[time-nuts] EFC divider resistors

Volker Esper ailer2 at t-online.de
Fri Jan 31 14:51:17 EST 2014


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
>
>
> 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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