[time-nuts] Some observations of the BG7T BL GPSDO (John Miles)

Attila Kinali attila at kinali.ch
Fri Mar 27 07:24:08 EDT 2015

On Thu, 26 Mar 2015 18:57:25 -0700
"Willis" <willis.hendley at gmail.com> wrote:

[u-blox LEA6-T]

> Is it possible that this also has a systematic offset or is this just 
> contributing to the observed noise level around the mean? 

No, the PPS is calculated from the time send by the satellites.
In worst case, the PPS is off from the real GPS second by transmission
delay (modulo calculation errors). As the error is bounded, the phase
offsest is bounded, which means the (average) frequency offset is zero.

> Being new to the 
> concept of GPSDOs I have no idea how much these intrinsic errors in the GPS 
> receiver itself bleed through to the performance of the output frequency 
> from the OCXO - or is it all masked by the long time constants and filtering 
> in the Software PLL loop?

Yes. Most GPSDO's use a PLL, measure the phase of the oscillator
relative to the PPS. There is at least one design that i know of,
which meassures the frequency, thus implementing an FLL. Of course,
this results than in a slight frequency error.

			Attila kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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