[time-nuts] Question on crystal jumps
SAIDJACK at aol.com
SAIDJACK at aol.com
Thu Oct 23 16:25:44 UTC 2008
Hi Bob,
they should and do, and if it jumps the Rb will pull-in the OCXO frequency
correcting for the error. This pull in time is determined by the loop time
(cut-off frequency) of the Rb PLL loop. I expect this loop time to be much
shorter than a GPSDO's loop time, so the effect is probably much less visible. One
should still be able to see it as a distinct offset when plotting the EFC
control voltage of the OCXO.
Considering that jumps often hit 1ppb offset, I am surprised how many folks
on this forum have discussed that their oscillators jump, and how little
research actually has gone into figuring out how to minimize these jumps. I would
not be a big fan of contraptions that use two or three OCXO's to minimize
the impact since all three could potentially jump at the same time. I would
rather attack the problem at the source - most of the time its the crystal.
There is probably something that can be done to minimize jumps, such as the
crystal cut, modifying the crystal operating temperature, modifying the
current through the crystal, comparing different mounting methods, changing the
direction (orientation) the crystal is mounted in, etc etc.
Here is a hint for experimentation: I once had an OCXO that jumped all the
time (many times per hour). Since this was a reject part (definitely a factory
RMA), I took a large screwdriver and gave the OCXO a good bang. The jumping
was immediately mitigated down to a couple of times per day. That causality
was a real eye opener, whatever stresses were on the crystal were released by
the shock.
bye,
Said
In a message dated 10/23/2008 08:39:18 Pacific Daylight Time,
quenbob5 at pacbell.net writes:
So why would they not show the same kind of jumps as plain OCXO's? Rick K.
mentioned atomic standards avoid jumps.
Bob Q.
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