Impact of LPF Mismatch on I/Q Imbalance in Direct Conversion Receivers

Low-pass-filter (LPF) mismatch is a main source of frequency-selective (FS) I/Q imbalance in direct-conversion receivers. Using a first-order Taylor expansion approximation, the relationship between the non-ideal resistors and capacitors (RCs) in the circuit and the LPF mismatch is analyzed. Close-form expressions are derived for the expectation of LPF mismatch and front end image-leakage-ratio (ILR), and further verified by simulations. It is shown that for RC matching inaccuracies and tolerances less than 5%, the LPF mismatch is dominated by the matching inaccuracy, while the influence of tolerances is negligible. FS I/Q imbalance caused by LPF mismatch varies significantly across the transmission bandwidth, which is extremely serious near the cutoff frequency. Therefore, compensation for FS I/Q imbalance is always recommended, especially near the cutoff frequency.