[USRP-users] Using GPSDO with gps lock

Marcus D. Leech patchvonbraun at gmail.com
Sat Feb 23 17:41:28 EST 2019

On 02/23/2019 04:04 PM, Sirkin, Joshua F. via USRP-users wrote:
> I had a question about the TCXO GPSDO module for the B200/B210.  I am interested in using it without a gps lock as a better clock than the tcxo that is internal to the bare board.  I am only interested in frequency stability, not time of day.   If I did use it in this way, I had a few questions:
> 1)   The spec on it says that it is 75ppb over temperature in unlock condition.  That is the only data point I can find about using it unlocked.  Do you have any better numbers on what should be expected for frequency stability cold booting the device without ever giving it any gps if I select my reference source as external?
What numbers are you looking for?

> 2)   Is the ref_locked sensor value for aligning the GPSDO to a gps reference?  Can I skip the ref locked check if I am not using gps?
The "ref_locked" sensor tells you when the reference PLL has achieved 
lock to an external source.  This usually happens within milliseconds
  of the "external source"  (whether that's the GPSDO or an external 
10MHz reference) being present.

> 3)   I believe I read somewhere that the GPSDO uses the same 10MHz lines as the REF in.  What happens if I have a 10Mhz source hooked up while the gpsdo module is installed when I do not have a gps lock?  What would happen if a 10Mhz input was hooked up while I did have a gps lock?
There are electrical cross-talk issues between the GPSDO's 10MHz line 
and the external reference line.  So, if you plan to use an external
   reference, you should remove the GPSDO module from the board.

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