The use of thermal converters for ac voltage metrology was introduced by Frank Hermach at NIST in 1952. The basic thermal converter is a thermoelement, consisting of a thermocouple positioned at the midpoint of a heater wire, enclosed in an evacuated bulb. The thermoelement senses the heat generated by an electrical signal applied to the heater. By comparing the heating effect of an unknown ac signal to that of the average of both polarities of a known dc signal, the rms ac quantity may be measured in terms of the known dc quantity, giving the ac-dc difference of the thermal converter. The traditional thermoelement has an input voltage of a few volts or an input current of a few milliamperes, with a best uncertainty of about 1 µV/V or µA/A over the frequency range from 10 Hz to 1 MHz. In combination with a precision resistor or shunt, the thermal converter is commonly used at voltages up to 1000 V and currents up to 100 A, and in combination with amplifier circuits, is used at voltages as low as 2 mV.
The fundamental errors in thermoelements are thermoelectric errors, produced when a current flows through a temperature gradient. These thermoelectric errors may be reduced by adding more thermocouples and using the thermoelement at a reduced input amplitude. The resulting device, the Multijunction Thermal Converter (MJTC), is the most accurate means of measuring ac voltage and current at voltages above 500 mV and currents up to 50 mA; uncertainties of about 5x10-7 or less are possible in the audio frequency. Unfortunately, traditional MJTCs made with wire are difficult to make and nearly impossible to obtain.
To address this problem, NIST is presently making MJTCs using semiconductor fabrication techniques. These devices fall into two broad categories; one type of MJTCs is designed for high frequency ac voltage metrology, the other for high current ac metrology. The high-frequency MJTCs (Fig. 1) are fabricated on quartz substrates, for reduced dielectric loss, and are predicted to have very small ac-dc differences up to 100 MHz at a few volts. The high-current MJTCs (Fig. 2) have very large heater structures capable of taking currents up to 1 A. We plan to parallel up to eight of these high-current MJTCs in a single module to achieve a 10 A current converter. Initial results are very encouraging, and we anticipate using both of these MJTCs in our measurement services before the end of the year.
In addition to new artifact standards, in collaboration with the Quantum Voltage Project, we are developing a quantum standard for ac voltage, based on pulse-programmable Josephson Junctions. We have already provided a quantum-based calibration to a customer with reductions in uncertainty of as much as 98 % over uncertainties obtained using traditional scaling techniques. The AC Josephson Voltage Standard (ACJVS, Fig. 3) promises unprecedented reductions in uncertainty over its operating range of 2 mV to about 275 mV and, coupled with MJTCs, should allow us to substantially reduce the uncertainties for ac voltage metrology.