Voltage traceability of Johnson noise thermometry by ac-dc transfer method

The absolute Johnson noise thermometer (JNT) being developed at INRIM traces temperature measurements to electrical units. A pseudorandom noise signal is employed for calibration: the traceability of its RMS value was given by commercial AC voltmeter. To improve the measurement accuracy, a primary ac-dc transfer method with a multi-junction thermal converter, is presently implemented. First results show that AC-DC transfer measurement is compatible with AC voltmeter readings and has a comparable noise; an improvement of an order of magnitude in the voltage measurement accuracy is expected and presently under validation.