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 <b...@evoria.net> >>>> To: Discussion of precise time and frequency measurement >>>> <time-nuts@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 >>>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.
<<attachment: DF9PL_Resistor_devider.gif>>
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