Controlled power line filters for suppressing interference

A power line filter comprising:

• a filter circuit for connection to a network supply, the filter circuit having a filtration characteristic that can be altered;
• monitoring means for monitoring a characteristic of the network supply which characteristic is representative of the interference content of the network supply;
• and
• a control arrangement connected to said monitoring means and to said filter circuit, said control arrangement comprising means responsive to said monitored characteristic of the network supply to alter said filtration characteristic of said filter circuit in a sense to adapt said filtration characteristic to better suppress the interference to which the input network supply is currently subjected, said control arrangement comprising memory means storing data representing a desired network supply characteristic, and at least one microprocessor connected to said monitoring means and to said memory means, said at least one microprocessor being operative to compare said monitored characteristic with said stored data and to cause alteration of said filtration characteristic of said filter circuit in a sense to minimize any difference between said monitored characteristic and said stored data.