AGC-T - Backing off to reduce noise vs. advancing to hear noise

Duane N9DG

All of the below assumes that the proper level of pre-amplification was previously set to reach the band’s noise level. But not too much that needlessly amplifies the natural band noise. That is a topic for another thread.

I see a lot of AGC-T setting comments where the recommendation is to start with the control far to the right and back off until the band noise "noticeably drops". I do it the exact opposite way. I start with the control far to the left with no audible band noise, and then advance it to the right until there is a solid and distinctly perceptible atmospheric/natural band noise present and stop there. This is done at a “empty” spot on the band.

My rationale for that approach is that any signal right at the band noise level will be heard with that setting because I can also just hear the natural band noise that is present. So a weak signal at about that same level is also audible. Meanwhile backing off from the high side till the noise reduces you still don’t actually know how close you are to the natural band noise. As such the AGC will likely be acting on the band noise and potentially masking signals right at the noise floor.

Also if you advance from no perceptible band noise to where there is distinctly perceptible band noise, you will never get the rushing noise between CW elements or pauses in speech on SSB.

As for volume level of the weak signal with the above settings, that is what the volume control is for. AGC-T should never be used to try and set audio level of the signal being listened to. But I suspect a fair number of people try to use the AGC-T as a pseudo volume control. Please don’t do that.

Perhaps it is a terminology and semantics thing. And those who say back off from above actually do arrive at the same place anyhow. But to me it just seems easier to grasp what is happening with the AGC to advance it from no band noise heard to just a little being heard as band conditions call for.

The advance from no noise to solidly perceptible noise approach is what I have always used going back to the early SDR-1000 days and PowerSDR. And also with most analog radio RF gain controls too for that matter.

And at the end of the day the AGC-T setting for this quickly becomes second nature and most will reach a point where they just do it automatically without specifically thinking about it. But for those starting new, I suspect that many of them think more band noise heard means more sensitivity, when in reality that is not true at all.

K2KXK

Very good advice. The acc-t is one of the best aspects of the Flex. I use my Flexcontol to adjust it and the audio level which allows me to fine tune both quickly and easily. Thanks.

Jay Nation

I completely agree.

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Varistor

Consider using the presence or lack of color noise in the pan adapter/waterfall. If you see random dots you have too much gain, regardless of what you hear.

Clay N9IO

Definitely DXers in this thread. Sound advice from all.

Eric Gruff

A related question - what do folks running FT8 do as far as AGC-T? It's not usually possible to hear weak signals that can be decoded, and I'm not sure if the waterfall or panadapter are useful either, so typically just set it around 40-45 and hope for the best.

Varistor

RF gain, or AGC-T, is something that needs to be actively managed at all times. This is not "set it and forget it". How much RF gain you need changes from band to band, day to day, and time of day. If you see too much color noise, i.e. random blue dots, you have too much RF gain.

Bill -VA3WTB

The AGC-T does not effect the display on the water fall or panadaper.
Adjusting the water fall gain in the display tab controls what you see on the waterfall.

K2KXK

VA3WBT is correct.  AGC-T and RF Gain are completely different  AGC is in the audio chain and does not affect the waterfall.  RF Gain and/or preamps (depending on your radio model) will affect the display although much of the effect will be filtered out over time.  However, Varistor is also correct in stating that there is no set it and forget it setting for AGC-T.  That is why I have one of the buttons on my Flexcontrol configured to adjust AGC-T.  Hope this helps.

Eric Gruff

I agree with both of you, but again, from a practical standpoint, what is the best way at a given time to determine how to set the AGC-T? Do I still go by ear, and assume it's also correct for the digital signals?

Bill -VA3WTB

I don't do much FT8 so I am not sure about this,,does the AGC-t effect digital modes?
Yes you set the AGC-t by listening to the noise floor,,just at the point of the noise going down or a tiny bit more is the sweet spot.

Jay Nation

With WSJT-X, I set the DAX channel gain at 50% (Default), AGC Fast, or Off (if there are really strong FT8 signal in the passband). My threshold level varies between 27% and 36%.

I also pay attention to the audio level indicator in WSJT-X, ensuring that with my 3k passband tuned to the nearest quiet, signal free area on that same band  results in the WSJT-X level meter reading about 30 dB, which is why I vary the AGC-T threshold level. This results in signals that are not visible on either SmartSDR's or WSJT-X's waterfalls, still being decoded.

I adjust both waterfall color gain levels so that very little, but some of the noise remains visible. If I'm missing any decodes they would be signals I can't see, or hear.  IOW they are undecodable, if they exist at all.

It's a little more work, but it works!

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Jay Nation

Ken
Agreed. The AGC Off setting might behave like a legacy RF gain control, but it's definitely not an RF gain control, because it's not controlling RF, the AGC-T function is being done in the AF stage of the receiver.

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Tim - W4TME

For FT-8, I set the AGC "aggressiveness" to SLOW and set the AGC-T for optimal SNR based on the band noise background I am hearing from the speakers.  I then adjust the DAX RX gain so that I get a 30 dB signal level in WSJT-X.

Don't set AGC-T from quiet to hearing noise; you are now increasing the gain of the noise which is not the optimal SNR.  You do not want the AGC to amplify noise

Jay Nation

Thanks Tim, I hadn't really evaluated SLOW with FT8, now I will. 

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