How does the Automatic Gain Control (AGC) work in SmartSDR

A Description of the SmartSDR AGC

Automatic Gain Control (AGC) is a feature that automatically adjusts the slice receiver’s audio gain (volume) based on the strength of signal levels 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) should be adjusted for optimum performance in noisy or quiet environments. The AGC-T sets the maximum gain (volume) applied under any circumstances. Since the noise floor is relatively constant on a given band at a given time, the AGC should be adjusted using the AGC Threshold control (AGC-T) so that the AGC never applies gain to or increases the volume of the band noise, but it will apply gain to signals that are slightly greater than the noise. In doing so, the AGC can reduce the ambient 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 first mode tracks and addresses short-lived, impulse type signals like lightning crashes very quickly while the other track mode is the more normal slow or fast attack rate based on changes to longer duration signal levels.  Both tracks work in a similar fashion, but each is targeted at different response times. This helps prevent fast impulse type signal spikes from dominating a slower changing signal (such as a sideband signal).

The speed or aggressiveness 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. With the AGC set to FAST with a strong signal, you can hear the gain (volume) pump up and down while on SLOW it recovers after a longer period once the signal stops. After the signal stops, you may hear the noise floor volume level increase as the gain returns.

The AGC in SmartSDR does not increase signal volume based on a slope factor.  We use a flat response. There are several implications to this. First, if two independent signals are evaluated one-at-a-time such that each of them is strong enough to hit the target level without hitting the maximum AGC gain, they will play at exactly the same audio level (i.e. the target). This is the AGC doing its 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 receiver passband (filter bandwidth), 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.

The AGC Threshold (AGC-T) slider control 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 volume level (if you want to try this, just slide the AGC-T all the way up to the highest setting -- it isn't pleasant).

Band conditions can also affect this since what is being adjusted is the maximum gain against the input signal. So if the ambient level of noise changes, this will definitely change how things sound.

Thus AGC-T is one of the most important adjustments to make for optimal reception.  It is used to achieve the highest SNR (signal to noise ratio) providing the maximum weak signal receiver performance out of the FLEX- 6000 series SDRs. 

Also, the AGC-T should be properly adjusted before using any of the other noise mitigation features such as the Noise Reduction (NR) and Noise Blanker (NB).  Eliminating excess band noise from the received signal is key for these two noise mitigation features to function optimally.  You may find that using the AGC-T alone will mitigate band noise to such a low level that NR and NB may not be needed. 


How to Adjust the AGC-T for Optimal Reception

There are three primary steps for optimizing the receiver signal to noise ratio and should be done in the following order:

  • Set the RF-Preamp to the correct setting for your location, antenna type, and band conditions.
  • Set the AGC Speed or "Aggressiveness" Mode
  • Adjust the AGC Threshold (AGC-T)

How to Set the RF Preamp for or Your Location, Antenna Type, and Band Conditions.

In general, once you set the RF Preamp for a particular band and antenna, you generally do not have to reset it unless band conditions change drastically.  Please use the procedure outlined in the HelpDesk article How to determine the amount of RF Preamp gain to apply for band conditions for setting the RF preamp for your FLEX-6000.

How to Set AGC Speed or "Aggressiveness" Setting 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 (volume) 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 (SNR) 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 for the listener.

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. Any benefits of increasing SNR with the AGC are lost in this mode of operation.  Another disadvantage of turning AGC off is that the operator must adjust the AGC 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.  It should be noted that is using the AGC OFF setting, you should significantly reduce the AGC-T value so that minimal gain is being added to the recovered signal's volume.

Adjusting the AGC-T (AGC Threshold) Setting for Optimal Reception

The following example is provided to convey a better conceptual understanding of why properly setting the AGC-T is important.  And remember, you will be changing the AGC-T more frequently as you change bands or band conditions change.

As noted previously, the AGC-T setting determines the maximum gain that can be applied to any signal within the receiver's passband filter.  This is known as the AGC Window, the area that encompasses the signal level strength where the AGC applies gain or volume. 

When the AGC-T is properly set using the procedure below, the AGC window, the minimum and maximum signal strengths where the AGC is applied is applying gain to primarily signals and not band noise as shown below.  


However, if the AGC-T is set too high, in addition to having the AGC apply gain to signals, it also applies gain to band noise from the noise floor because the AGC Window includes signals and band noise.


To adjust the AGC Threshold, do the following:

  • Check the AGC Speed setting for the mode and band conditions.  When in doubt, start with MED (medium).
  • Tune the slice receiver to a quiet spot between stations where you are listening so that you are only hearing the noise floor.
  • Starting with the AGC-T at a high value; 50 is a good starting point.  Adjust the AGC-T slider slowly to the left to lower threshold values until the background noise level just begins to decrease and then set it a little lower.  Make the adjustments slowly and wait a little bit after making a change so the AGC has time to react to the changing threshold levels.
  • Once you find the threshold level where the volume of the band noise starts 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 25, you may have to compensate for the loss in audio gain (volume) by increasing the slice or master AF volume to a higher value.  For very quiet bands, the AGC-T may be as low as 15 or 20.
  • Tune the slice receiver back to the frequency where there are signals of interest present.

When you set the AGC-T and AF volume adjustments correctly for the band conditions, it will keep the volume of strong signals constant which will allow weaker signals to be more easily heard even with AGC in FAST mode.

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