[ARC5] AGC in the AN/ARC-5 receivers.

[Kenneth G. Gordon]

On 3 Nov 2013 at 9:00, Mike Hanz wrote:

> > work. I am still not convinced that isn't a misprint or typo.
> 
> Not to put too fine a point on it, Ken, but the question comes to
> mind, "So what?"

The "so what" in my mind, Mike, is that perhaps this is one reason so many 
hams had "marginal" results when using those receivers for more than for 
what they were designed.

>  Just about all the commonly used WWII military
> receivers used delayed AVC of one sort or another.

Except, as you say below, the early models of the "ARC-5" receivers and 
some few others.

>  Because of
> different AVC design schemes among them that use different voltage
> levels as the trigger, it seems to me that the only meaningful measure
> of the AVC start point is the antenna input that begins to reduce the
> output voltage.

Oh, I heartily agree.

>  The same paragraph in the manual that you mentioned
> says 30uV for the comms receivers, which seems to be in the same
> 20uV-30uV ballpark as some of the examples in Langford-Smith.

Yes.

>  The
> primary purpose for delayed AGC is to provide maximum gain at all
> inputs less than that,

Of course.

> and I could understand either a designer's
> choice or even a contract specification that referred to a specific
> input level where it would begin to kick in. I suppose we could debate
> the selection of 30uV, but I have a feeling it wasn't just a random
> figure.

Again, I agree. As I have repeatedly stated here, I think the ARC engineers 
were pretty much geniuses and did nothing randomly.

>  I would think that it was most likely based on experience
> with the ARA and SCR-274N receiver performance in combat aircraft
> noise/antenna limitation environments, and with real signals used in
> both wartime theaters. Since the ARA and SCR-274N receivers used a
> much cruder form of output limiting, there obviously had to be a
> change that was driven by some specific shortcoming in that older
> design.

Indeed!

> Or perhaps I misunderstood your puzzlement...?

Perhaps. I have been working with "pre-hacked" receivers for the past year 
or more, and have wondered about the effectiveness of the AGC system in 
the ARC receivers. It never seemed to me to be particularly effective, and I 
was simply wondering why. Call it another example of my inordinate 
curiousity, I guess.

It has become abundantly clear over the past several months that using an 
ARC receiver withOUT also installing an effective AF gain control keeps the 
receiver from reaching its most useful potential, at least for AM work. 

In order to reach the AGC threshold, one must crank up the only gain control 
there is, the RF gain, to ear-splitting levels, since the AF gain is always 
maxed out.

I thought perhaps a too-high AGC threshold was one reason.

Obviously, it isn't.

Ken W7EKB