[time-nuts] GPS W/10KHz

[Chris Albertson]

On Mon, Feb 10, 2014 at 2:37 PM, "Björn" <bg at lysator.liu.se> wrote:

> The sawtooth correction is the difference between where the receiver would
> wish to place the edge and where its known limited resolution electronics
> lets it put the edge.

I've plotted the sawtooth function reported by my older Moterolla
receivers.   It does not look like it would be hard to apply to a
GPSDO.

What you have to do is model how an known error in the GPS' PPS signal
would effect the value you send to the DAC and take that out.  If we
are using a PID algorithm to control the DAC,  I think the sawtooth
error could be used to adjust the I and D terms.    Yes you could push
the sawtooh signal forward to drive some kind of analog hardware but
that is not easy and limey expensive.  Using the sawtooth to adjust
the I and D terms is nearly free.

The trouble is molding this and finding away to test.  You'd need a
clock that is much better than a GPSDO to test this and most of us
don't have that.

I makes some sense, a sawtoth that has been positive for some number
of second really should be used to adjust the integrated error (i
term) and the rate that the sawtooth moves is a directive of the error
or d term.     The phase error that you measure every second between
the PPS and your OCXO is the P term.  But mostly this is just a one or
maybe two bit value. so we tend to integrate them over time, sawtooth
should tell you the error in that integration.

-- 

Chris Albertson
Redondo Beach, California