Audio AGC questions

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WA9ZVF
Posts: 3
Joined: Sat Jul 28, 2018 8:44 pm

Audio AGC questions

Postby WA9ZVF » Wed Oct 10, 2018 8:21 pm

Given that there are multiple AGC loops, things get confusing. The following questions are regarding the Audio AGC.

The Rx settings box has parameters for Fast to Slow AGC. Could someone elaborate on these parameters.

What specifically are Attack, Hold, and Release and what are the units?

What is a slow release. A small number or big number?

The EX Control window has a Threshold setting for AGC. Is this the Audio AGC? How does the threshold work along with attack, hold and release?

For CW and SSB what are "reasonable " settings to start with for these parameters?

It would be great if someone knowledgeable did a YouTube or some type of documentation on the audio agc.

Thanks,
Ray Mikula
WA9ZVF
Last edited by WA9ZVF on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Roger
Posts: 139
Joined: Fri Dec 29, 2017 7:59 pm

Re: Audio AGC questions

Postby Roger » Sun Oct 14, 2018 7:46 pm

Ray,

You are correct; there are two AGC loops in the RSP. The first is an IF AGC loop which ensures that the A to D converters (ADC) are getting the best signal level possible which does not cause ADC overflow. The output from the two ADC streams (I and Q) is then digitally downconverted to the frequency selected by the user and sent to the demodulator (i.e. SSB, AM etc.) software in the host PC. The output from the demodulator software is digital audio but the level will be low for weak signals. That is where the Audio AGC comes into play.

The role of the Audio AGC is to "level out" the signals coming out the speakers or headphones. In order to do this in an SDR receiver low level signals must be digitally amplified and strong signals maintained at close to their original level. However we don't want the gain change to be instantaneous because of the way signals are received and perceived by the operator. For example lets say two hams are having a conversation on 20M - one is weak and the other is strong and you are listening to the conversation. If the strong signal was being received and then then transmission stopped there would be a large jump in noise coming out of the speaker until the other ham started transmitting. Once the weak ham started talking there would be another instantaneous gain change. From this simple example you can see that there needs to be a way to gradually change the gain based on the type of signals being received and their levels.

The strategy used is to have an attack time, delay time and AGC threshold (" knee") as parameters in the amplifier feedback loop. From the 2017 ARRL Handbook...

There are two primary AGC time constants. AGC attack time describes the time it takes the AGC system to respond to the presence
of a signal. AGC decay time describes the response of the AGC system to changes in a signal that is present. The optimum time
constants for the AGC system depends on the type of signal being received, the type of operation being conducted, and the operator’s preference.


These are discussed further in the following articles....

https://www.qsl.net/va3iul/Files/Automatic_Gain_Control.pdf
http://www.cqham.ru/forum/attachment.php?attachmentid=243878&d=1474916250

Some SDR receiver software (SDR-Console) allows the user to set the attack, delay and AGC threshold. SDRuno has 4 settings (Off, Slow, Medium & Fast and an AGC level. Slow, medium and fast refer to the speed at which the AGC responds (attacks) to signal changes. Unfortunately there is nothing in the manual which describes these settings in detail and the user is left to experiment to determine what works for them. It would also be nice to know something about the AGC level control.

The choice of whether to use slow, fast or medium depends on the type of signal (CW,AM SSB etc.), background noise, nearby signals, fading conditions and personal listening preference. Here is a debate on another group about effective use of AGC settings.

https://groups.google.com/forum/#!topic/rec.radio.shortwave/RUqV2Dl9bgA

Hopefully SDRplay tech support will join in this discussion and provide more details on the settings in SDRuno.

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