DAX RX Gain After Changing AGC for Digital Operation

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  • Updated 3 years ago
When running digital modes using FLDigi or similar programs I typically turn off AGC and move the AGC/Threshold slider fully to the right in SmartSDR.  This 'redlines' the RX Gain indicator in the DAX Control Panel, suggesting a significant decrease in the RX Gain setting.  Going back to voice modes requires the DAX Control Panel gain to be changed back to around 50%.

I would appreciate advice on how others are setting their AGC threshold for digital modes and whether they are then changing the RX Gain in the DAX control panel.  Is the 'best-practice' for digital work to find the 'knee' in the noise level using the AGC slider, move the AGC slider all the way to the right, and/or turn AGC off?  I looked through the forum history but could not find any comments that linked AGC operation back to the DAX control panel.

Thanks, Ron
K2RAS
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Ron Schwartz

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Posted 3 years ago

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KM4CQG

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I put mine on Fast and the slider full right in my testing this worked best for me.

Ian
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George Molnar, KF2T, Elmer

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For digi modes, I run with DIGU selected, AGC OFF, with slider at the "knee" (generally around 45-52, depending on Band), and then adjust the DAX channel level for the appropriate level in the digital Application (green Diamond lights in FLDIGI, 30 db in WSJT-x, 0 db in WSJT). Use a unique DAX channel per digital app. No need for DAX on SSB, unless you have a special situation?
(Edited)
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NX6D Dave

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I'm with George on this one.  I set AGC to OFF and set the AGC-T control to about 30, which is approximately the knee when my 6500 is in SSB mode.  The only reason I choose 30 is that it is my conventional number.

After that, I adjust the DAX audio level with the DAX control panel.  Most of the time I have plenty of adjustment room, and I can always change AGC-T up or down as needed.
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Al K0VM, Elmer

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For digital modes like WSJT and PSK ( where there is more than one signal in the pass band)  I would like AGT threshold to be reached only on the strongest of signals. Therefore I leave the leave AGT on but set the slider down into the 20-30 range,  DAX is at default value and the WSJT-X signal level indicator indicates about 10 on RX noise.    To me it seems as if its best if the digital program does most of the leveling.   If AGC is run close to threshold, strong signals supress the weaker ones. 

Just my way of doing it..

AL, K0VM
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W5XZ - dan

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That's curious, I see settings all over the ball park!  here, with WSJT-x, I run the rig a lot like i do on cw.  AGC-t ON, fast, < 50 typically...NB, sometimes...adjust DAX audio for '30' on the wsjt-x display..does WSJT-x have an agc function in its software? 

must just be me, but on CW, when i turn off AGC, and start digging for weak signals,somebody comes along and blows my headphones off my head! digital decoding software doesn't overload, or clip, or go into gain compression, so it doesn't need AGC?

would sure like to decode some more weak sigs, though, so will try the above suggestions.

73, w5xz, dan
(Edited)
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Ron Schwartz

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Actually, George has a point.  When I started this posting I just assumed that I should be setting the RX Gain in the DAX control panel back to a 'normal' level when I went back to voice mode.  Since voice modes do not use DAX, I really should not worry about the DAX setting when running voice.

It is interesting that there is no consensus yet on how to set AGC for digital, so I was not missing something.  I would be interested in Flex's opinion on the 'best practice'.

Ron
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Tim - W4TME, Customer Experience Manager

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So here is my answer, not necessarily the FlexRadio answer.

It depends on two factors; the receive bandwidth filter and band QRN / QSB conditions.  It really helps to understand how AGC algorithm is implemented in the radio since it is a dual track AGC that works on both fast and slow increases in gain.  I highly recommend reading the section on Noise Mitigation in the SmartSDR for Windows Software User's Guide.

In general, I do not want a stronger signal in my receive passband "capturing" the AGC where the audio gain will decrease, making weak signal reception more difficult.  If I am using a narrow filter relative to the mode I am operating and the bandwidth of that filter is such that the probability of another strong signal entering the passband is low, then I use the Fast or Medium AGC and adjust the AGC-T to the point where I have maximum signal to noise (SNR) based on band conditions (see the section in the manual on how to do this).

If band conditions are fluctuating or I am using a wide RX filter, then I set the AGC to Slow or sometimes Off.  Again if the AGC is set to Slow, I adjust the AGC-T for maximum SNR.  If I use the Off setting, you have to be a little more careful.  When AGC is off,  it really isn't off.  It means that a fixed amount of gain will be applied to both fast and slow increases in noise.  I use these setting so that a strong signal in my passband does not capture the AGC and trigger an AF gain reduction.
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W5XZ - dan

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well, my settings can't be THAT bad...i did pull vp8alj, falklands, out of the mud on a noisy 160m band, with modest antennas...twice, now...full credit, though, to my HiZ 4sq, on receive..

'just sayin', as the kids say?

73, w5xz, dan
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George Molnar, KF2T, Elmer

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As the Good Book says, there's more than one way to skin a cat. Or, your mileage may vary. If it works well in your unique Installation, it's good enough. Tweak on!
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Jay / NO5J

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My mileage varies from session to session. The settings get adapted for whatever tweak I'm using with that session. I use whatever works, heavy on the "whatever", and it usually works. I used to believe, I'd find some sort of magic sweetspot setting. Then I remembered I don't believe in magic.

73, Jay - NO5J  
(Edited)
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Mike KM4LQP

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For psk 31 I use AGC off, slider set to 45 or so, usually i adjust agc based on waterfall in fldigi.
Fldigi I have viewer squelch set to 1.5 with Receive Squelch level set 1/4 from bottom

I found today that while working Feld Hell that I needed to crank the agc slider to 90 or so to make out the information
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Jay / NO5J

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"Today" I'm  running preamp off, agc off, threshold 16%, and DAX RX 70%, 4k passband, with WSJT-X 1.6.0 -r5881. 
Works as I expect it too.

73, Jay - NO5J
(Edited)