AGC again Not AGC-T

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  • Updated 5 years ago
I have asked about this before but all you folks do is explain how to set up AGC-T

I don't have a problem with that.  It seems that when I am listeniing to a weak station and then a

strong station comes on the volume gets so loud and a the agc does not cut back     it makes it very uncomfortable trying to ride the volume control up and down   appreciate any help with this    I tried my Icom 7600 on the same stations and it works great.  I am using the 6500


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Posted 5 years ago

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Photo of Tim - W4TME

Tim - W4TME, Customer Experience Manager

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The AGC system in SmartSDR is a dual track system; meaning that it can track both slow and fast increases in signal gain, making appropriate gain correction decisions in the presence of each.  The speed of the AGC (FAST, MED, SLOW) sets how quickly or slowly the AGC recovers after a strong signal.  You can easily hear this by tuning into 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.

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, then, provide faster levels of recovery for 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 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 rate that is less distracting.  If the User 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, this means that 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 adjust the AGC-T to the right, the more gain is applied and the louder the signal and noise will be.  The benefits of increasing SNR with AGC will be lost in this mode.  The other down side of turning AGC off is that the operator must to pay attention to the 'RF Gain' manually to avoid distortion due to overload by strong signals. 

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

The Automatic Gain Control Threshold (AGC-T) adjustments are located with the slice audio controls; it is the bottom control in the audio panel.  Each slice receiver can have its own settings for the AGC timing and threshold.

The proper setting for the AGC threshold is determined by the band noise and the signal strength of the desired signal.  Reducing the threshold can significantly improve the signal-to-noise ratio of the desired signal.  Increasing the threshold will increase the gain in the receiver, which is sometimes necessary for weak signals.  Properly adjusted, you can virtually eliminate band noise and have a very clean signal that is very pleasant to hear.  

To adjust the AGC Threshold, tune to a quiet spot between stations and 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 "knee" of the AGC algorithm.  When you get the knee adjustment correct for the band conditions, it will make the signals come up out of the noise, even with AGC in FAST mode.  This 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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Thanks everyone that responded

I will give all a try

Photo of Alex - DH2ID

Alex - DH2ID, Elmer

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Thanks, Tim. Best explanation found here.
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Thanks Tim, GREAT explanation.