[time-nuts] EFC divider resistors

[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


Am 31.01.2014 20:25, schrieb Volker Esper:
> The ratio tempco is the point! It's hard to design a divider yourself,
> since temperature differences between the two divider resistors is the
> problem.
>
> Resistor networks with very low ratio tempcos are fairly expensive. I
> don't know your budget and how far you want to go. If you want to try it
> yourself, use voltage divider resistors of only one value and combine
> the required number of resistors. That is to have each resistor at the
> same power dissipation (and therefore at the same drift), then put them
> together very closely. Isolate them with insultation tape or so.
> Example: 10V to 2.5V: use 4 resistors of say 1k and pickup the 2.5 volts
> at the resistor next to ground.
>
> By the way: in a temperature sensitive environment where every microvolt
> counts I would not recommend using a potentiometer. Use it for
> determining the exact values and then replace the pot by the required
> number of resistors, see above.
>
> Tell us something about your intended budget.
>
> Volker
>
>
>
> Am 31.01.2014 18:40, schrieb Robert LaJeunesse:
>> 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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