After setting all the necessary parameters as described here https://community.bistudio.com/wiki/Arma_3_Spectrum_Device the spectrum device works just fine.
If i then switch for example to a rifle and back to the spectrum device, then the min.max frequency and the min.max sensitivity bar on the x and y axis are not shown anymore. The defined signals set in the #EM_Values variable are still shown properly.
When i change the min/max frequencies, then the description of both axis appear immediatelly (only changing one of them is enough). Setting the same min/max values as before doesn't make the axis description reappear.