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This paper presents a new dynamic quadrant swapping scheme that reduces crosstalk due to I/Q path mismatch for DQPSK receiver. The scheme reduces crosstalk by selectively transforming/swapping the I, Q components of the received symbols. The scheme's effectiveness is demonstrated by applying it to the IF digitizer part of the receiver. In the IF digitizer the incoming IF (first IF) signal is mixed and then converted to two digital signals, the I and Q signals, both at a second IF. These digital signals are then fed into a post conversion block. In this block dynamic quadrant swapping is applied, which reduces the crosstalk. Using an IF digitizer that includes two separate mixers with two separate low pass sigma-delta modulators for I and Q paths, the scheme's effectiveness is tested. Both simulation results and measured results show a significant improvement in image rejection ratio and BER, when the scheme is applied. With a SNR of 10.8 dB, at a 160.16 MHz incoming IF, and with a data rate of 36 kbps, the scheme improves the measured BER from 4.2 Ã 10 to 1.4 Ã 10- 3. At a 10 MHz incoming IF, and with a data rate of 2 kbps, the scheme improves the measured image rejection ratio from 41 dB to 65 dB and the measured BER from 1.3 Ã 10-3 to 0.7 Ã 10-3. The post conversion block is layout in 0.09 Â¿m CMOS technology. It occupies an area of 0.00092 mm2, and is simpler than previous reported schemes. The IF digitizer is fabricated in 0.35 Â¿m CMOS technology.
Date of Publication: Feb. 2010