[time-nuts] Voltage standards

Will Matney xformer at citynet.net
Thu May 26 18:42:22 EDT 2011


Hello all,

I was reading some old posts about voltage standards from Dr. Kirby, and
some others, and one that was mentioned was the older Fluke standards. I
did some research on this, going back to the Fluke 730A transfer standard,
which I have one of, and found out what the voltage reference and the
differential amplifier was from an old Motorola parts book dating from
1968.

The voltage reference in the Fluke 730A was most likely the Motorola part
number: MCA1914N or the MCA1924N in a can package. Both are rated at 6.8
Vdc, but there is a difference in the maximum voltage change between the
two. The MCA1914N is the tightest, with a change of only 0.005 volts from 0
to 75 deg C. The MCA1924N is made for a broader temperature, with a change
of only 0.010 volts from -55 to 100 deg C. These are not an oven zener, but
a voltage reference. Motorola first produced these using two diodes and a
transistor sealed in a can, literally. They used a temperature compensated
zener diode, consisting of a zener, and a regular diode, turned with their
cathodes facing, and connected together. These were placed inside the can,
along with a transistor across from them, then sealed. They used this same
reference transistor on up through several transfer standards, including
the 732A.

The Fluke 730A did not use an op-amp after the reference, but used a dual
transistor in a can, and created their own comparator amp. Motorola made
several of these along with Fairchild. There are two possibilities here in
the parts. I think they used a MD918A or MD918B, which are dual NPN silicon
annular transistors, with a Vceo of 15 Vdc, and an Ic of 50 mA. The MD918
series was to be used for "differential amplifier applications requiring a
matched pair of transistors with a high degree of parameter uniformity
under varying environmental conditions".

The other possibility on the dual transistor was a 2N2914, which was a
"dual npn silicon annular transistor, especially designed for low-level,
low noise, differential amplifier applications, featuring very high Beta,
guaranteed from 10 uAdc to 1 mAdc, and excellent noise characteristics".

Next, the Fluke 731B used the same voltage reference, but it did not use
the dual transistor amp. Fluke changed this to an op-amp, which turns out
to be a LM308. This op-amp was then used in the newer models after the
731B. Also, this op amp used a few of the same resistor values, and
circuitry, as the two transistor amp.

Now, a word about "voltage transfer standards". They are not meant to be as
stable as a true primary voltage standard, as they are meant to be used to
"transfer" a voltage from a known source to other equipment being
calibrated. The transfer standard is made to be recalibrated in one year
increments or closer. Tight tolerances in the short term, in one year or
less, are all that is needed. The Fluke 730A, 731B, and 732A and B are all
transfer units. The small voltage transfer standard boards, now sold on
ebay, are meant for calibration every six months, and are not meant to be a
long term primary standard. They should be calibrated from another transfer
standard, which has been calibrated from a primary voltage standard, such
as a Josephson type, by Fluke, or the NIST. The ones calibrated by using a
DMM, no matter who made it, are not calibrated correctly. To calibrate a
board correctly requires the use of a voltage transfer standard, that was
calibrated by a primary voltage standard, and a very sensitive null
meter-detector such as a Fluke 845A, etc.

Hope this helps the ones who had a few questions.

Will





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