I wanted to find out about the T.C. behavior of my HP3458A, i.e. what's the influence stemming from the T.C. of the reference board, and which one from the A/D converter assembly U180.
The specification "with ACAL" assigns 0.15ppm/K to the reference board, and "without ACAL" are 0.5ppm/K for the overall T.C., i.e. the sum of both.
This overall T.C. makes manual transfer and stability measurements over a longer time difficult, especially when measuring at sub ppm level.
I pulled the reference board out of my 3458A, and assembled it into an aluminum box, with 18V supply for heater and reference circuit, reference output to KS34465A DMM, and a precision NTC, tied to the 75k resistors on the board, and to be measured by an HP34401A.
The oven temperature is lowered to about 70°C, by the 100k parallel to the 15k resistor.
Also notice, that the T.C.-correction resistor R417 = 200k is assembled, which should NOT be used for the A-version, and probably creates an unwanted T.C. of the board.
The LTZ1000A on the reference board is in upright orientation inside the aluminum box, when latter is closed and placed with its bottom plate on the electrical heater.
Such low TC measurements are quite delicate, and I was now missing another "silent" and stable DMM like the 3458A.
The KS34465A has an LM399 inside, quite noisy, and might drift too much.
So I used its RATIO function, and a FLUKE 7000 as a reference. This deletes temperature and time drifts of the DMM.
The noise of the 34465A is mitigated by taking data each 0.1K, or each 30 sec.
The aluminum box was first cooled down to an inner temperature of < 18°C, and then during the slow warm-up, the whole assembly was nearly in thermal equilibrium, so the T.C. could be determined to about +0.95ppm/K (linear regression or box method)
For the regular operation temperature range inside the 3458A, usually 35..36°C in my lab, I heated the aluminum box to about 43°C, and let it slowly cool down.
This gave a T.C. of about + 0.11 ppm/K.
So this T.C. of about +0.1ppm/K is well inside specification.
I intended to bring its T.C. to zero, by removing or changing R417, improve its long term drift by further reducing the oven temperature, and would have replaced the ceramic capacitors by film capacitors, to reduce possible noise generation.
As timely and temperature drift is very low anyhow, I finally left the reference board as it is, for now.