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

[Attila Kinali]

On Wed, 29 May 2019 14:53:07 +0200
Attila Kinali <attila at kinali.ch> wrote:

> > 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.

Addendum: It's quite important here that you are aware what you optimize
for. It makes a difference if you use ADEV or MDEV or HDEV. You will
get different cross over point and thud different time constants.
This is especially true, if you use something more complex than a 
first order PI loop.

			Attila Kinali

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