[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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