The airspy SDR has a somewhat temperature stabilized oscillator, I say somewhat because it needs re-calibration every time you turn it on. So how do you do that? I rely on 4 or 5 frequency standards on the HF and the VLF, decode them in CW and I interpret the spectra. You can adjust the PPM correction of the airspy and the spyverter separately, and this procedure needs to be repeated once per day or so if you want any accuracy. Some references for the time standards are (there are more of them, but I use these):
- DCF77 is at 77.5 kHz : https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77.html
- RWM is on 14996 9996 and 4996 kHz : http://www.irkutsk.com/radio/tis.htm
- MSF is at 60 kHz http://www.npl.co.uk/science-technology/time-frequency/products-and-services/time/msf-radio-time-signal
The SNR for all time standards is between 30 and 35 dB, occasionally there is some QSB. Below you see the observed spectra as seen on 19-nov-2016 at 8:30.