Hi Ok, You have a thermocouple junction at the + post on the DVM. You have one at the - post on the DVM. You have a junction at the + connection to the board. You have a junction at the - connection to the board. There are indeed more than that, but those four are pretty much a sure thing. The ones near the hot OCXO are also worth looking at.
The metals involved are unknown, so we have to guess a bit. You *can* get some alarming junctions with very normal test lead materials. Getting low thermal EMF connections requires special attention. If your junctions have a 1 mV / K coefficient, then you need temperature data that is good to 0.001C at each junction to work out what is going on at the junctions. That assumes you are after 1 uV on your “data” plot. It’s not terribly hard to get a contact at one (or more than one) junction that messes you up. Bob > On Nov 7, 2016, at 12:20 PM, Bob Stewart <[email protected]> wrote: > > Hi guys, > First of all, thanks for the additional responses. I was a bit angry and > rude yesterday, and I figured this thread was over. Thanks for staying with > me. > I haven't had time to look over the data etc in your responses. I'll do that > and get back to the list if appropriate. > > I spoke to Attila and Azelio offline last nite and from their input, I > decided to hook up the 3456A and collect some data, which is in the plot > attached. As usual, I've modified one of my standard plot scripts, so there > is some extraneous data that wasn't removed. > > First for our purposes is the thin red line, which is the DAC value locked at > 0x734B0. The orange trace is the temperature adjusted so that each step of > 10 on the right hand Y tics is one degree F. The dark blue trace is the EFC > value read by the 3456A. It has been multiplied by 100,000 and then had > 282600 subtracted. This leaves just the LSD scaled at 1:1 on the right hand > Y tics. > There's something interesting on the far right hand side where the > temperature goes low and stays there. The DVM value follows it down, but > then recovers while the temperature stays down. I'm not sure what to make of > this. Either then OCXO is making up for the temperature change by increasing > the temperature, or the 3456 is compensating for it after the fact. In > either case, the EFC seems to only follow the transient temperature changes, > and doesn't actually track the temperature on the board. > > So, to my eye, after 14 hours, there is only a dependency on thermal > transients. I'll leave it running for some time yet, but the EFC doesn't > seem to be drifting in any meaningful way at this point, other than in > relation to temperature changes. > > Note: I used a shielded twisted-pair with the usual clips attached to ground > and EFC in my GPSDO. On the 3456, the two leads go to the appropriate volts > inputs, and the shield goes to the ground input. The "guard" switch is out, > which is the off position. There is no shield connector on the DUT side. > > Bob > > ----------------------------------------------------------------- > AE6RV.com > > GFS GPSDO list: > groups.yahoo.com/neo/groups/GFS-GPSDOs/info > > > > <PLL.png>_______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
