Patent ID: 8626090

Claim:
An interference reducing circuit for a radio frequency transmitter, wherein the radio frequency transmitter comprises: an amplifier; an antenna port; an isolator configured to isolate an output of the amplifier from an interfering signal of the antenna port; a transmission line comprising a first part, a second part and a third part, the first part comprising a first end connectable to a signal source and a second end coupled to an input of the amplifier, the second part comprising a third end coupled to the output of the amplifier and a fourth end coupled to an input of the isolator, the third part comprising a fifth end coupled to an output of the isolator and a sixth end coupled to the antenna port; and a linearization loop configured to reduce a non-linearity of the isolator, the linearization loop comprising: a first directional coupler coupled to the third part of the transmission line; a loop impedance coupled at a first port to the first directional coupler; and at least one further linearization loop element coupled to a second port of the loop impedance via a loop feed line and configured to adjust a linearization signal and to feed the adjusted linearization signal to the transmission line; and wherein the interference reducing circuit is configured to reduce an influence of the interfering signal of the antenna port on the linearization loop and comprises: a second directional coupler configured to extract a compensation signal from the third part of the transmission line; a variable gain and phase shifter coupled at a first port to the third part of the transmission line via the second directional coupler and configured to: adjust an amplitude of the compensation signal to match an amplitude of the interfering signal of the antenna port passing through the loop impedance; and adjust a phase of the compensation signal to be of opposite phase as the interfering signal of the antenna port passing through the loop impedance; and a third directional coupler coupling a second port of the variable gain and phase shifter to the loop feed line and configured to feed the adjusted compensation signal to the loop feed line.