[time-nuts] Thermal impact on OCXO

Bob Camp kb8tq at n1k.org
Mon Nov 7 12:49:51 EST 2016


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.


> On Nov 7, 2016, at 12:20 PM, Bob Stewart <bob at evoria.net> 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
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