therefore reduces the specifications placed on the duplex filter 43 with respect to its stop band attenuation reduction of the stop band attenuation requirement in the duplex filter reduces the number of filter poles required reducing the number of filter poles reduces the cost of the duplex filter since each additional pole adds a significant cost to the overall circuit in addition each pole removed lowers the insertion loss of the filter which thereby reduces the power output requirement placed on the transmit power amplifier since the noise cancellation circuit is arranged for single point in line insertion in the path on which cancellation is to be performed the circuit can also be implemented as a self contained stand alone unit which can provide noise or interference signal reduction at the output of any system for example and referring now to fig5 such noise cancellation circuit can also be inserted on the receive path of an rf transceiver in order to perform cancellation at a frequency at which a troublesome interferer is detected once again the noise cancellation response can be dynamic to adjust according to changes in the receive frequencies in use and the frequency band of identifiable interferers in the example shown an output is fed from the receive filter 414 in the receive path via an attenuator 420 to a tracking loop 430 arranged to provide the pilot tone input to the ancc 10 a located between the receive path filter 414 and the low noise amplifier 415 the attenuator sets the threshold level that an interference signal.