[time-nuts] Tbolt temperature sensor

[SAIDJACK at aol.com]

Hello Tom,
 
it's pretty straight forward to calculate the effects of ground-loop due to  
cable loss; we can calculate this based on the impedance of the wire.

 
>From Wiki (search for AWG table) we get 16.14 Ohms/1000Ft, or 16.14mOhms/  
foot for a 22 Gage wire. We have seen that the OCXO current varies by 4mA over a 
 couple of days. This 4mA generates a ~65 microvolts delta across the ground  
wire.
 
The OCXO we use has about 40Hz delta per 5V change in EFC, or 8Hz/V, thus  we 
get:
 
   8Hz/V * 65microvolts = 516 microhertz change due to ground  loop, or 
~5.2E-011.
 
That's a fairly significant change just due to the ground loop.
 
This change is linear to OCXO current, which we can see  is inverse-linear to 
EFC voltage to keep the OCXO locked. So as the OCXO  current rises, the 
voltage drop across the ground wire would increase, so the  OCXO would see less EFC 
voltage on it's EFC pin.
 
So it could theoretically actually reduce the thermal sensitivity of the  
system, since the OCXO EFC needs to go down with rising OCXO current as can be  
seen in the plots I had sent out earlier!
 
BTW: the unit that was having large phase errors now stays locked within  
-10ns to +13ns, and has a SD of only 3ns over the last 52 hours. The unit now  
had enough time to learn, and is compensating very well now, you can see it in  
the attached plot or online.
 
bye,
Said
 
 
 
 
 
 
In a message dated 2/8/2009 15:07:08 Pacific Standard Time,  
tvb at LeapSecond.com writes:


I  still don't like unintended consequences. Did anyone try the
experiment to  move your OCXO off-board by a foot of cable?

That might help isolate  the root cause of the effect that you're
compensating for. Also, do you  know if all single/double OCXO
show the same correlation of frequency  output change vs. oven
current consumption change? This might be relevant  for anyone
taking a Fury board and using their own external  oscillator.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: gpsstat1.gif
Type: image/gif
Size: 11107 bytes
Desc: not available
URL: <http://febo.com/pipermail/time-nuts_lists.febo.com/attachments/20090208/c5c9cedc/attachment.gif>