[time-nuts] EFC divider resistors

EWKehren at aol.com EWKehren at aol.com
Fri Jan 31 22:25:59 UTC 2014


What is your cost goal?
Bert Kehren
 
 
In a message dated 1/31/2014 3:19:50 P.M. Eastern Standard Time,  
bob at evoria.net writes:

I'm  trying to put something together that is generic enough for the guys 
on the  VE2ZAZ GPS Standard board, but that will be relatively free of 
temperature  issues.  Yeah, I know: an impossible task.  The precision of the  
GPSDO is probably not in the same league as what this board is used to.   The 
DAC is actually a 10-bit PWM with 4 bits of dither.  The output from  the 
board comes from an op-amp.  The OCXO (for me) is a Trimble  34310-T.  Its 
range across 0 to +5V is about 7.5Hz.  I'm trying to  reduce that to 2Hz.  In 
spite of those limitations, it works well.   I'm just trying to wring every 
last bit of performance out of it, but in a way  that my results could be 
generally repeatable by someone who was willing to  make a moderate effort.

I didn't have any problem with finding 5PPM/C  resistors for the 20K and 
6.8k.  The others were a different story.   I found a 15PPM for the pot, and 
5PPM and 10PPM for the others in values that  should be close enough.  I 
think I'll look around on Mouser a bit more  before I press the buy button, but 
this is probably what I'll wind up  getting.


Bob




>________________________________
>  From: "GandalfG8 at aol.com" <GandalfG8 at aol.com>
>To:  time-nuts at febo.com 
>Sent: Friday, January 31, 2014 1:18  PM
>Subject: Re: [time-nuts] EFC divider resistors
>  
>
>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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