FlightAware Discussions

Flightfeeder: Noise Figure vs. gain setting

Retired RF engineer here.

I’ve been slowly adjusting the FlightFeeder Orange box gain in an attempt to get the best results. Like most, just based on the number of ‘overloads’. I’m curious how FA implements the gain function. Does lowering the gain affect the front-end noise figure? Or is it simply an adjustment further down the RF chain where it has minimal impact on NF? I’d like to lower the gain a bit more, but not at the expense at raising the front-end NF.

I guess another question would be “Is there even a FE amp?” or does the antenna feed right into the A/D chip and the NF is determined by the A/D chip?

Sure would be nice to see a cascade analysis of the RF chain! :slight_smile:

Sorry if this has been discussed, I checked around the forum and didn’t see any comments pertaining to this question. Plenty of NF comments, just none for gain vs. NF.

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The Orange FF internally uses a Prostick. All of the variable gain is in the tuner (R820T2).

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I presume you’re running graphs1090?

I also struggle to find “the best” settings for my setup etc…
I know it will always be a balance, but without having a stable reference, it’s very hard to tweak things, unless you use the auto-gain feature by @wiedehopf to achieve a balance.

I would love to get a fast oscilloscope and probe everything in the chain to see what effects what etc :nerd_face:

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It’s a flightfeeder, so no he won’t.

Don’t think front-end NF is really the issue with this setup, main issue is the ADC at some point running out of dynamic range probably.
If you’re anywhere with other receivers there is usually no need for super long range, thus reducing the gain to get the local stuff is exactly what you want.

Before going below -32 dBFS or so for the weakest signal on rtl-sdr SDRs typically not much if any range is lost.
The RSSI will drop lower but typically it’s accompanied with slightly reduced range.

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Technical:
The NF of the receiver chip R820T2 (A/D) is about 3.5 dB, but the FF has an extra amp chip on board, with lower NF (SKY 7150). However, it doesn’t have a SAW filter.
However, longer antenna cables (attenuation) will degrade that NF. I am using a filtered LNA at the antenna and lower the gain at receiver.

This is the FF receiver stick.

Practically the NF and gain don’t make too much of a difference. Too high of a gain can overload receiver, but once you fix that, the ADC dynamic range is more important.

The range limiting factor in most cases are the obstructions at local horizon. The ADS-B signals are fairly strong, when I “lose” a plane is not because the signal dropped below a threshold, but because is suddenly obscured. Line of sight.

Look at the map, for signal levels at the edge of reception versus signals at closer range.
I have max 3dB level drop between 50 miles and 200 miles. Some smaller planes have lower levels close-by than the far-away ones when they disappear.
So it’s not the level that makes them disappear.

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Thanks SoNic67,

Your comment pretty much told me what I was looking for. The LNA sets the NF, and the gain setting likey doesn’t impact the NF much (can’t be positive since I don’t have a cascade analysis). The Skyworks 7150 has a pretty impressive NF!!

wiedehopf - Yep, understand the in the impact of overloading the ADC. I was concerned that by lowering the gain to reduce overloads, that I was potentially impacting the NF which would affect the my distant plane count. With a seperate LNA and the gain setting being inside the R820T2, overall NF is likely still OK, regardless of the gain setting (within reason).

So the takeaway here is that it appears I can lower the gain based on overloads and not be too concerned w/the noise figure being adversely affected. That’s good to know.

A sidebar: In the ‘request thread’, I mentioned it would be nice to see a plot of total RSSI (to spot overloads) on the stats page. Like all w/a FlightFeeder (not a Pi setup), it’s sort of ‘set gain’, ‘wait’, then ‘check overload count’…then rinse and repeat. If we had RSSI stats for a 24 hr period, that would be nice to see to help set gain. I should add, the FlightFeeder is in an awkward location, so seeing the info on the stats page would be a lot easier for me that checking the FlightFeeder itself. I guessing the Pi guys probably can already do something like this.

Thanks to everyone that chimed in, much appreciated!

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Myself I got a higher Dynamic Range ADC converter, in the Airspy receivers.
With a LNA filter up front.

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