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AGC Slope

Tim K9WX
Tim K9WX Member ✭✭✭
edited June 2020 in SmartSDR for Windows
I’m two weeks and 500 QSOs into my Flex 6600 with a Maestro.  Most of those QSOs are CW with some FT8 and RTTY.

I have a number of questions based on my experiences to date, especially today when I operated SO2R CW for the first time.  I will raise those questions here as a series of postings, so that anyone with advice or insight can respond selectively.

The operating setup is identical for all questions.

·        SmartSDR 2.2.8

·        N1MM+ 1.0.7201.0

·        Windows 8.1

·        Headphones are plugged into the radio, not the Maestro, speakers are not used

·        Paddles are plugged into the key jack on the radio and the Iambic setting set to Enabled and to B

Q2: I’ve become familiar with the use of the AGC-Threshold control but find there is too much variability or range between the weakest and strongest signals, requiring me to turn the AF knob up to hear weaker signals and down for louder signals.  Some radio manufacturers incorporate a “slope” function in their AGC so that signal response beyond a certain level can be relatively flat and signals that exceed this level are close in AF output level.  Is there a similar adjustment available in the Flex?  

Tim K9WX




Answers

  • Varistor
    Varistor Member ✭✭
    edited June 2018
    It is important to point out that setting AGC-T, or the RF gain of the radio, is not a "set it and forget it" step. That is, it must be done as band conditions change and be actively managed.

    Required RF gain is also different from band to band. For example, it is a lot less on the lower bands and higher on 15 and 10.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited June 2018
    True dat!
  • Matt NQ6N
    Matt NQ6N Member ✭✭
    edited April 2019
    I've been curious about the question of AGC Slope with the 6000 series for a while so I was happy to see this question asked.  (There is a definition of AGC Slope provided in the below article):

    http://www.qsl.net/va3iul/Files/Automatic_Gain_Control.pdf

    From my google search it appears that PowerSDR had a configurable "Gain Slope" for AGC. But in reading the description of this it's not clear whether it's different from the AGC-T setting in SmartSDR.

    It seems that by definition the AGC-T is the "knee", and the AGC-Slope determines how much gain reduction is applied based on the strength of the input signals which exceed the threshold. 

    I'd be very appreciative if someone could describe in detail the architecture of the 6000 series in the context of the diagrams on page 2 of the above paper.  Does the AGC-T implicitly adjust the slope? What was the thought process for not including a user-adjustable AGC-Slope?

    73,
    Matt NQ6N






  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
    edited April 2019
    Hi Matt

    Did you get a chance to review the reply to the original question?  It did cover most of what you ask.

    Mike

  • Matt NQ6N
    Matt NQ6N Member ✭✭
    edited June 2018
    Mike - I did, it talks about AGC-T but not about AGC-Slope at all, or about the design decisions in whether to make that user configurable.  That's why I typed up my question :)

    73,
    Matt NQ6N
  • Tim K9WX
    Tim K9WX Member ✭✭✭
    edited June 2018
    Matt, thanks for posting this info. What you describe is how I have been setting the AGC-T and I am happy with the way that works. But I agree with Matt NQ6N that the info you provided does not address my question about AGC slope. He has expressed the question more eloquently than I did.
  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
    edited June 2018
    I poked one of our awesome engineering team.  They will dive under the covers for you on this.  :)
  • Eric-KE5DTO
    Eric-KE5DTO Administrator, FlexRadio Employee admin
    edited April 2019
    So AGC is a complex animal.  Anyone who thinks otherwise hasn't ever implemented one.  ;)  The AGC system in SmartSDR is a dual track system.  Both tracks work in a similar fashion, but each is targeted at different response times.  This helps prevent fast impulse type spikes from dominating a slower changing signal (such as a sideband signal).

    The AGC-T slider is actually setting the maximum gain that can be applied to any signal.  We always attempt to bring the incoming signal up to some target level, but at times, it doesn't make sense to apply that much gain.  For example, when amplifying the noise floor, you don't want to hear that at the full target level (if you want to try this, just slide the AGC-T all the way up -- it isn't pleasant).

    As others have mentioned, band conditions can affect this since what is being adjusted is the max gain against the input signal.  So if the ambient level of noise changes, this will definitely change how things sound.

    Today there is no slope applied.  We use a flat response.  There are several implications from this.  First, if 2 independent signals are evaluated one-at-a-time such that each of them are strong enough to hit the target level without hitting the max AGC gain, they will play at exactly the same audio level (i.e. the target).  This is the AGC doing it's job.  Even if one of these signals is significantly weaker than the other, they will sound the same (again -- this is when they are evaluated one-at-a-time).

    Second, if you put both of those signals inside the same passband, the stronger signal will dominate the AGC and the weaker signal will sound quieter.  If one signal is 10dB stronger than the other, the weaker signal will sound 10dB weaker in our audio.  This is the direct no-slope mapping in play.

    I agree with Mike that a combination of adjusting the filter (to eliminate an offending signal) and adjusting the AGC-T should give the desired result of a comfortable audio level for a wide dynamic range of signals.

    tl;dr SmartSDR does not use a slope AGC.
  • Alan - KA4B
    Alan - KA4B Member ✭✭
    edited June 2020
    Eric, I am interested in knowing more about the 'dual track system' and how slow, medium and fast relate to the two tracks.  My question has to do with minimizing the impact of lightning crashes on ssb signals on 80 meters and increasing intelligibility without damaging the hearing.  Thanks!
  • Ken - NM9P
    Ken - NM9P Member ✭✭✭
    edited June 2018
    If you have lots of lightning on 80, I surely wouldn’t use Slow. But medium should be OK.
  • Mark  K1LSB
    Mark K1LSB Member ✭✭
    edited June 2018
    Eric, I have good results with lightning crashes on 75 meters by setting the AGC Speed to Medium and sliding the AGC-T control toward the left -- keep in mind that the crashes will be reduced but so will your ability to pick up weak signals.
  • Bill -VA3WTB
    Bill -VA3WTB Member ✭✭✭
    edited June 2018
    Such is life in the world of communications...
  • Matt NQ6N
    Matt NQ6N Member ✭✭
    edited June 2018
    Thanks for the excellent explanation, Eric!   I very much like the AGC-T control and have had great success using it in combination with the narrowest filter bandwidths to dig out incredibly weak CW signals. 

  • Marty Ray
    Marty Ray Member ✭✭
    edited June 2018
    To mitigate the effects of static crashes when using CW, I set the sidetone to a very low frequency, (less than 400Hz), and use a very narrow filter. 73, Marty N9SE
  • Eric-KE5DTO
    Eric-KE5DTO Administrator, FlexRadio Employee admin
    edited June 2018
    Sorry for the delayed response as I have been out of the office for a week.  So the dual track has 2 different time constants that it tracks and both of these are affected by the AGC Mode (Slow, Med, Fast).  It isn't really any more complicated than that the time constants are longer for Slow than Med, and for Med compared to Fast.  For any one of these settings, the dual track has a "fast" track with a much shorter time constant for handling impulses, static crashes and such without capturing the AGC making the radio appear deaf for many seconds while it recovers.

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