IQ PHASE IMBALANCE CORRECTION METHOD IN CARTESIAN LINEARIZATION FEEDBACK PATH WITH DUAL PHASE SHIFTERS

To implement phase training in a Cartesian transmitter, the phase training module 530 adjusts the Q phase shifter 546 to minimise the Q feedback signal Qfb with a test signal on the I input only and then the module 530 adjusts the I phase shifter 544 to minimise the I feedback signal Ifb with a test signal on the Q channel only, thereby effecting separate and independent phase corrections for the I and Q loops. The technique is useful in counteracting the not insubstantial IQ imbalance exhibited by local oscillator (LO) phase shifters using separate mixers for the I and Q LO signals (figure 4). The initial setting for the I phase shifter 544 during phase training may be set in accordance with the phase shift determined for the Q phase shifter (because the I and Q phase shifts will often be comparable), thereby reducing calibration time for the I phase shifter and reducing overall phase training time.