Wideband notch filters

Some commercially wideband notch filter (Sfew grands) has passband frequencies from DC up to 18ghz, anyone know what they are made of? and what is the theory behind it?

THx

For wide band notches you can use a low pass and high pass filter in parallel signa paths that get added togther.

Another neat trick is to use I and Q down conversion to DC and then pass both sides through high pass filters and then up convert with same I and Q local oscillators.