ABSTRACT
Long before the modern-day engineer was born, Nyquist thought of the electrons in a conductor as behaving like a gas and applied his understanding of thermodynamics to predicting the nature of random voltages generated from the electrons jiggling around. Subsequently, van der Ziel added the first term, which is a linear function of frequency. The result is expressed most usefully in terms of a power spectral density, which might be observed by instruments having infinite input impedance but finite bandwidth, connected across resistance R. The result is often written as
P f R h f h f
e f
( ) ,= + −
2
2 1kT
where |f | assumes that one will be integrating over positive and negative frequencies with equal contributions and the “2” out front assumes such integration.
