the writeup.. :), so I visited

http://www-users.cs.york.ac.uk/~fisher/lcfilter/

a site that allows one to design various kinds of filters.  I set the thing
up for Butterworth, 5-pole, shunt input, with 2Mc cutoff, 300-ohm
input/output impedance, and synthesized a filter very similar to the
BC-band filter that's in the receiver.  The values calculated by the
program are a bit different, but close.

The other (4Mc) filter is a different story.  This is a series-element
input/output filter of 7th order.  The on-line program produces values that
are fairly different from those used by TMC, the main difference being that
the center inductor, which is quite a bit smaller than the outer pair of
inductors.

Given the relatively rough phase response of the simulation I ran for the
existing filter in the radio, I wonder if it might be possible to improve
on this filter, and whether it could actually improve the receiver's
performance.

If I'm reading the schematic correctly, the 4Mc filter is only used on the
top band, which most of us probably use rarely, if ever..

73, John K4OZY


On Sat, Mar 15, 2014 at 1:08 AM, Richard Knoppow <1oldlens1 at ix.netcom.com>wrote:

>
> ----- 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
>