High-level noise source for the calibration of Johnson noise power thermometers

A new high-level noise source was developed specifically for the calibration of Johnson noise power thermometers (JNPT), but it is useful in other applications requiring a stable, wideband, well-defined noise power spectral density. The design concept for the noise source is based on the fact that the open-circuit thermal noise voltage of a feedback resistor, connecting the output to the input of an inverting amplifier, is available at the amplifier output from an equivalent low output impedance caused by the feedback mechanism. The noise power spectral density level at the noise source output is equivalent to the density of the open-circuit thermal noise of a 100-Omega resistor at a temperature of about 64 000 K. The noise source has an output power spectral density that is flat to within 0.1% (0.0043 dB) in the frequency range 1-150 kHz which brackets typical passbands of the signal-processing channels of JNPTs. The noise source was evaluated by comparing its output noise power with the thermal noise power generated by a platinum resistor of known ohmic value at the temperature of the aluminum freezing point (933.52 K). The measured noise power was within 0.06% of the value computed from the fundamental thermal noise equation of Nyquist. The power spectral density varies less than 0.03% for ambient temperature fluctuations in the range 15 degrees -35 degrees C.