[TMC] GPR-90 RXD Tuning

[WQ9E at btsnetworks.net]

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

-------- Original Message --------
> From: jvendely at cfl.rr.com
> Sent: Tuesday, March 18, 2014 2:48 PM
> To: "tmc at mailman.qth.net" <tmc at mailman.qth.net>, W2HX <w2hx at w2hx.com>
> Subject: Re: [TMC] GPR-90 RXD Tuning
> 
> Hi Eugene,
> 
> The early 1960s TMC DDR-5 series, a double conversion receiver with 1st if at 1750 kc, and 2nd IF at 250 kc was an example of a toob receiver without in-band IF.  The contemporaneous National AN/FRR-59 was another example, which had 1750 kc and 80 kc IFs. Of course, both were very large, very high-priced, state of the art communications receivers.  The DDR-5 had a very elaborate 4-stage tuned RF amplifier to obtain its minimum 80 dB image rejection.  Its tuner with 1st IF alone was larger than a GPR-90.  And both these receivers only tuned down to 2 MHz.  This would not have been a suitable choice of 1st IF for a for a 1955 vintage, median-market, general coverage receiver of modest performance such as the GPR-90, which had to tune the standard broadcast band and up through the MF and HF range with no coverage gaps.  
> 
> The compromise was therefore to stick with the traditional 455 kc IF for the lower bands, and add a conversion stage ahead of this for the higher bands.  This was necessary because with a 455 kc IF, its relatively low-cost front end could not provide sufficient selectivity to obtain an acceptable image ratio much above 5 Mc.  The crystal controlled first conversion on the upper bands remedied this without adding excessive cost, but this inevitably required the added IF be in-band.  In summary, like so many engineering problems, it was a cost/performance compromise. 
> 
> Ultimately, HF receivers went to the upconvert/downconvert architecture with no in-band IFs, but this had to await the development of low noise VHF frequency synthesis and stable, high quality VHF crystal filters, which first became practical about 1970.  
> 
> 73, 
> 
> John K9WT 
>