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Accurate modeling of transmission lines becomes increasingly important in high-speed interconnect system design. However, it is rather difficult to obtain broadband transmission line models, in particular using frequency-domain measurements. This paper points out two potential accuracy issues. First, inaccurate DC values of the frequency-domain data cause a severe error in the time-domain simulations. Second, it is difficult to characterize the characteristic impedance over a wide frequency range due to the reflection caused by the port discontinuities. This paper proposes the combination of both time and frequency measurement data to mitigate the DC accuracy issue. For the characteristic impedance model, a new de-embedding technique is presented to mitigate the port discontinuity issue. Several numerical examples, such as MCM-L coplanar lines and package microstrip lines, are studied to validate the accuracy of the proposed method.