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The paper presents a discussion on the experimental characterization and microwave modelling of coplanar electro-optic modulators on lithium niobate substrates. To introduce the issue, a discussion is presented on the design criteria for travelling-wave modulators. In particular, we show that attenuation is a main concern if modulation bandwidths in excess of 20 GHz are sought, while velocity matching can be achieved with comparative ease, as a survey of the available structures shows. Finally, the experimental characterization and modelling of some conventional modulator structures is presented, with the aim to assess their operating limits in terms of modulation bandwidth.