Flightaware Prostick + AGC behavior

[skip20092017]

I searched threads here and couldnt pick out one where the below was discussed- if you know of any, please hit me with a link to the thread ?

I noticed an interesting phenomena when running gain scripts. During a reboot when returning the config to -10 ( AGC) the stats momentarily soar, hitting over double the message rate and plane count- then stats zero out and climb back to a lower, more expected level.

Its as if the dongle gain throttles way up to the max and then cuts off and allows gain to build back to some software determined level. However, there is no manual gain setting that can attain those message rates. I don’t believe that the current versions support the MAX command.

So that makes me wonder if the 49.6 gain setting we have access to is lower than what the card is actually capable of by a good margin. Also, it appears that the software is using some unknown to me algorithm to set a gain threshold, perhaps based on signal to noise ratio or just a plain ole clip level. In any event its clear that AGC is not bound by the 49.6 limit, but I have no idea what gain it actually uses.

[biekerc]

AGC sets a gain value that is a bit higher compared to what you can set manually. This is actually a kind of a bug, according to a post by @obj.

AGC also ends up setting the highest possible gain value. This is due to the nature of the ADS-B signals, many short messages, compared to the TV signals theses dongles are meant to receive.

@obj

It’s pretty horrible actually…

The R820T tuner has three gain stages: LNA, mixer, VGA.

If you ask librtlsdr for a manual gain value (or “maximum”, which picks the largest manual setting) there is a table that sets fixed gains for each stage based on the requested total gain.

Setting --gain -10 asks librtlsdr to use automatic gain settings in the tuner. For a R820T tuner, librtlsdr uses these settings:

• LNA gain is set to be controlled by the tuner’s AGC
• Mixer gain is set to be controlled by the tuner’s AGC
• VGA gain is set to a fixed value

The tuner’s AGC (in hardware) samples the input power (probably at the tuner output? I can’t remember…) and adjusts the LNA/mixer gain based on that. Unfortunately it is not well suited to ADS-B traffic (which has short bursts and highly variable power levels), it’s designed more for DVB-T signals (continuous signal, stable power level). So what usually happens is that the tuner’s AGC sees what seems to be a very weak signal and so sets LNA / mixer gain to maximum.

So the net effect is that “–gain -10” actually sets a fixed gain (!)
The reason that it can produce better results than the maximum manual gain used by librtlsdr is that the VGA gain setting used in AGC mode is slightly higher than the maximum value you can set with a manual gain (this is arguably a bug, you should be able to access this gain setting manually)

So you should interpret “–gain -10” as something more like “–gain 55” (I can’t remember the exact value, it’s a little higher than the usual maximum of 49dB)

[skip20092017]

Thank you for that. So it seems that what I am observing is some sort of brief inter-stage result where after the Pi boot, the dongle tuner boots and LNA gain and Mixer gain are not yet in sync. Im using the FA Pro + dongle- Do the other RTL-SDR dongles behave the same way on startup ?

Adding that to the pile of things to check out.

[evangelyul]

I witnessed the same every time I change antennas
or unplug / plug the dongle, in the form of a spike.

[skip20092017]

I power the Pi down before plugging/unplugging anything - the dongle itself or the antenna. If I had the abilty to graph the graph would show that" spike", dip to zero then climb back to operational levels. It happens quickly but its still a curve however steep.