[time-nuts] EFC divider resistors

[Bob Stewart]

Hi Nigel,

I didn't give any thought to thermal issues when I bought the resistors.  They're just standard carbon composition.  I see some metal film resistors with a temp coefficient of 5 PPM/C, so I'll get some of those and put them in.

thanks,

Bob





>________________________________
> From: "GandalfG8 at aol.com" <GandalfG8 at aol.com>
>To: time-nuts at febo.com 
>Sent: Friday, January 31, 2014 11:35 AM
>Subject: Re: [time-nuts] EFC divider resistors
> 
>
>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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