Fully differential direct conversion receiver for W-CDMA using an active harmonic mixer

A fully differential direct conversion receiver IC for W-CDMA is presented. The receiver IC consists of a complete active portion of a W-CDMA receiving system, such as an LNA, quadrature demodulator, low-pass filter (LPF), and variable gain amplifier (VGA). In order to suppress the DC offset, which is the most important issue in a direct conversion system, an active harmonic mixer is applied to the quadrature demodulator. Furthermore, the receiving system, including an LNA and RF filter, adopts a differential architecture to reduce local signal leakage, which generates DC offset. The performance of the entire receiving system was evaluated and the DC offset in steady state was measured at only 40 mV. Moreover, the DC offset variation in the LNA gain change, which has the largest affect on the receiving performance, was limited to 60 mV, which is less than -10 dB compared to the desired signal strength. It was confirmed by computer simulation that the DC offset variation in the LNA gain change did not degrade the bit error rate (BER) performance at all.