Analytical Investigation of LPF Mismatch in Direct Conversion Receivers

This paper analytically investigates the impact of low pass filter (LPF) mismatch in direct conversion receivers, which is a main source of frequency selective (FS) I/Q imbalance. Assuming a practical Elliptic LPF realized by active RC (resistor and capacitor) architecture, the LPF mismatch is mainly resulted from the non-ideal RCs in the circuit. The relationships between the imperfect RCs and the LPF mismatch and also the I/Q imbalance are analyzed using a first order Taylor expansion approximation. Close-form expressions are derived for the LPF mismatch variance and front end image leakage ratio (ILR), which are further verified by simulations. It is shown that for RC matching inaccuracies and tolerances less than 5%, the LPF mismatch (or the FS I/Q imbalance) is dominated by the RC matching inaccuracy, while the influence of RC tolerances is negligible. Moreover, simulations are carried out to investigate the impact of I/Q imbalance on the BER performance. It is shown that for both orthogonal frequency division multiplexing (OFDM) and single-carrier-frequency division multiple access (SC-FDMA), the bit error rate (BER) is seriously degraded by FS I/Q imbalance and an error floor will occur. In addition, the BER of OFDM is even worse than that of SC-FDMA, due to the poor image attenuation near the cutting off frequency. Therefore, compensation for the FS I/Q imbalance is always recommended, especially for OFDM systems.