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 <ail...@t-online.de> >> To: Robert LaJeunesse <lajeune...@mail.com>; Discussion of precise time and >> frequency measurement <time-nuts@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 >>> > _______________________________________________ > 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.
