A Frequency-Domain Calibration Method for Local Oscillator Leakage and I/Q Imbalance in Direct-Conversion Transmitters

Direct-conversion transmitters (TXs) exhibit performance degradation resulting from the local oscillator leakage (LOL) and in-phase/quadrature-phase (I/Q) imbalance. Many calibration methods suffer from the time alignment in the estimation process. This article introduces a frequency-domain approach to estimate and mitigate these detrimental effects. The method features no time alignment and simple training signal only by closed-form relations and power measurements in specific frequencies. Without time alignment, both the estimation precision and complexity are improved. The frequency-independent (FI) I/Q imbalance can also be calculated simultaneously when estimating the dc-offset mainly contributing to the LOL. To detect frequency-dependent I/Q imbalance, the method transmits single-tone training signals with different frequencies. The I/Q imbalance is compensated by using a complex finite impulse response (FIR) filter. The simulation and laboratory experiments conducted on the commercial 5G transceiver with a 491.52-MHz sample rate demonstrate that dc-offset and I/Q imbalance can be accurately estimated and compensated with the proposed method. The measurement results indicate that LOL can be suppressed to 85 dBc, and the image reject ratio (IRR) can achieve 70 dBc across 450-MHz bandwidth.