[time-nuts] GPS Locked and Unlocked Performance Comparison
SAIDJACK at aol.com
SAIDJACK at aol.com
Wed Feb 13 19:56:40 EST 2008
Hi Bruce,
I wonder how the analog switches' tempco would affect the performance. They
usually do have 10 - 100 Ohms series resistance if I remember correctly.
The Temex OCXO is achieving STS of parts in E-013, it would be unfortunate
to lose that performance due to external circuitry.
At these extremely small values, I would guess that the finite
off-resistance of the switch, and the leakage currents through the switches' transistors
would also degrade the performance. Maybe a mechanical relay would work
better? But that would cause a mechanical vibration that could jerk the crystal.
Also, I guess one would have to add a small RS-232 micro to decode the
Validity bit from the GPS and do the actual switchover.
Here is an interesting factoid: Magnus mentioned that many GPS receivers do
strange things when they go into holdover. I can only concur to that. The M12+
and M12M are really excellent in this, but other receivers are not and may
give you a couple of 1PPS pulses that are way off, say 200ns or much more
before they shut up. Not so nice.
bye,
Said
In a message dated 2/13/2008 15:09:54 Pacific Standard Time,
bruce.griffiths at xtra.co.nz writes:
It would also be necessary to use an analog switch in series with the PI
filter input resistor and for extended holdover keep the PI filter
feedback cap charged at the DAC voltage to minimise transients on
reverting to normal operation.
Bruce
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