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Impedance/admittance transfer matrix characterization of voltage source converters is an essential issue in frequency domain stability studies. Several dq- and pn-frame approaches are proposed to obtain this matrix with experimental or numerical simulation measurements. Although these are well-established approaches, various issues require further study. The paper presents a detailed review and discussion on the dq- and pn-frame approaches for converter impedance/admittance transfer matrix measurements and contributes to clarifying the influence of the mirror frequency effect and the starting point of the FFT window on these measurements. The contributions are validated by PSCAD/EMTDC simulation with a VSC model containing PLL and outer controls.