[TMC] GPR-90 RXD Tuning

jvendely at cfl.rr.com jvendely at cfl.rr.com
Tue Mar 18 15:48:15 EDT 2014 

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 

---- W2HX <w2hx at w2hx.com> wrote: 
> Just to clarify my comments. The question really was why TMC didn't design the receiver so the IF would not be in the middle of its RF receiving range. Re-reading my comments, it seemed like I was asking why we don't move it. Obviously that would not be possible. So my question is really one of the TMC design.
> 
> Thanks
> Eugene
> 
> 
> -----Original Message-----
> From: tmc-bounces at mailman.qth.net [mailto:tmc-bounces at mailman.qth.net] On Behalf Of W2HX
> Sent: Sunday, March 16, 2014 7:01 PM
> To: John Vendely; tmc at mailman.qth.net
> Subject: Re: [TMC] GPR-90 RXD Tuning
> 
> Forgive my ignorance on this subject, but instead of having to deal with the IF frequency in the RF section, why not move the IF above 30 MHz so as not to have to worry about it leaking through to the IF? I am sure there are million reasons, but I am a novice when it comes to receiver design hihi
> 
> Eagerly awaiting some schooling on the subject
> 73 Eugene W2HX
> 
> -----Original Message-----
> From: tmc-bounces at mailman.qth.net [mailto:tmc-bounces at mailman.qth.net] On Behalf Of John Vendely
> Sent: Sunday, March 16, 2014 5:00 PM
> To: tmc at mailman.qth.net
> Subject: Re: [TMC] GPR-90 RXD Tuning
> 
> Hi John,
> 
> The 4 Mc highpass would be used on all bands which utilize double conversion, since it's needed to provide rejection of RF input signals around 3.955 Mc which will work their way directly into the 1st IF.  So this filter would be used on the top three bands, any frequency above
> 5.4 Mc.
> 
> The different simulation component values might stem from differences in the termination impedances you chose for the simulation vs. what actually exists in the receiver.  This could result in significantly different values.  I've never seen measured input impedance curves for this receiver.  The IF rejection filter response could vary a bit without affecting receiver performance too adversely.  They may  have tolerated some passband response variation, provided the filter provided good IF rejection and didn't hurt sensitivity too much.  Quite a bit of RF phase response variation over wide frequency ranges might also have been tolerated, provided the phase variation over small bandwidths 
> (equal to the widest IF BW) wasn't too large.   BTW, it looks like some 
> versions of the GPR-90, presumably the earlier ones, had only the BCB reject filter.  The 1st IF rejection filter may have been added later after problems were noted.  I would expect it was a necessity, and it's surprising it wasn't used in all versions.
> 
> 73,
> 
> John K9WT
> 
> On 3/16/2014 3:42 PM, John Poulton wrote:
> > 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
> >>
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> 
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