Thanks Dr. Frank for your answer. I can only learn from that, what happens (t)here.
I know, the scale of noise and of the dip is really not obvious. ![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
I don't want to mix this dip problem with that TC of my meter. That's a 34461a.
Is it right, that this meters have TC of 1ppm/k by itself? So, that we can only measure not better than that TC with it?
Best regards
Well, at first, the '461A and the '465A probably have the same topology of their reference around the LM399, and also the ADC circuit should be identical.
If you look into the specification, both instruments are nearly identical in DCV, without the ACAL feature of the 465A.
The 465A may have a bit more stable reference , 30ppm/yr. over 35ppm/year, due to a possibly better selected LM399.
The T.C., though, is identical, i.e. 5ppm/K. That is due to the FineLine resistor network, which is used around the reference to generate the diverse ADC reference voltages, and in the ADC itself. See 34411A schematic, which is probably identical.
The LM399 itself is specified 1ppm/k, so the '465A can autocal itself, which gives 2ppm/K. But even that is of limited use, when making T.C. measurements in strongly varying environment.
btw.: d-smes complained about the assumed bad quality and high T.C. of ´his new KS '465A, but he obviously measured 1ppm/K in his experiment, which is very good, compared to the specified 5 ppm/K. I still do not understand his criticism, one can't expect more from that class of instruments, and also I think, that the new DMMs are even better than the old 34401A.
Anyhow, it's very difficult to determine the LTZ1000s T.C. by an instrument, which is specified to have 100 times higher T.C., but it's not impossible.
At first, you may also estimate the T.C. of your '361A by means of your LTZ1000 circuit, as d-smes has done it. As a proper designed LTZ1000 has at least < 0.1ppm/K, you would have a first guess by variation of room temperature, because the 361A is expected to have a factor of 10..50 higher T.C.
Maybe you also see 1ppm/K only, like d-smes instrument.
Your basement may offer much more stable temperatures, I achieve +/- 0.1 °C over many hours there.
So you could monitor the 361As internal temperature. I don't know if it has that parameter available over the bus. otherwise you'll have to mount a thermometer inside the instrument, and then you might be able to detect changes on the order of 0.1ppm.
If you change the LTZ1000s temperature over +/-5°C quite quickly, that would be 100 times more than the 361A, you might be able to estimate the LTZ1000 T.C.
You will notice some correlations to amount and direction of both temperature changes, so it might be possible to assign these correlations either to the 361A, or to the LTZ1000. If you have a first rough estimate of your LTZ1000 circuit, you may use it as the new baseline for further measurements.
You may also try to improve the LTZ1000s T.C. by trimming the T.C. compensation resistor.
If you are lucky to have a 2nd LTZ1000 circuit, you can bootstrap these T.C. measurements, and T.C. trimming by the same scheme as above, but your 361A would only be used for relative / ratio measurements further on.
I use my 3458A for that purpose, that may seem much easier than using a 361/365A, but it's not.
It has a (measured) T.C. of 0.5ppm/K (as specified), but that's only a factor of 2 (maybe) better than the 6 1/2 digits instrument.
I estimated my LTZ1000 circuit to have a T.C. of about 0.02 ppm/K, as that you cannot distinguish any more between both changes in temperature.
So this trimmed LTZ is at least 10 times more temperature stable than the 3458A, and will serve as the new baseline for the next LTZ circuit, I just have assembled. Goal will be to trim the next reference to ~ 0.01ppm/K, and afterwards to determine the T.C. of the first LTZ with much higher confidence.
Frank