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

[Mike Hanz]

On 11/3/2013 1:42 AM, Kenneth G. Gordon wrote:
> On 3 Nov 2013 at 5:06, Fuqua, Bill L wrote:
>>    Yes it is delayed AGC. I was simply saying 20 Volts is way too much.
>> a few volts perhaps.
> Maybe I didn't describe it properly: the manual says that the cathode bias on
> the 12SF7 is 20 volts and that the signal from the IF amps has to rise above
> that voltage before the AGC starts to 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?"  Just about all the commonly used WWII military receivers 
used delayed AVC of one sort or another.  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.  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.  The primary purpose 
for delayed AGC is to provide maximum gain at all inputs less than that, 
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.  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.

Or perhaps I misunderstood your puzzlement...?

Very 73,
  - Mike