  Tunable electromagnetic sources, such as voltage controlled oscillators
(VCO), micro electromechanical systems (MEMS) or diode lasers are often
required to be linear during frequency-sweep modulation. In many cases, it
might also be sufficient that the degree of the non-linearity can be well
controlled. Without further efforts, these conditions are rarely achieved using
free running sources. Based on a pre-distortion voltage ramp, we develop in
this letter a simple and universal method that minimizes the non-linear
frequency response of tunable electromagnetic sources. Using a current-driven
Quantum Cascade Laser (QCL) as an example, we demonstrate that the
non-linearity can easily be reduced by a factor of ten when using a single
distortion parameter $\gamma$. In the investigation of the IR absorption
spectrum of ozone at 10\,$\mu$m, an even better reduction of the frequency
scale error by two orders of magnitude is obtained by using the pre-distortion
method to generate an essentially purely quadratic sweep frequency dependency
which can be inverted easily to retrieve precise molecular line positions.
After having tested our method on a variety of electromagnetic sources, we
anticipate a wide range of applications in a variety of fields.
