Airspy Mini vs R2 Notes

It's not that well known, but the Airspy Mini can actually run at the higher sample rates usually reserved for the Airspy R2 (e.g. 10 MHz IQ / 20 MHz real-mode sampling). The official drivers for the Airspy don't allow running the Airspy Mini at these rates, but when using KA9Q-Radio, we can just specify samprate = 20000000 and off we go..... with some caveats!

The main issue is spurs. The Airspy Mini is a cheaper SDR - it uses a cheaper clock reference, and I think it has less power supply filtering.

The following plot shows the noise floor of an Airspy Mini vs R2 running at 20 MHz, both running at maximum gain (lowest noise figure) and terminated with a 50 ohm load:

The noise floor is generally higher on the Mini (1dB or so), and there are some obvious repeating 'bumps' in the spectrum along with some large-ish spurs. What impact this will have on you really depends on your local RF environment. If you don't have any sondes near where the spurs end up, then this is probably a non-issue. If an external preamp is used, the Airspy will be running at a lower gain setting, so the slightly higher internal noise will probably not have an impact either.

As a bit of an example, here's a spectrum plot of an Airspy Mini running at maximum gain, terminated vs plugged into an antenna at my house:

I'm already limited by my local noise, so that slightly higher terminated noise floor really doesn't matter!

So... what to make of all this?

• In most situations, the Airspy Mini is probably fine for covering the entire radiosonde band.
• If the spurs end up over where a radiosonde is, you can try adjusting the VFO frequency a little, as the spurs should remain fixed relative to the VFO frequency. e.g. change frequency = 407m0 to frequency = 407m1 to shift the spurs by 100 kHz.
• If you want to try and optimise out the very last dBs of performance, then consider an Airspy R2.