  We developed an absolute Johnson noise thermometer (JNT), an instrument to
measure the thermodynamic temperature of a sensing resistor, with traceability
to voltage, resistance and frequency quantities. The temperature is measured in
energy units, and can be converted to SI units (kelvin) with the accepted value
of the Boltzmann constant kb; or, conversely, it can be employed to perform
measurements at the triple point of water, and obtain a determination of kb.
The thermometer is composed of a correlation spectrum analyzer an a calibrated
noise source, both constructed around commercial mixed-signal boards. The
calibrator generates a pseudorandom noise, by digital synthesis and amplitude
scaling with inductive voltage dividers; the signal spectrum is a frequency
comb covering the measurement bandwidth. JNT measurements at room temperature
are compatible with those of a standard platinum resistance thermometer within
the combined uncertainty of 60 ppm. A path towards future improvements of JNT
accuracy is also sketched.
