AirSpy vs FA SDR with Parallel RF Path
I set up parallel systems to compare the performance of a 12 bit AirSpy mini and an 8 bit RTL-SDR.
A normal FA omni antenna feeds a cavity bandpass filter and then an LNA. From there the RF signal is split in a microwave power divider with > 25 dB isolation between the 2 output ports. One port drives the AirSpy, the other the RTL-SDR.
The two SDR dongles each are connected to two Raspberry Pi 3B+, each running 3.7.1. Each of the two Pi is connected via Ethernet for FA access.
The gain settings for each SDR were adjusted over time to maximize the received signal count.
The Airspy averaged about 1400 messages / second in busy times, while the RTL-SDR tended to run about 1100 at the same times.
I plotted the total aircraft count for the two systems for several months. The Airspy side consistently performs better in both total message count and total AC seen.
Here’s a 30 day snapshot of aircraft counts:
The AirSpy channel has about 6% more aircraft on average.
I’ve since shut down the parallel path and the AirSpy channel performance has improved some more. Power supply noise reduction at the LNA seems also to have helped by about 1%.
1 Like
Which Airspy model are you using?
In my case the numbers are closer.
Yesterday:
Airspy mini: 3,018 planes 479,924 positions (gain at #20)
FA Pro dongle: 3,003 planes 474,221 positions (gain at 15dB)
1 Like
What about the sample rate, is it at 12 or 20?
For me, with the sample rate at 20, the Airspy will get about 10% more planes as shown below (transition day Aug 10th):
If you subtract the somewhat useless “Other” aircraft and possible fluctuations, it’s still 5% probably.
Anyway with an RPi 3B+ and good range, he’s most likely running 12 MHz, 20 MHz doesn’t run well on the 3B+ with that many planes.
The 4 day bump at “other” is probably when I had DX-mode enabled I guess
Even without DX, the “other” count will go up a bit with 20 MHz.
But even if it’s legitimate aircraft, comparing the number of aircraft you actually get positions from makes more sense.
The sample rate for the test was 12 MHz.
Mine is at 12MHz too… Just a Pi3 here.
For now.
Thanks for posting this, it’s good to see your results. This 6% makes it worthwhile for me.
Instead of getting corrupted files with your Pi4 you should use it for the Airspy and go to 20 MHz 
I do have more than one Pi4 
But you’re right, I need to get this sorted. I’d really rather wait until the fix hits mainstream release before I do the swap though.
There was an update, i would assume it’s in there.
Just do an update and plug in an rtl-sdr receiver.
Try rtl_test, should tell you if it’s working.
Also for power you’ll now need some adapter i guess.
Or just supply the IO pins directly if you trust your power supply 
Good stuff, that’s my plan for the next week then. Power should be fine, all I need to do is use the little adapter I have for micro-usb to usb-c.
It is and running fine
Linux ADSB-pi4 4.19.66-v7l+ #1253 SMP Thu Aug 15 12:02:08 BST 2019 armv7l GNU/Linux
