Hi,
i've to design a circuit able to use dc component of ac biased voltage in order to power two different kind of load (ac load as lamp and a dc one).
So you need one filter to pass the DC and you need a filter to pass th AC.
The one is called low pass filter, the other high pass filter.
Each filter has to suppress the other frequency component.
What electrical/electronic divices do you know that have frequency dependent resistance?