[time-nuts] EFC divider resistors

[Bob Stewart]

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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