[time-nuts] New NTBW50AA

[Charles Steinmetz]

>I still get generally about a 400 ppt swing with 
>the OSC, about 20 ns swing with the PPS and the 
>other things are just what they are. Different 
>filters and surveys don't seem to change anything that I've seen.
>      *   *   *
>Is this acceptable?

You seem to have a reasonably decent view of the 
sky, judging by the numbers of satellites you are 
able to track, but the signal quality is lower 
than I expect (note: my only Trimble GPSDO 
experience is with TBolts).  I'd expect the 
signal reports to be mostly from 40 dBc upwards, 
while yours run from low 30s to barely 40 (IIRC, 
this hasn't changed materially as you've moved 
the antenna multiple times).  Your results look 
consistent with those signal readings.  I don't 
recall -- have you tried a different 
antenna?  It's also possible that the front end 
in that particular GPS is noisier than the TBolts I'm familiar with.

Also, I see that the oscillator loop gain is set 
for 1.4 Hz/v.  I have no idea what is normal for 
the OCXOs used in that unit.  Have you verified 
this yourself?  (Turn disciplining off, then 
manually run the DAC voltage up and down while 
watching the frequency on a counter.)  Lowering 
the GPSDO loop gain may improve matters.  Or 
not.  (IIRC, this may mean setting "gain" to a 
higher number -- the "gain" does not SET the loop 
gain at that value, it tells the digital loop 
"expect the oscillator to have this EFC characteristic.")

>I did figure out the temp thing and found a link 
>where it's explained, but I have a new chip 
>coming and expect to change it, but I doubt if 
>anything will change except the reported temp 
>will most likely track to less than 1° C unlike it's doing now.

That would be my expectation, too.

>Would it be worth looking into the MV89A OCXO as 
>a replacement to get better ppt?  I haven't done 
>any looking into my OCXO yet to see what it's spec sheet may be.

That depends on whether or not the remaining 
errors you are seeing are attributable to the 
oscillator.  The remaining errors can be classed 
broadly as (i) noisy/wandering OCXO, and (ii) 
adjustments to the DAC that the GPS thinks it 
should make.  In your case, they look more like 
class (ii) than class (i) to me.  Before you 
switch out the OXCO, let the crystal that is in 
the unit now settle down for a few months of 
uninterrupted running.  The class (i) errors 
should go down by themselves.  If the errors you 
see do not go down, it is more evidence that your 
limitation is noise in the GPS, not the OCXO.

Best regards,

Charles