[time-nuts] Fury Interface Board: 5MHz needed?

[Bruce Griffiths]

Hal Murray wrote:
> [from two separate messages]
>
>   
>> Any ground loop error above about 10 - 50 microvolts would affect the
>>  systems' accuracy on good double oven OCXO's.
>>     
>
>   
>> George for a single ground pin OXCO you might want to consider
>> compensating  for the oven's internal ground bounce. The voltage drop
>> caused by a normal  heater current is around 1/4 to 1/2mV for an MTI
>> 260 oven.
>>     
>
> I'm missing something.  MTI has been in this business for a while.  I'm 
> assuming they aren't dumb.  Why are they still making (otherwise) very high 
> quality parts with only a single ground pin?
>
> I'm not picking on MTI.  They are just a handy example.  Many of the data 
> sheets I've looked at have only one ground pin.
>
> My first guess is that it's a backwards compatibility mess.  Ages ago 
> somebody made a part with a single ground pin and somebody used it in some 
> telco gear and everybody has been using the same footprint ever since.  If 
> that's the case, I'd expect there would also be a variation with a second 
> ground pin to get better performance.
>
> How much better would their part be if they had a second ground pin?
>
> Are they running temperature tests with the EFC pin open so they don't see 
> any problem as the heater current changes?  (or something like that)
>
>
>
>   
Even the FTS1200 only has a single ground pin.
However the heater current is a function of ambient temperature, so it
is possible to correct for the variable ground voltage drop by sensing
either the ambient temperature or even better the supply current and
having the microprocessor "learn" how to correct the EFC voltage for
this effect.

>From Peter Schmelcher's earlier post:

Normal ground drop due to heater current is around 0.5mV max at room
temperature.
This corresponds to about a frequency off set of 1.5 -4.5 E-11 depending
on the model.
Since the heater current could be 5x this at low temperatures the
corresponding frequency offset will be around
7.5- 22.5E-11 depending on model (neglecting the decrease in the wiring
resistance as the temperature drops).

Since the temperature drift is 1-2E-9 the effect of the varying ground
voltage drop is 5 -10 x smaller, it doesn't really matter whether the
EFC pin is grounded or not during temperature testing.

Bruce