Filter

A method of filtering a differential signal comprising:

• passing a positive part of the differential signal through a positive arm;
• and
• passing a negative part of the differential signal through a negative arm;
• wherein passing the respective part through the respective arm comprises:
• passing the signal along a signal path from an input to an output through a first reactive component;
• from a first node on the signal path, shorting the signal to ground through a first series resonant circuit at the resonant frequency of the first series resonant circuit, the first series resonant circuit comprising a second reactive component in series with a third reactive component;
• and
• using an active circuit comprising a voltage controlled current source connected to a second node on the signal path between the first node and the input of the signal path such that the voltage controlled current source draws current through the second reactive component, the active circuit changing the current flow through the second reactive component in dependence on a voltage at the second node;
• wherein the first series resonant circuit of the positive arm and the first series resonant circuit of the negative arm are connected together to form an AC ground.