AGC Slope

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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

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Tim K9WX

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Posted 3 months ago

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Michael Walker, Employee

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Official Response
Hi Tim

I had one of the engineers explain to me the best way to set up AGC-T.  I need to do this in a video, but in the mean time.

  • Select your antenna and pre-amp/attenuation 
  • Select a frequency void of signals based on your RX bandwidth
  • Start with the AGC-T turned up and slowly bring it down until you hear a reduction in noise.  This will only affect the noise, but not the received signal.
That is now the correct setting for AGC-T.  Correct setting of AGC-T should manage this.  I pulled the following from the manual just in case you hadn't had a chance to review it and for others who  might not have seen it.  (page 52)


The AGC-T has 2 modes of attack.  1 mode attacks the short lived hi burst signals like lightning and the other is the more normal slow or fast attack rate.  

Automatic Gain Control (AGC) is a feature which automatically adjusts the Slice Receiver’s audio gain (volume) based on the strength of signals in the receiver’s passband filter. The goal of AGC is to amplify weak signals and attenuate strong signals so that they all lie within a comfortable listening range.

The receiver Automatic Gain Control Threshold (AGC-T) can be adjusted for optimum performance in noisy or quiet environments. The AGC-T sets the maximum gain applied under any circumstances. Since the noise floor is relatively constant on a given band at a given time, the AGC can be adjusted using the threshold control so that the AGC never applies gain to noise, but it will apply gain to signals just out of the noise. In doing so, the AGC can reduce the level of noise you hear, and help signals pop out of the noise.

The AGC system in SmartSDR is a dual track system, meaning that it can track both slow and fast increases in signal strength, making appropriate gain correction decisions in the presence of each. The speed of the AGC (FAST, MED, SLOW) determines how quickly or slowly the AGC recovers after attenuating a strong signal. You can easily hear this by tuning to a CW signal and going through the three settings. On FAST with a strong signal you can hear the gain pump up and down while on SLOW it recovers after a longer pause once the signal stops. When the signal stops, you will hear the noise floor increase as the gain returns
How to Set AGC for Different Operating Conditions

If you are listening to a loud voice signal, AGC SLOW will resist increasing the gain between syllables and therefore reject most of the noise which is at a level far below the signal. FAST and MED provide faster levels of recovery for situations when you want the system to more closely follow the dominant signal in the passband. Any time you have a very strong signal that causes the gain in the AGC to be reduced, you could experience a loss of gain to a weak signal you are listening to. The filter passband edges, which are continuously adjustable, and TNFs can be used to eliminate signals that might interfere with AGC operation.

The operator might prefer to use SLOW settings when rag chewing in a high signal to noise environment where there isn't much QRN and the noise floor is stable. This keeps the gain at more of a constant level that is less distracting. If the operator is trying to pull a weak CW signal out of the noise, they may prefer to use FAST mode to quickly ensure that the long-term average of the noise floor doesn't overcome the signal and prevent it from being heard. MEDium is a reasonable compromise.

When AGC is set to OFF, a fixed amount of gain determined by the AGC-T setting will be applied to both fast and slow signals regardless of their level. The more you increase the AGC-T, the more gain is applied and the louder the signal and noise will be. The benefits of increasing SNR with AGC are lost in this mode. Another disadvantage of turning AGC off is that the operator must adjust the 'RF Gain' manually to avoid distortion due to overload by strong signals.

Operating with AGC turned OFF may be desired by operators who want to avoid having a strong signal drive a weak nearby signal into the noise floor resulting from AGC audio attenuation, such as when operating digital modes.

To adjust the AGC Threshold, tune to a quiet spot between stations. Starting with the AGC-T at a high value adjust the AGC-T slider to the left (lower gain values) until the background noise just begins to decrease. This is the AGC-T "sweet spot" or the "knee" of the AGC algorithm. Depending on band conditions, if the AGC-T is set below 50, you may have to compensate for the loss in audio gain (volume) by increasing the Slice or master AF volume to a higher value. When you get the knee and AF volume adjustment correct for the band conditions, it will keep the volume of strong signals constant which will allow weaker signals to be heard even with AGC in FAST mode. Thus AGC-T is one of the most important adjustments, and often overlooked, to achieve the maximum weak signal receive performance out of the FLEX- 6000 series SDRs.
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Eric - KE5DTO, Official Rep

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Official Response
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.