[TMC] GPR-90 RXD Tuning

[Richard Knoppow]

----- Original Message ----- 
From: <WQ9E at btsnetworks.net>
To: <jvendely at cfl.rr.com>; <tmc at mailman.qth.net>; 
<w2hx at w2hx.com>
Sent: Tuesday, March 18, 2014 2:05 PM
Subject: Re: [TMC] GPR-90 RXD Tuning


> Or the approach Hallicrafters used for the SX-88 was to 
> use a first IF of 2.075 Mhz. for all except the second 
> range where 1.550 Mhz. was used to avoid having the IF in 
> the tuning range.  Later members of this 50 Khz. final IF 
> receiver settled on 1.650 Mhz. for the first IF and had a 
> coverage gap here.
>
> The Hammarlund PRO-310 operates as a dual conversion 
> receiver with a first IF of 1802 Khz. and a second IF of 
> 52 Khz. except for the first two bands covering .55 to 2.2 
> Mhz. where it is single conversion using the 52 Khz. IF 
> and this lowers its rated image rejection from -70db on 
> the middle bands down to -50db for the lower two bands.
>
> Although the TMC GPR-92 conversion scheme is pretty 
> standard with 3.955 and .455 Mhz. IF frequencies it has a 
> rather strange IF passband probably because it was able to 
> be used with a multiplex adapter providing 4 voice or 64 
> RTTY channels.  It provides 6 bandwidth choices from .5 to 
> 15 Khz. @ -6db but at -60db the .5Khz. selection is 23 
> Khz. wide and the remaining choices are 25 to 28 Khz. 
> wide.  Corresponding figures for the GPR-90 range from 9 
> to 23 Khz. wide. Even though the GPR-92 has a built in 
> product detector I have a second GSB-1 that stays attached 
> to the GPR-92.
>
> Rodger WQ9E
>


      The standard GPR-90 and RXD seem to have used three 
conventional IF stages with a 6db down bandwidth of about 
6khz but they had an alternative IF with much wider 
bandwidth for use with the receiving adaptors for 
multiplexed signals. Very wide IF bandwidth was available on 
a few other receivers. The RCA-AR-88 has about a 15 khz 
bandwidth but the F version also has an IF output after the 
second IF stage. I have never measured mine but the tuning 
for the IF is such that its probably around 16 khz.  The old 
Super-Pro with variable coupling IF would also go to 16 khz. 
I think getting more than this with a 455khz IF might be 
difficult although I've seen designs for high fidelity AM 
receivers with about 20 khz bandwidth. They are very 
elaborate and I have no idea how practical they were.
     TMC seems to have used the same frequencies as in the 
Hammarlund SP-600. Perhaps they needed to meet specs written 
for it or, perhap these were just good fequencies to use for 
the amount of image rejection desired. It is tempting to 
compare the GPR-90 with the SP-600 because both were used 
for similar applications but the SP-600 is a much more 
elaborate receiver and its original price was double that of 
the GPR-90. Its too bad we don't know more about the 
internal history of both receivers.


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