I just checked the frequency offset of the Ettus B210 with internal and external reference using the 23 cm beacon DB0VC which is obviously locked to GPS or another reference signal. A fellow ham did check the beacon frequency with his GPS-locked sig gen like I did with my CMU200.
With the external reference the B210 offset is +2,7 kHz on 1296,920 MHz and of course +900 Hz on 70 cm.
With the internal reference the offset is -80 Hz on 1296,920 MHz.
I don’t know how I came to the 950 Hz offset on 70 cm. Maybe a typo… 95 Hz!
Yesterday I checked the offset with SDRConsole which to my surprise has a switch to enable the external reference of the B210.
I use the Precision GPS Reference Clock by Leo Bodnar. No problems with my OpenHPSDR-TRX and Kuhne XVTR.
I will check the B210 a with reference source at work on tuesday.
So, it mean, your B210 has internal reference not worse then GSPDO? How, then, can be explained the measurement results with external reference? IMHO, there is an mistake in the interpretation of the measurement results.
What I figured out is that seemingly I have a problem using the external reference depending on the uhd firmware version. I will do some more testing during the next days, before I do write more comments. I don’t want to worry others more than neccessary