[time-nuts] VRefs - I'm a bigger Voltnut than a timenut..
Dr. Frank Stellmach
drfrank.stellmach at freenet.de
Sun Nov 30 15:18:14 UTC 2008
there's currently no easy way to convert a frequency into Volt, i.e.
other than by cryo Josephson junction array or realizing Ohm by a
Klitzing quantum hall effect device. Otherwise, FLUKE would have
implemented that already in their Calibrators/Standards. They still rely
on Artefact Calibration, i.e. ovenized longterm stable Zener References
and Reference Resistors.
The 5720A uses a stacked double Zener reference on a proprietary hybrid
circuitry, giving 1-2 ppm/year stability, the 7001 and 732B use the
LTZ1000 (Zener above BE junction), as the 3458A, the 732A uses the
Motorola device (Zener below BE junction), and the elder 332D and 335D
use similar, but less longterm stable references.
The LM199, AD587 and I think also the Geller device may only be used as
transfer standards, having poor longterm stabilities of 20ppm/yr. or more.
Therefore, the LTZ1000 only may serve as a really affordable standard,
i.e. below 8ppm longterm stability, for private use.
I was lucky to get some LTZ1000 samples, and used the standard circuit
from the LT datasheet for creating a small PCB. I purchased commercially
available wire wound resistors (TK 3, 25ppm/yr) for the 5 necessary
reference resistors and two OP07. Those resistors should give less than
TK 1 and 1ppm/yr additional drifts.
Compared to my HP34401A, the circuitry showed quite good short term
stability below 1ppm, but shielding still has to be improved, as the
LTZ1000 circuit is susceptible to external EMC disturbances. This Volt
reference then costs less than 100 Euros/Dollars in total.
I designed further circuitry (with TK 0.3 and TK 0.1 precision metal
foil resistors from Alpha and Vishay, chopper amp, precision switches
and Teflon insulated wires) to have the possibility for precision
transfers of the 7,2V reference voltage to 100mV, 1V, 10V, 100V, 1000V
Only then, a Volt Standard is complete.
All this stuff will emulate or replace the commonly needed 732A (ref),
720A (Kelvin Varley), 752A (Reference Divider) and 834B (Null Detector).
Got no time yet to finish and improve the system, and to do the repair
on an old 332B standard, but would like to share my knowledge, if
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