Re: {Collins} AGC - A question.

AGC systems are pretty simple in principle, but the job they have to do (and therefore the job of designing a good one) is rendered more difficult by the wide range of conditions they have to deal with in an HF receiver. In particular, SSB service presents challenges because the signal goes to zero between words.

Some extreme (but realistic) examples:

1. The band is quiet, so the AGC system is "wide open" and all you hear is a bit of front-end and galactic noise. A strong signal appears; if the AGC system doesn't "attack" fast enough, the final IF and detector circuits can overload, producing audible "whiskers".

2. Again, a quiet band, and a noise pulse appears (as in storm static). These can be extremely strong, but of short duration. If the attack time is too fast, the radio will block up until the AGC voltage decays. So this requirement tends to conflict with example 1.

3. You're listening to an SSB signal, and the decay time is too fast. You'll hear the background noise come up in the brief pauses between words. Intelligibility isn't greatly affected, but it's annoying to listen to.

4. You're listening to a net; a strong station turns it over to a much weaker one (just above the noise level). If the hang time is too long, the receiver won't resensitize fast enough, and you'll miss the first word(s) from the weaker station.

So, if you listen to the receiver under these four conditions, and it performs well in all cases, you have a well-balanced AGC system.

In practice, it's difficult to find a hang time that's ideal for all situations, so some radios offer a choice. In the case of attack time, the AGC system isn't the only thing involved; if the detector exhibits soft, symmetrical clipping, a certain amount of overload can occur before it becomes noticeable, which makes the job of the AGC system easier.

I hope this provides some insight.


On 2014-11-16 8:35 PM, Walter E. Cheatham wrote:
Hello all,

For quite some time I have had a a question nagging me about AGC in the various Collins receivers, specifically the S-Line. I will now go ahead and ask it now.

I have recently seen some questions about various AGC functions of different receivers, the most recent was the 51S-1.

My big concern is that maybe I do not understand how I can actually hear the difference on the AGC setting, specifically the fast or slow. In addition the hear-able difference between fast attack and slow decay.

Maybe is it simply my ears but I thought I'd ask this anyway.

Frankly I feel a bit shy to even ask but since I do not know everything about electronics and radio. even though I have known electronics since 1963 or so (when I first was learning electronics) I find that my knowledge is limited when I compare to Bill Carns and others (I am NOT an engineer).

So, what is or are the answers????

Walter Cheatham K7CCA
Laveen AZ
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