NO AGC when AGC-T Properly Adjusted. See Video

Steve N4LQ

Flex-6500 et al. SmartSDR 1.4.16 or any version

I perceive this as a problem. Others may not.

Adjusting AGC-T "properly" results in no AGC at all.

Reason: AGC-T is really RF Gain and the AGC Threshold is fixed at such a high level that constant volume changes occur due to fading and various signal strengths.
I demonstrate this in this video.
https://www.youtube.com/watch?v=9z_c4S-LV0g 
We end up "riding" the AGC-T control to compensate. It's exactly like using a receiver from the 1930's.

I would like to see AGC-T to be a true AGC Threshold setting. We should be able to move it down - left and lower the threshold so that AGC worked on weaker signals. 

Threshold = Point where things start to work.
Low threshold should provide AGC at LOW signal levels. 
What we have is not a threshold at all. 

Turning AGC-T all the way up - to the right should result in NO AGC. Threshold raised!
The current design is faulty. 

I wasn't list this as a problem or an idea so I did both.

WA2SQQ

Interesting demo, will be waiting to see how flex explains it.

Ken - NM9P

Steve, it seems to me that you are adjusting the AGC-t backwards.

You are starting at the left and bringing up until you start to get noise, which leaves most of the signals way below the point where AGC is doing anything, and compensating for lack of volume by turning up the AF volume.  This means that MOST, if not all of the signals are not being controlled by AGC.

Rather you should start at the right and turn it to the left until the noise JUST starts to DROP. 
In your demo, you needed to reduce the AF volume because you were getting signals too loud.  This is as it should be, because once the noise starts to drop as you turn the control to the left, then anything that is above the noise floor threshold that you just set will be acted upon by the AGC.

You mentioned that if you want any AGC action that you need to turn the control further to the right, which is correct.  If you follow the procedure I outlined, you will experience AGC action the way it was designed to operate.  The "Knee" or "Sweet spot" is found by turning it from the right to the left until the background noise just starts to drop.  Then adjust your AF volume for a comfortable listening level.

if you want to quiet the receiver a little more, you can go a little further to the left, but just a LITTLE.  If you turn it too far to the left, you will have just taken a lot of signals out of the AGC zone and you will be compensating by turning your volume up only to be blasted by strong signals that are actually strong enough to get to the AGC Threshold.

Hope this helps.

Ken - NM9P

Steve N4LQ

Thanks Ken. You are very observant. I must admit that I didn't interpret the instructions that way and since you said "start at the right and turn it to the left" it puts a different perspective on the procedure. Something in my psychic caused a mental block when it came to which direction a "knob" is turned mattering but I should have known better. I suspect I'm not alone. 
  
I'll do some more testing.  I see another video coming shortly. )

Steve N4LQ

Is there a way to delete an entire posting? I think It would avoid confusion. 

W9OY

what is thew point of AGC?  It is to turn down the gain automatically.  What happens to weak signals when you automatically turn down the gain?  They disappear.  Since you cant turn down the band noise the best you can do is set the automatic threshold so some point barely above where the band noise is.  This is the point where your receiver is most sensitive and the reason weak signals tend to "pop" out of the noise  This is hardly a 1930's style RF gain control and I'm not sure why you continue to think of it this way.  Another way of looking at this is presume you have a window both width and height.  The width is the filter and the height is the dynamic range.  AGC-T is used to lower the bottom edge of that window exactly on the band noise.  It makes no sense to have half the window and hence half your dynamic range buried in the noise, and have ACG turning the gain way down because of your mis-tuning.  You pretty much can't hear under the noise (maybe a little) so why listen to it?

73  W9OY 

Steve N4LQ

Well the problem with that is this. If you happen to adjust AGC-T too far to the left and think to yourself ....."gee, must be good because the band is now so quiet" .....Then when you do receive a moderate signal, the AGC does nothing and you experience volume level fluctuations and blasting. 

So Ken's point was to begin with the AGC-T slider fully to the right then gradually reduce it just to the point that band noise begins to decrease a tad. 

I was starting at the left and sneaking up to the right until I heard band noise.

The manual does call AGC-T RF Gain but  only when AGC is turned off. 

Tricky stuff!

k0eoo

I would think either way would work. I have used both directions myself.  For me it works best if I put the AGC in the FAST mode so I can detect the change quicker....

Dennis, k0eoo

Steve N4LQ

Ok well you saw what happened when I came in from the left....No AGC. 
The "knee" is on the upper edge and is pretty touchy. 

Al_NN4ZZ

It seems like the option to automate the AGC-T would make it easier to use and more consistent.  Set the threshold you want and not have to make adjustments continually as band conditions change or when your change bands, etc.

https://community.flexradio.com/flexradio/topics/option-to-automate-the-agc-t-setting

Regards, Al / NN4ZZ  
al (at) nn4zz (dot) com


W9OY

AGC off is basically fixed gain ie not variable.  I use this all the time on 160 because of the static and thunderstorms at my QTH.  I have a DDUTIL macro that immediately switches me to AGC-fast with AGC-T set to 35 if a strong station opens up on top of a weak station I'm listening to,  If you want to set AGC-T to some level on any band just use the macro.  You can use FRStack and it will remember the AGC-T for a given memory/band as long as you set it where you want it.  I use FRStack as a really advanced bandswitch and I love it.

73  W9OY

k0eoo

Hi Steve,

I like your test setup and video.  I see what you are talking about now...  As you have said many times, looks like the 6K series has little to no real AGC action when the AGC-T is set low vs. when its set high.  I have to say I never noticed that before....   There have been some interesting suggestions put forth here.  It'll be interesting to see what Flex plans to do, if anything....

It looks like the Flex AGC-T does not work similar to the Collins AGC-T as I thought.  I stand corrected....

Regards, Dennis, k0eoo

Ken - NM9P

The "wisdom of experience". I.e. I went down this road myself a while back! Ha. Good luck...

Steve N4LQ

Right on....But I still say the AGC-T control is backwards. A lower setting number should result in a more agressive AGC. That's how the K3 works. Turn down the threshold and you get more AGC. Well the Flex version at first seems like it is working that way...Move the AGC-T to the left, band noise gets weaker...You think AGC is acting on the band noise but Nooooo....When a strong signal pops in it blows you out of the room...No AGC....
See what I mean Ken?

Ken - NM9P

I understand.  The FLEX AGC-T functions differently from any other rig I have used.  Once I got used to it, I began to really like it.  But I wish someone from FRS would create a quick graphic to describe exactly how the AGC-T control interacts with the signal levels.  Here is a word picture description as I understand it:

Imagine the panadapter screen in front of you.

Imagine also a horizontal dotted line across the panadapter that moves up and down according to the setting of the AGC-T control.

When the control is all the way to the right (largest numbers), the line is at the bottom of the screen.  (the signal strength numbers are also the "largest" with -127 dBm, etc)

As you move the control to the left (smaller numbers) the line goes UP, (corresponding to "smaller" numbers in the Signal strengthi.e.  -65 dBm)

Now here is the "Magic" of the SDR AGC programming.....

All signals that are ABOVE the line set by the control will be heard at the AGC-controlled level.  Set your AF volume to a comfortable listening level on signals that are stronger than the threshold.  They won't get any louder than this.

Everything BELOW the threshold line will be heard at a reduced volume proportional to the distance below the line.  This includes baseline noise and/or radio signals.  The further below the threshold line, the weaker the signal will be heard, because they are BELOW the AGC Threshold.

As you move the line UP (by moving the control to the left),  The effect is consistent.  Signals that are ABOVE the new threshold line will be heard at the AGC-controlled maximum volume.  All signals and noise that are BELOW the threshold line will be heard at lower volumes, PROPORTIONAL to the distance below the threshold line.

So... if you run the line up high by turning the AGC-T control too far to the left, everything BELOW the line will act as of there is NO AGC on them, because they are below the AGC Threshold.  SO if the threshold is set to S9+20 DB, only signals above that line are controlled.  Everything below that line act ash though there is NO AGC on them, because they are below the threshold.

If you set the line high, and compensate for the weaker signals that are below the line by turning up your AF volume gain, then stronger signals that are still below the threshold line will be much louder than the weaker ones because they ARE stronger, but are not controlled by the AGC since they are below the threshold line.

This is why setting the AGC-T correctly is so important.  When you start at the right and adjust it to the left so that the background noise starts to reduce, what you are doing is moving the AGC-Threshold line up from the bottom until it is above the standing background noise, but weaker signals will begin to pop out of the noise.

As you move the control further to the left, you raise the line, and weak signals begin to drop out below the line into the "weaker, uncontrolled zone." 

Finding the "Sweet spot" or "knee" is critical for best performance.  and the actual position of the threshold line will vary depending upon band conditions, antenna, noise sources, etc.  This is why AGC-T is persistent according to band selected.

I find that it is easier to fine tune the AGC-T with the FlexControl Knob.  There is a setting that allows the Knob to adjust AGC-T on the active slice.  I also have programmed this into my DJ Midi Controllers to make it easy.

I hope this helps visualize the function of the AGC-Threshold.  I may try to edit a picture to illustrate it once I figure out how to produce one.

It may not be 100% accurate but it is the way it appears to work for me.
As I said, once I understood how it worked, I really began to like it.

Ken - NM9P

James Del Principe

Great description, Ken.  It may be nice to have a momentary graphic appear during setting of the AGC-T to show this visually. 73, Jim

Steve N4LQ

This is the only explanation I've seen published here that is correct. Virtually every time a question is asked about AGC-T the answer has been to "turn up the slider until you barely hear band noise". Doing that results in zero AGC. 

Flex definitely needs to adapt the AGC horizontal line as an option. 
Trying to explain this in words is useless. 

Slider to right, line goes down noise goes up, volume stays same on all signals. Slider to left, line goes up, noise goes down, weak signals sound weak, strong signals sound loud, no AGC. 

Now one thing that bugs me about this......With the slider to the right we have maximum AGC however certain signals have an "overloaded" or distorted sound and this is most noticeable on CW signals with "hard" keying. You end up with a twirpy, crunchy or slightly chirpy note in the speaker. The cure is to slide AGC-T further to the left however that defeats the AGC. These signals aren't super strong...S9 or so. 

The S meter behavior is still a problem. I do not care if it's tied to the AGC or not however it does need some persistence so it can be averaged without guessing.

Thanks 

Steve N4LQ

Here's a video showing how to adjust AGC-T per SmartSDR manual page 36 and per Ken's NM9P instructions.
https://www.youtube.com/watch?v=Hwm6rszuMis&feature=youtu.be

Ken - NM9P

Nice.  Now you have your volumes set about where I usually have them.
Master speaker volume at 100%
Headphone volume about 50%
Each slice volume about 50%
Then my Bose Speaker knob is adjusted about halfway because that is the comfortable listening point in my shack.  I think you will have a lot more fun now!  It is a great rig once you learn it's unique personalities. 

If you don't like as much background noise as you have, you can use the mouse and click to the left of the AGC-T slider and adjust it one-by-one until the background noise drops off a bit more, but not too much.  Most weak signals will pop out of that noise just fine, and with a little better SNR.  (easier on the ears)  If you get a really weak one, you can bump the threshold open a bit more for that contact.  Then go back to a little quieter rig for standard ops...

Ken - NM9P

W9OY

Excellent video!

73  W9OY

Dan -- KC4GO

He is another way set  AGC-T
using FlexMeter by Denley Barnette W3XY available at http://www.denzone.com/
See video at: https://drive.google.com/file/d/0B5gpcTVs7PtocmhzQWNKSUpZWUU/view?usp=sharing
Select a location on the band near where you wish to operate but only has noise.
Turn off all DSP set AGC-T to 100
In the FlexMeter Select the slice 0=A 1=B 2=C etc...
Select SLC then AGC+ and set for Average
Let it settle, then reduce the AGC-T so the average is down 3 dB
This is also where you start to hear the audio reduce.
Then I turn on the DSP as needed or desired. 

Dan -- KC4GO

W9OY

Dan

I didn't know about FlexMeter.  Your method works perfectly and would lend itself to an automated AGC-T set.  

73  W9OY