DC offset and IM2 removal in direct conversion receivers

DC offsets cause interference in homodyne receivers, caused by nonlinearity and imbalances in the receiving amplifier and mixing circuits, as well as local oscillator (LO) leakage into the RF input port and RF leakage into the LO port. The latter causes a squaring action (hence, second order intermodulation distortion (IM2)) in the mixer which generates a DC offset (low frequency) that varies with the amplitude of any high power RF interfering (jamming) signal, whatever its applied frequency. A method for removing this DC offset is described. The signal immediately after the mixer is squared to obtain a distortion estimate of the jamming signal, and then subtracted from the main signal path to remove the interference from the desired signal. The experimental results indicate an 11.5 dB improvement in jamming margin for systems with 30 kHz RF bandwidth. In the experiment an adaptive equaliser is used to equalise the two paths and improve the dynamic response