A BST Varactor Based Circulator Self Interference Canceller for Full Duplex Transmit Receive Systems

An analog closed loop feedback system for improving the transmit to receive isolation of a generic circulator is described. The RF section of the circuit utilizes barium strontium titanate (BST) varactors to realize an antenna port tuner that reflects transmit power back into the circulator at the correct amplitude and phase to cancel the transmit signal leaking into the receiver port. Closed loop operation of the tuner is demonstrated using an Op Amp controller and a logarithmic detector at the receiver port. More than 33dB of additional self-interference cancellation was observed over that of the circulator for a 30MHz bandwidth QPSK transmit signal centered at 930MHz.