  We investigate the spectral features of the emission from a superluminal
polarization current whose distribution pattern rotates with an angular
frequency $\omega$ and oscillates with an incommensurate frequency $\Omega
>\omega$. This type of polarization current is found in recent practical
machines designed to investigate superluminal emission. Although all of the
processes involved are linear, we find that the broadband emission contains
frequencies that are higher than $\Omega$ by a factor of the order of
$(\Omega/\omega)^2$. This generation of frequencies {\it not} required for the
creation of the source stems from mathematically rigorous consequences of the
familiar classical expression for the retarded potential. The results suggest
practical applications for superluminal polarization currents as broad-band
radiofrequency and infrared sources.
