[TangerineSDR] Tweaking rubidium

[Phil Karn]

As I mentioned on our zoom call this past evening, I'm having fun 
comparing a ERC 130 Rb oscillator to my Leo Bodnar GPSDO.

I've tweaked the Rb so it's within about 2.5e-11 of the GPSDO but I see 
regular step changes to the frequency difference with a period of maybe 
a minute10 sec or so. I suspect this may be cycling of a OCXO heater. Or 
maybe it's the tracking algorithm in the GPSDO. Or maybe both?

I can see it gets tricky at this level. Still, it's fun to see 
cumulative 10 MHz phase differences of only a couple of degrees after 5 
minutes or so.

Phil