[time-nuts] EFC divider resistors

[GandalfG8 at aol.com]

It's hard to be sure without knowing the characteristics of the  DAC ouput 
and the oscillator EFC, but if you're only making a one off I wonder  if it 
might perhaps be possible to simplify that arrangement  to just a potential 
divider using selected values and being fed from  the DAC and tapped off 
into the EFC?
 
Regards
 
Nigel
GM8PZR
 
 
In a message dated 31/01/2014 18:04:39 GMT Standard Time, bob at evoria.net  
writes:

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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