[TMC] GPR-90 RXD Tuning

[John Poulton]

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


On Thu, 13 Mar 2014, jvendely at cfl.rr.com wrote:

> Hi Steve,
>
> Back in those days, a high premium was placed on HF receiver sensitivity, so designers went for low noise figure.  This marketing-based obsession continues today in the ham radio market, to the detriment of some receivers.  Over most of the HF band, and particularly below 10 Mc or so, atmospheric noise is generally the limiting factor, so modern commercial and military receivers are usually optimized for dynamic range rather than noise figure.  So the old receivers usually had lower noise figures than the modern ones.  As such, the GPR-90 has plenty of "excess" sensitivity on 40M, so you might want to simply try adding attenuators ahead of the receiver to reduce the intermod.  6 to 10 dB of attenuation might do the trick, but you could probably add as much as 20 dB attenuation on 40M at night, if you have a reasonably efficient antenna, and still have adequate sensitivity, but drastically drop the intermod product levels.
>
> 73,
>
> John K9WT
>
>
> ---- wb9tow at egr.msu.edu wrote:
>> Excellent information on the TMC Receivers!
>> Just a present day observation, I use a GPR-90 with
>> GSB-1 on 40m CW about every day. The combination is
>> not too bad depending on the "time of day". I live about 1 1/2
>> miles from a 5KW MW BC station. Their station is off the air
>> at night, which helps reduce the IM....
>> I run a resonant antenna system and an ATU but still
>> find the IM rejection quite poor.
>> I've often wondered if there is a way to improve this design
>> without destroying the value of the rx, but I think the best
>> step is to acquire a GPR-90RXD or GPR-92....
>> 73
>> Steve w8tow
>>>
>
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