[RTTY] AGC: on or off? [long tutorial]

[Robert Chudek]

Peter,

That's a good question. And I see several fellows have provided an answer for you. Hopefully I can help by providing an explanation of how this all works. There's a couple of pieces of information you need to know to help you decide whether AGC should be on, off, slow, medium, or fast for a particular situation.

First the foundation information: Everyone knows that AGC is the acronym for Automatic Gain Control. This is a circuit in the receive section of your transceiver that monitors the strength of the signal you are listening to. When you turn the AGC on and tune across the band, the AGC circuit tries to keep all the signals at a consistent level for you. That's the automatic part of it! It adjusts the receiver sensitivity up or down, depending upon how strong a received signal is.

Another benefit of AGC is to prevent the receiver circuits from being overloaded by strong signals. This reduces the chance of distortion or other nasty consequences of overdriving circuits. AGC is your friend... most of the time!

When you turn the AGC off, this puts you in control of the receiver RF sensitivity. You will need to operate the RF Gain control yourself. As others have mentioned, this can help you hear (cw/ssb) or print (digital) a weak signal. The statements and experiences are true, but it will help you to know why this is...

Let's say you're on 20 meter rtty during a contest, you have your AGC turned on, and you hear a weak station calling CQ. Let's also say this station is S-3 on your meter. You should be able to print this fellow without any trouble. But wait... there's trouble brewing... another station starts calling CQ about a half KHz away and he pushes your meter to 20 dB over S-9. What happens next?

Your friend Mr. AGC hears this strong signal and automatically cuts your receiver sensitivity back. And now the desired station cannot be heard anymore due to the lack of receiver sensitivity. The next thing that happens is your decoder stops decoding because the strong signal is not tuned in properly. You've got the proverbial double whammy!

Turning your AGC off can help in this scenario. You will need to manually adjust the RF sensitivity control up to a level where you can hear the weak station and it begins decoding again. Of course you're going to hear the strong station's audio blasting through, but it will be ignored by the decoder because the tones are not at the right frequency. This "AGC off" technique takes practice. Your success will also depend somewhat upon the characteristics of the receiver you are using. But in general, this is your AGC on/off decision.

It just occurred to me to mention this... You do need to be listening to the receiver audio to be effective during a contest! Although the rtty decoder is generating the information on your screen, you as the operator still need to listen to the signals and determine the band conditions.

Okay... on to the slow/medium/fast... Slow is good for casual ssb rag-chewing... Medium is good as a marketing gimmick... and fast is good for contesting and DXing. This AGC setting determines how quickly the receiver sensitivity will adjust to follow a change in signal strength.

Here's an example: You might be working a strong station that has a lot of QSB on the signal. If your AGC is set at the slow position and the signal strength drops quickly, the receiver sensitivity might not recover quick enough for a consistent audio level for your decoder. If your decoder can't hear any audio, it stops decoding. Setting AGC to fast is a good operating practice. This becomes especially important when working DX stations that have rapid QSB or flutter on their signals.

Knowing why you would choose one setting over another is half of the battle. Practice and experience is the second half. Combing these will help you increase your rtty contesting results.

73 de Bob - K0RC



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Message: 7
Date: Mon, 30 Jan 2006 19:26:34 -0600
From: Peter Laws <plaws0 at gmail.com>
Subject: [RTTY] AGC: on or off?
To: RTTY <rtty at contesting.com>
Message-ID:
<2538cc000601301726u73f98748he45dc808bab0158e at mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

I keep seeing mention of AGC.  That's the one control on my IC-746Pro
I admit to not knowing how to set.  I've read the Handbook section on
AGC and the manual entry and, of course, fiddled with it ...  so? 
Where do I set it for RTTY?  SSB?  It's got a fast/med/slow setting
for each mode and each preset is adjustable, including to "off".

--
Peter Laws | N5UWY/9 | plaws0 gmail | Travel by Train!

"They that can give up essential Liberty to
obtain a little temporary safety deserve
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