Problems using probes in Transient Solver using Human Voxel Modell

Hello, I'm currently researching the effect of high frequency electromagnetic waves on specific brain centers in CST MWS. To do this i have positioned probes at the different brain centers. I'm using a plane wave and using a custom excitation signal witch is a 450 MHz sinus aprox. 100ns long. Using transient solver, my probes gets a signal that is aprox. 900 MHz, i.e. the double of the frequency of the excitations signal. I can't figure out why this is happening.

I have also positioned probes outside the human modell and they also reads a 900 MHz signal. Does anybody know why this happens?