The influence of DC offsets on the digital cancellation of second-order TX intermodulation distortions in homodyne receivers

Intermodulation distortions are produced when a modulated blocker signal with a nonconstant envelope gets processed by a stage with a nonlinear characteristic. Especially in homodyne receivers (RX), where the RX signal is directly downconverted into the baseband, even-order intermodulations might overlap the wanted signal in its baseband. In Frequency Division Duplex (FDD) systems, where RX and transmitter (TX) are active at the same time, the own TX is the source of major intermodulation distortions, since the TX signal power can be 130dB larger than that of the RX signal [1]. Usually the isolation of the duplexers, which are used to seperate TX and RX path at the antenna, is limited. As a consequence parts of the TX signal are leaking into the RX path. Due to nonlinearities, especially of the downconverter, intermodulation distortions are produced, degrading the performance of the RX. The quadratic term in the transfer functions of those nonlinear stages produces the major distortions, the so called second-order intermodulation distortions (IMD2). One approach to avoid IMD2 is to attenuate the TX leakage before the downconversion in the RX takes place. This would require analog filters which cannot be integrated into the radio frequency (RF) transceiver chip, increasing the production costs of the wireless communication device. Furthermore, in multiband RX, separate bandpass filters are needed for different signal bands.