[time-nuts] Effects of filter delay

SAIDJACK at aol.com SAIDJACK at aol.com
Sun Dec 24 19:49:13 EST 2006


 
In a message dated 12/24/2006 15:05:52 Pacific Standard Time,  
bruce.griffiths at xtra.co.nz writes:

SAIDJACK at aol.com wrote:
> Hi Bruce,
>  
> yes  the heater circuit is problematic, and will affect stability a   lot.
>  
> For example, some OCXO's don't have separate  (Kelvin sensed) ground pins  
for 
> the heater, and EFC  voltage.
>  
> The result is that any changes in the heater  current will cause a voltage  
> change in the ground pin of the  OCXO from the OCXO body to the PCB ground 
plane  
> (due to the  finite resistance of the ground pin).
>  
> This I2R loss is  very small, but will superimpose onto the EFC voltage, so 
 
> it's  as if this voltage is placed onto the DAC output.
>
> We have  measured this change of voltage to be in the order of 10's of  
>  microvolts for some OCXO's, and that translated into 10's of EFC DAC  
steps  worst 
> case (during power-on for example when the heaters  are running at maximum  
> current).
>  
> Again  the problem is not the steady-state heater current, but changes in   
> heater current due to thermal tracking, and loop stability of the  heaters, 
this  
> in turn causing changes in EFC voltage. We are  talking about 1E-012 or  
better 
> being desirable stability, so  these marginal effects almost sudden become  
a 
> real  issue.
>  
> The best approach is to separate the ground  return current of the heater  
> circuitry completely from the DAC  ground circuitry. But that's not 
possible on  
> many  OCXO's...
>  
> Bye,
> Said 
>  _______________________________________________
> time-nuts mailing  list
> time-nuts at febo.com
>  https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>
>    
Said

I guess this means you have to model the effect of the changes  in IR 
drop due to heater current changes and modify the EFC voltage to  compensate.
The complicating issue being that the value of R will vary with  ambient 
temperature (copper has a resistance tempco of about  0.4%/K).
Since the a compensation accuracy of around 1% or so will suffice,  
analog compensation circuitry could be used, however it would have to be  
adjusted to suit individual  oscillators.

Bruce



Hi Bruce,
 
well, there are two things going for us: the required offset for any given  
temperature typically does not vary over time (if we compensate for aging),  
and the GPS disciplining can give us an absolute reference of EFC versus temp  
required to get an exact frequency. These facts allow us to compensate without  
an underlying understanding of the exact parameters of the hardware (which  
change board-to-board).
 
So for any given temperature, if we are well locked to the GPS, we can get  
the required absolute EFC setting for that particular temperature (again this  
only works if we compensate our data for aging).
 
This temperature-to-EFC offset typically does not change much over time,  and 
is well correlated, e.g. there is a nice curve of the requied EFC  offset 
versus temperature for a given output frequency.
 
The advantage of this method is that it also measures and compensates for  
all other tempco's such as DAC and reference tempco, PCB tempco, etc!
 
The closer this compensation is to the perfect values, the better the  OCXO 
will behave in the face of temperature changes. Of course the overall error  
will eventually be minimized by GPS Disciplining anyway.
 
bye,
Said
 
 
 
 
 
 
 
 


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