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We demonstrate highly compact optical add-drop filters based on silicon-on-insulator microring resonators. The microring resonators have a small radius of 2.5 mum and a very large free spectral range ~ 32 nm at the 1.55 mum communication band. The propagation loss in such small micoring resonators was experimentally determined and shown to be extremely important in designing microring add-drop filters with low add-drop crosstalk, low drop loss, and maximally flat drop passband. For box-like channel dropping responses, second-order optical add-drop filters with two coupled microring resonators are designed and demonstrated, and the simulation matches well with the experiment. Devices were patterned with electron-beam lithography. Two fabrication procedures utilizing different polarity of resists were introduced and compared, and the process with negative resist resulted in much smaller sidewall roughness of waveguides, thus reducing the propagation loss in microring resonators.