[time-nuts] GPS 1PPS, phase lock vs frequency lock, design

[Attila Kinali]

On Tue, 28 May 2019 18:39:34 +0000 (UTC)
life speed via time-nuts <time-nuts at lists.febo.com> wrote:

> Attila,
> Thanks for taking the time to respond and share your practical experiences in this area.  Your explanations are very helpful, I have a few more questions if you don't mind:
> 1)  Would you elaborate on the "saw-tooth" correction, is this related to the 
> "correction" data made available by the GPS receiver that is in addition to 
> the 1PPS output, which apparently has limited accuracy by itself.

The saw-tooth correction is the error of the PPS signal, as generated by
the hardware, and where it really should be. The clocks of most GPS receivers
are in the order of 20-60MHz and are usually unsteered TCXOs (or even XO for
the cheap ones). Hence the granularity at which the PPS can be generated
is fixed. The saw-tooth correction gives you a higher accuracy (or removes
noise) from what you would get without. The u-blox timing appnote has
a nice graph showing the distribution of the offset of the PPS with and
without saw-tooth correction.

> 2)  I think I understand this.  Further I will have to understand  what the 
> optimal PLL BW is in light of the OCXO short-term ADEV being potentially 
> better than GPS 1PPS with correction.  Perhaps this means the loop BW should 
> be on the order of a milliHertz, if it takes 1000 seconds for the OCXO to 
> drift worse than the GPS.

Usual time constants for OCXO based GPSDOs are in the order of a few 100s
to ~5000s. As a rule of thumb, you set the time constant to the cross
over point of the GPS ADEV[1] to the ADEV of your OCXO. At these time-scales
most people talk of time constant instead of BW of the loop, though it's
the same thing.


> 3)  The nominal design uses a low-noise 16-bit current output DAC, your point 
> regarding LDAC to enable precision (in time) updates to the output would 
> enable a more-rigorously correct loop.  I am considering using two DACs, one 
> scaled to the OCXO tune range of 5V or 10V, one scaled with much smaller 
> range to provide additional bits of resolution, summed to provide the extra 
> bits necessary.

This would also work.

> Does your sigma-delta modulator comment apply to the numbers written to the 
> traditional 16-bit DAC as a dithering technique?  I understand your comment 
> regarding a direct implentation of an SD 1-bit DAC, again dithering the 
> digital input presumably to "smear" quantization noise?

Yes. Exactly.

> 5)  When you refer to "timestamp PPS", is this the technique that is used to 
> extract extra resolution from the GPS signal at the receiver?  Mostly I'm 
> following your (very helpful) descriptions, but some of this terminology is 
> new to me.

time stamp as in "measure the arrival time of the PPS using the local clock
(aka OCXO) as reference". It's the same as a phase detector, but using 
(relative) time as the measured value instead of (relative) phase.


			Attila Kinali

[1] see e.g. http://www.leapsecond.com/pages/m12-adev/
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