Some Pitfalls with the Kramers-Kronig Relationships [Education Column]

The properties of the Kramers-Kronig (KK) relationships relating the real and imaginary parts of the frequency-domain permittivity are well known, but not at every electrical engineer's fingertips. This tutorial presents some surprises (e.g., incorrect prediction of the imaginary part from the real part for a Drude-model cold plasma), and explains not only why but how these come about.