Optimal AGC-T values for noise level

KD0RC

I have been wondering where the optimal point for the AGC-T is for various levels of band noise so I did some experimenting with a signal generator and my Flex 6400 running V3.1.12.51. It is late, so I am listening to a very noisy 80 band, but I think I have a rough calibration. I will try to refine this if 10 meters opens up (It is quiet here on 10). To get -130, I switched to RX A with no antenna attached.

Noise Level AGC-T

-130 35 - 40

-125 30 - 35

-120 25 - 30

etc...

-105 10 - 15

So it is the absolute value of the noise minus 90 to 95. I was really surprised how low the AGC-T needs to be set under high noise levels.

I find that a noise reduction of 15 or so coupled with these AGC-T numbers really makes things sound good. If there is a really weak one down there, I sometimes turn off the NR.

Any thoughts on this?

Thanks & 73,

Len, KD0RC

Erika - KØDD

Brilliant Len! Thank you. See you on the band. Trying your setting right now on 160.

Erika DD

Erika - KØDD

One more thing, how does this change with frequency? My Radio seems rather numb on 6, 10, 15 meters and a lot of times I have the thing all the way to 100 up there to even see signals? Opinion? DD

KD0RC

Hi Erika, You may need to increase the RF Gain (ANT button on left side of SSDR screen). I look at the noise level, then switch to an unused antenna (usually RX A) and watch for a drop of around 10 dB. If it drops much more than that, decrease the gain. If more, then increase the gain. It is funny, they recommend 8 - 10 dB, but the RF Gain steps are in 8 dB increments so don't worry about getting this too close, just get in the ballpark.

To answer your question, it doesn't exactly change with frequency. It is a function of your antenna system and band conditions. So when I am on 10 Meters during a poor propagation time (like late at night), my noise is around -130 dBm. When the band is hopping, it can be closer to -120 dBm. When my neighbor turns on his million LED light Christmas display, I lose a few more dB. On 80 at night, I am lucky to get -100 dBm. So I check the noise and adjust my RF Gain every time I sit down to operate and any time I change bands. If I notice the noise changing, I re-adjust, but I don't worry about it too much once I adjust it for a session.

Hope that helps.

73,

Len, KD0RC

Erika - KØDD

Thank You Len,

I have adjusted everything in the radio's toolbox trying to find optimum settings to get the best results. Using the signal generator was such a great idea and to find optimal setting for different conditions is great. trying to get some extra performance using weak stations on the band isn't very precise. It just takes JOE with a stronger signal coming up in the passband to throw off the result.

Already with 160m dxing I've seen a huge increase in performance. I've gone from about 45 down to 10-12.... The result has been very interesting. Testing with FT8 where I don't even put headphones on it works great, but on CW I have been controlling the volume with the control agc, headphone audio is nearly all the way up.

Have a great Weekend,

Erika DD

KD0RC

Great! I am glad that is working for you. Most of us think in old radio terms, so understanding what the controls really do in an SDR rig like the Flex is important. I had the AGC-T cranked way too high for quite a while (trying to use it like RF Gain), then I saw a video where Mike, VA3MW interviewed Tim, W4TME about the AGC-T setting, what it does and how to set it. What a difference it made when I cut it back! Until I did the sig gen trick, I was still setting it too high. Now I look at the noise, subtract 90 and adjust a bit from there if needed. If I am on a net with all strong sigs, I cut it back even further. I can hear great and NO noise!

73,

Len

Tim K9WX

Len, could you clarify you this statement from one of your earlier posts above? You said, "I look at the noise level, then switch to an unused antenna (usually RX A) and watch for a drop of around 10 dB. If it drops much more than that, decrease the gain. If more, then increase the gain."

Shouldn't one of those "mores" actually be a "less?"

Tim K9WX

KD0RC

Hi Tim, I think I have it right. To see this in action, put your Flex on an unused antenna port, then change the RF Gain up and down to see the effect on the noise floor.

Flex quotes 8 - 10 dB, so I just shoot for anything remotely close to that. Changing the RF Gain with an antenna connected, does not illustrate the behavior; it needs to be an open port (even a connected dummy load via coax can skew the results a bit).

73,

Len

Robby

I thought I had this figured out but this has prompted me to re-read the help desk interview. And then again the above and I now find that I was less than half way there. If you are running 3W to a wire please call me. I need the practice.

Robby/HP3SS

Tim - W4TME

Please read this: How does the Automatic Gain Control (AGC) work in SmartSDR