Patent ID: 8278918

Claim:
A method for operating a metal detection system comprising a balanced coil system with a transmitter coil that is connected to a transmitter unit, which is adapted to generate transmitter signals having a transmitter frequency that is selected from a group of at least two transmitter frequencies, and with a first and a second receiver coil that are adapted to provide output signals to a signal input of at least one amplifier unit included in a receiver unit, wherein the output signals are adapted to compensate each other so that the system is in balance, said method comprising: providing a control unit that generates a control signal according to the transmitter frequency of the transmitter unit; providing the control signal to a control input of at least one controllable impedance unit, which is coupled to the signal input of the at least one amplifier unit; and controlling an impedance value of the at least one controllable impedance unit with the control signal in such a way that the impedance value is increased or lowered when the transmitter frequency is increased or lowered.