[volt-nuts] HP 3420B calibration assistance

Dr. Frank Stellmach drfrank.stellmach at freenet.de
Thu Mar 18 22:00:45 UTC 2010

Hi Dick,

I'm sure, I met the 3420B long time ago..that's a nice unit,.. but I do
not have the schematics of the 3420B at hand, also did not find them on
the agilent site, only the HP catalogue from 1970.

The instrument - as far as I remember - compares the input voltage
directly against 0-10V or 0-1V given by an internal reference and a
Kelvin-Varley divider. 100V and 1kV input are divided first down to 10V
and/or 1V.

So you have to calibrate the linearity of the KV first, best done in the
10V range.
Then you have to calibrate the 10V, 1V range, then the 100V and 1kV range.

Using the 5440B only is the best way to handle this process, even better
and faster than using an internal Wheatstone Bridge for linearity
calibration. As far as I remeber, the missing card did not contain the
necessary precision resistors for the Wheatstone Bridge, the user had to
solder the Bridge on his own.

Linearity of the 5440 is ~0.5ppm, sufficient for the 3420Bs KV.
Range uncertainty is also superior.

First two decades should be binary coded, so they should be calibrated
each by a 1-2-4-8 sequence from the 5440B.
The 3458A may be used for adjustments in the 100mV range.

I do not remember the construction of the KV, especially how the
adjustment from one decade to the other is designed. I.e. if the next
lower decade is applied directly to the predecessor, or if an adjustment
resistor is used (probable, if you mention 5 resistors per decade).
Perhaps you may send me a scan of the KV, with the adjustment pots.

I think, this should work if no additional decade resistor is used:

Use the normal 10V range of the 3420B for linearity calibration.

Start with the third decade, which cannot be adjusted, all its digits
and lower ones to 0.0999X, which gives 100mV. Set 5440B for this assumed
voltage of 100mV in the divider mode, freshly calibrated by 3458A, and
set to operate.
Adjust 10V range cal of 3420B , so that differential VM of 3420B shows
Zero to 1µV or less.

Set DVM un-sensitive and change 3420B setting to 0.10000, ´1`being the
first decade resistor which can be adjusted.
Apply 100mV again, set DVM to sensitive, and adjust this resistors
adjustment to Zero deflection. This calibrates the transfer from the
third decade to the 2nd.

Now, apply 200mV, 400mV and 800mV, and adjust the appropriate resistors
of the 2nd decade at 0.20000V, 0.40000V, 0.80000V settings acoordingly
to Zero, 1µV or less.

Set 2nd decade to 0.X0000, or to 0.9999X if the first setting is not

Apply 1V from the 5440B in the normal mode (not divided, control by
3458A) and adjust 3420B 10V range to zero deflection again, to have a
more precise adjustment.
Set 3420B to 1.00000V and adjust the first resistor of the 1st decade to
a Zero deflection,<1µV.

This calibrates the transfer from 2nd to 1st decade.
May also be different, if an additional decade resistor is designed in.

Now calibrate 2V, 4V, 8V from 5440B against 2.00000V, 4.00000V, 8.00000V
of 3420B to Zero deflection to 2µV, 4µV,8µV, or better, if not too noisy.

Now apply 10V, setting 3420B to 9.9999XV or X.00000V, whichever is
possible, finally adjust 10V range.
Apply 1V, 100V and 1kV and adjust those ranges at the 3420B.
As far as I remember, 1000V adjustment may not be not necessary, as this
is accomplished already in the 100V range.

Hope,this description helps.


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