How to create a source signal that has constant magnitude over a frequency range?

Hello there!

I'm trying to do a plane wave simulation and I want to create a plane wave source that has constant magnitude over the frequency range from 0.3 GHz to 3 GHz. The Harmonic and Guassian modes don't work for this, so I'm thinking the only remaining option is "User Defined". However I'm not sure how I would create the equation for such a signal. Do I have to do some kind of parametric sweep across this frequency band? Thanks!