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We propose an equivalent circuit model for the coplanar waveguide (CPW) which serves as the traveling wave electrode to drive carrier-depletion-based silicon modulators. Conformal mapping and partial capacitance techniques are employed to calculate each element of the circuit. The validity of the model is confirmed by the comparison with both finite-element simulation and experimental result. With the model, we calculate the modulation bandwidth for different CPW dimensions and termination impedances. A 3 dB modulation bandwidth of 15 GHz is demonstrated with a traveling wave electrode of 3 mm. The calculation indicates that, by utilizing a traveling wave electrode of 2 mm, we can obtain a 3 dB modulation bandwidth of 28 GHz.