[time-nuts] EFC divider resistors

[GandalfG8 at aol.com]

The problem is certainly real enough.
 
What you're seeing is a drift of approx 2500ppm, and Vishay shows the  
temperature coefficient of their standard carbon film resistor, for  example, to 
be approx 200ppm per degree C for a value of 20K.
 
If yours is carbon film this would imply your measured resistor is  seeing 
a temperature rise of around 10 or 12 degrees C, which might be a  bit on 
the high side but is certainly feasible given a body temperature of  around 35 
degrees C.
 
Standard metal film resistors have a lower temperature coefficient, say  
around 50ppm per degree C, so this might be one option for reducing the 
effect,  another could be temperature control of your EFC network.
 
There are more specialised resistors with much lower temeparature  
coefficients, the Vishay RCME series of metal film resistors for example can be  as 
low as 5 to 6 ppm per degree C, although I don't know what the cost  
difference might be.
 
And no, I don't have any shares in Vishay:-)
 
Regards
 
Nigel
GM8PZR
 
 
In a message dated 31/01/2014 17:04:24 GMT Standard Time, bob at evoria.net  
writes:

I put a  divider network in the EFC line of my GPSDO to restrict the OCXO 
range to  2Hz.  Now I'm seeing heat-related drift that wasn't apparent  
before.  I put a 20K resistor from the same strip on my 3456A, and the  warmth of 
holding it between fingers moves it by about 50 ohms.  What  type of 
resistors should I put in there?  Or am I chasing a problem that  doesn't exist?  
Totally out of my league here.

Bob -  AE6RV
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