[time-nuts] GPS Locked and Unlocked Performance Comparison

[Bruce Griffiths]

SAIDJACK at aol.com wrote:
> 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
>  
>   
Said

If your worried about analog switch tempco then the classic solution of
using an noninverting buffer after the selection switch works well.
The tempco of the analog switch in series with the PI filter elements is
relatively insignificant in that it only affects the PLL natural
frequency and damping and not the filter dc output.
Leakage current shouldn't be a problem in the selector switches, unless
the switches get really warm.
A custom discrete switch could be used to eliminate leakage problems by
ensuring that the voltage across the last off switch in the switch chain
is very small.

One could always use the output of the tracking ADC to drive the EFC
input, disabling tracking during holdover and reenabling when the GPS
reference is again stable.

If one monitors the message containing the estimated variance of the PPS
output one may be able to circumvent such effects.

Bruce