image rejection mixer
During the receiver idle time (e.g. when the transceiver is being warmed up, or during the intervals betweenreceive frames,) the LNA is replaced by an image tone generator which injects a tone at the image band into the receiver. If the I and Q channels were perfectly matched, the image tone would be totally rejected leaving a null signal at the ADC output. In practice, a small digitized signal will appear at the ADC output reflecting the amount of existing IQ mismatch. This signal is then processed by an external digital signal processing (DSP) unit which in turn sends in correction signals through the DAC input pads. By adjusting the mixer gains and controlling a pair of phase shifters, these correction signals are used to compensate for the IQ mismatch. The DSP continues this process until the ADC output signal is minimized, indicating an optimized image rejection.