[TMC] GPR-90 RXD Tuning

[jvendely at cfl.rr.com]

Richard,

The GPR-90 was noted for front-end overload and intermod problems, as you pointed out.  I suspect the RXD version was intended for use in larger, point to point stations with big antennas (e.g. rhombics)for which the GPR-90 would have been poorly suited.  The tuned 1st RF stage of the RXD was likely added to improve intermod and image rejection.  I would guess they used a manually adjusted 1st RF stage tuning capacitor to allow use of the same ganged tuning capacitor as originally used in the GPR-90. 

It would be interesting to compare measured intermod and image rejection performance of the two versions of the receiver.  JP and I had discussed this, but neither of us has gotten around to actually doing it.  

Regarding manually-tuned receiver preselectors, an example of a military receiver using this method was the early 1960s vintage AN/FRR-59 designed by The National Radio Company.  No doubt many receiver designers (and especially users) considered this arrangement inconvenient and a detriment to "browsing", but it did have the advantage of reducing front end tracking errors, and was cheaper.  

73,

John K9WT

---- Richard Knoppow <1oldlens1 at ix.netcom.com> wrote: 
>      The GPR-90RXD has a tuned first RF stage unlike the 
> original GPR-90 which has a broad-band input. I don't have 
> either kind of receiver so must ask of someone who does.  It 
> appears that the first IF of the RXD is not tracked with the 
> LO as in most receivers but must be manually peaked with the 
> "antenna tuning" knob. Is this the case?  Another puzzle: in 
> the original GPR-90 the antenna tuner is actually in the 
> grid of the second RF so it can't peak the antenna tuning 
> except for the broadcast band where the first RF is not use. 
> In the RXD version the antenna tuning cap is in the grid of 
> the first IF in the conventional way, but again, there is no 
> additonal section on the main tuning cap so it appears the 
> antenna peaker is the sole tuning for the first RF.  Of 
> course a lot of receivers and transceivers these days have 
> manually tuned pre-selectors.  I am not sure when this 
> started with modern receivers but the Collins S line must 
> have been about the earliest.
>      Another question:  The GPR-90 is sometimes criticized 
> for having poor overload rejection. It would appear that the 
> addition of another tuned circuit in the RF was to remedy 
> this.  Is this the case.
> 
> 
> 
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com 
> 
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