rf self mixing in a cmos mixer
RF self-mixing occurs when a strong out-of-channel blocking signal leaks to the LO port of a down-conversion mixer and becomes downconverted with itself. The resulting distortion component has a DC term and a spectrum, which depend on the amplitude modulation and average power of the blocker. In addition, the phase shift between the blocking signals at the RF and LO ports of the mixer affects the resulting distortion component. When there is no phase shift, the result is similar to the second-order distortion . The resulting offset at the mixer output is in the range of some millivolts . DC offset cancellation schemes are effective only in removing the constant DC offset. Since it is very difficult to remove the in-channel baseband distortion component due to RF self-mixing of an amplitude modulated blocker after the downconversion, the leakage of a strong signal to the LO port of a mixer should be suppressed to a sufficiently low level. In UTRA/FDD, the transmitter may be on simultaneously with the receiver. Although the transmitter signal is attenuated in the pre-select filter, it is probably the strongest interfering signal at the input of the LNA. If the leakage is not filtered out using an off-chip bandpass filter between the LNA and mixers, it sets the specifications for the isolation between the mixer ports.