Patent ID: 8565708

Claim:
A filter for filtering a received signal from an amplifier, in order to attenuate an interferer therein, the interferer having a component at an interferer frequency, and the filter comprising: an intermediate filter providing a passband and a stopband; a first frequency converter configured to form a first intermediate signal by frequency-shifting an input signal derived from the received signal such that a component of the input signal at the interferer frequency is shifted to a frequency in the passband of the intermediate filter, and to input the first intermediate signal to the intermediate filter so as to cause the first intermediate signal to be filtered by the intermediate filter to form a second intermediate signal; a second frequency converter configured to form a cancellation signal by frequency-shifting the second intermediate signal such that a component of the second intermediate signal in the passband of the intermediate filter is shifted to the interferer frequency; a cancellation unit configured to cancel the cancellation signal from the received signal to attenuate the interferer therein; and a signal strength detector configured to detect the strength of a signal at the output of the intermediate filter and to, if the detected strength is below a predetermined threshold, deactivate the filter; wherein the interferer frequency is detected in dependence on said received signal at the output of said amplifier.