[ARC5] A discussion on AGC.

[Kenneth G. Gordon]

On 8 May 2017 at 22:53, K5MYJ wrote:

> Modern CW/SSB rig us PRODUCT DETECTOTS

True, but those have nothing to do with AGC. In my opinion, the almost universal switch to 
product detectors was a very significant advance in receiver technology. I really like product 
detectors.

> and the AVC is generated by rectified 
> audio. Not from the rectified RF.

Not true for most modern rigs.

Audio-derived AGC has some, few, advantages, but very significant dis-advantages. For one 
thing, it is too slow to react.

All SSB transceivers I have been working with for the past many years use an IF derived 
AGC. This includes all of Heathkit's SSB products (SB-100/101/102, HW-100/101, 
HW-12/22/32, SB-300/301/303 receivers) among other makers stuff. Collins, Drake, Swan, 
Kenwood, Yaesu, Icom, all use IF derived AGC. In fact, I don't know of ANY major 
manufacturers of SSB equipment which use audio-derived AGC, although I am sure there 
are some.

The "secret" is to take your AGC voltage from some place which is NOT effected by the BFO, 
and most specifically NOT from the detector. Usually it is taken from some stage well ahead 
of the detector.

In the AN/ARC-5s, that appears to be the case with those receivers which derive the AGC 
voltage from the diode in the 12SF7.

I always thought it was very interesting that someone (ARC?) prevailed on someone else 
(military, tube makers?) to make or have-made a special tube which was (apparently) ONLY 
used in the AN/ARC-5 receiver, the 12SF7.

I know of no other piece of equipment which ever used one, although I am sure there were 
some, probably all military.

Which reminds me: I told someone here some time ago I would send two NIB 12SF7s to 
him. So many things have intervened in the meantime that I have forgotten who that was. If 
you know who that was, please remind me.

They have been sitting on my operating bench in their white boxes for many months.

Ken W7EKB




---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus