[time-nuts] EFC divider resistors

Bob Stewart bob at evoria.net
Fri Jan 31 18:02:25 UTC 2014


I just realized that I have a pot in the mix.  I see one on Mouser with 25PPM/C, so I suppose I'll try that.  Here's the circuit I'll use with the replacements.  What I have in there now has the values of R3, R4, and R5 multiplied by 10 and R2 is 3.9K.  I suppose it won't matter so much for the pot.

http://www.evoria.net/AE6RV/GPSstd_PLL/EFC%20Divider/EFC2.png

Bob





>________________________________
> From: Robert LaJeunesse <rlajeunesse at sbcglobal.net>
>To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>Sent: Friday, January 31, 2014 11:40 AM
>Subject: Re: [time-nuts] EFC divider resistors
> 
>
>
>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
>>
>
>


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