[time-nuts] EFC divider resistors

[Robert LaJeunesse]

Jellybean resistors can have a tempco in the 100s of PPM per degree C. Some precision resistors are as low as 25 PPM/C while really good resistors can be had at 5 PPM/C. Better yet look for a resistor array where part tracking is called out. Some arrays get below 5 PPM tracking so the division ratio holds much better than the absolute value. (I find the DigiKey part search tool helpful in finding the low PPM parts I want.) 

Another possibility is an integrated resistive divider like the Maxim MAX549x series. The have pre-trimmed dividers in a number of ratios and values, typically with 35 PPM/C absolute tempcos but ratiotempcos as low as 2 PPM/C.

Bob LaJeunesse



>________________________________
> From: Bob Stewart <bob at evoria.net>
>To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>Sent: Friday, January 31, 2014 12:01 PM
>Subject: [time-nuts] EFC divider resistors
> 
>
>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
>