[time-nuts] TIC resolution impact on GPSDO's performance
SAIDJACK at aol.com
SAIDJACK at aol.com
Mon Dec 25 14:20:06 EST 2006
In a message dated 12/25/2006 06:18:57 Pacific Standard Time,
df6jb at ulrich-bangert.de writes:
In my own GPSDO i discipline the LO's pps to have a MAXIMUM phase offset
of 500 ms against the receiver's pps. That avoids lots of ambiguity
problems and leaves the widest phase measurement range to make the loop
even lock when the OCXO is initially far apart its setpoint.
Otherwise you are correct that one can optimize for different
parameters. This gives immediate rise for the following publications:
1) A guide to discipline your frequency source to a phase source with
minimum phase offset
2) A guide to disipline your frequency source to a phase source with
minimum frequency offset
3) A guide to discipline your frequency source to a phase source with
best phase stability
1) A guide to discipline your frequency source to a phase source with
best frequency stability
If you can deliver or send some links: I will surely devour stuff like
this!
Best Regards
Ulrich Bangert, DF6JB
Hi guys,
this brings us to an interesting discussion: what is more useful:
* best frequency accuracy
* or best phase lock (minimum 1PPS phase offset to UTC).
We have one customer at Jackson-Labs telling us most implementations prefer
frequency accuracy over phase accuracy since these units are often used in the
field to drive frequency counters, signal generators, and receivers etc,
that don't require tight phase offsets, but rather need a very accurate
frequency.
It's a trade off of short-term frequency stability (lower for best phase
performance) or long term frequency stability, with longer phase drifts away
from UTC but better overall frequency accuracy.
I suspect the time nut's would rather have a very accurately locked 1PPS to
UTC and don't mind larger short term frequency offsets needed to steer the
1PPS onto the UTC 1PPS.
When doing frequency measurements, one would prefer to always have the most
accurate frequency possible.
Two different settings in the loop performance are required here. On our
Fury GPSDO, we allow the user to set the loop parameters by hand (with some
suggested defaults) to optimize the unit to be either tightly locked, or more
loosely locked to UTC. We also have two OCXO options available to address the
problem: a double oven high-stability unit that can be much more loosely
coupled, and a more cost-effective single oven that wants to be tightly controlled
by the GPS with higher short-term frequency wandering.
On top of the ADEV loop time constants cross-over point discussion Ulrich
described, it's also a question of how much UTC (as delivered by the GPS) phase
offset these units can achieve in say 1-Sigma measurements (<5ns and <<15ns
respectively typically).
bye,
Said
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