[time-nuts] EFC divider resistors

[Volker Esper]

Don't underestimate the effect of resistors being at different
temperatures. The tempco isn't linear at all, so your drift caused by
ambient temp won't be the same at the different resistors. It is best to
have them at the same value!

I'd been experimenting quite some days to find a reasonable solution
with single resistors. It's essential to have them at the same
temperature and to couple them thermally as good as you can.

Precision resistor dividers do it just the same way: very small resistor
pills very tightly placed on a very good thermally conducting
substrate.  If you find the right value/ratio, and are game for paying
about 30$ you can get a ready-to-use divider from Vishay or so.

Another suggestion: replace the divider R3/4/5 by a precision reference
at the desired voltage. "5V" sounds like a TTL supply - that would be no
good idea.

My personal story: When I had my divider placed on the PCB I was
wondering the oscilloscope showing sudden milli volt jumps from time to
time (where I expected a noise of some micro volts...). After hours of
searching I noticed a correlation with the wind blowing outside my
shack. It was a small crack in the window that let a tiny airstream flow
across my PCB, so tiny, I couldn't sense.

Volker


Am 31.01.2014 21:23, schrieb Bob Stewart:
> Hi Volker,
>
> Thanks for the ideas.  The resistors are all within a short radius of each other.  I hadn't thought about bundling them all together.  I suppose I could epoxy them together as a last resort.  I've got some EPF foam, so I could try putting that on the board right against them.  In fact, I could try that while waiting for the new parts.  I've already used foam to fill most of the space in the box, except for around and above the OCXO.
>
>
>
>
>
>> ________________________________
>> From: Volker Esper <ailer2 at t-online.de>
>> To: Robert LaJeunesse <lajeunesse at mail.com>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>> Sent: Friday, January 31, 2014 1:51 PM
>> Subject: Re: [time-nuts] EFC divider resistors
>>
>>
>> Sorry, missing picture...
>>
>> Am 31.01.2014 20:47, schrieb Volker Esper:
>>> Here's how to place the divider resistors. The picture is from 1992, I
>>> didn't use SMDs then. But the results are promising at a very low price.
>>>
>>>
>>> Isolierband = insulation tape
>>> wärmeleit. Kern = thermoconducting core
>>>
>>> The picture shows two examples, the left one if you need three
>>> resistors, the right one if you need six. Use metal film types with low
>>> tempco. Keep in mind: it's the fluctuating _difference_ between the
>>> resistor values, that'll kill your stability. So avoid everything, that
>>> leads to different temperatures at the resistors. In particular avoid
>>> placing them into an airstream.
>>>
>>> Volker
>>>
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