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Microwave photonic filter (MPF) is one of the key fundamental subsystems in microwave photonics, and it shows great potentiality in numbers of applications such as optoelectronic oscillator, microwave frequency measurement, and so forth. A narrowband bandpass MPF is highly desirable with its high selectivity in microwave photonic applications. However, a resonator with a high quality factor (>10^6) is very difficult to fabricate on the mature silicon photonics platform without optimization, thus restraining the applications of the MPF. In this paper, we propose and experimentally demonstrate a tunable sub-gigahertz bandwidth MPF based on an ultrahigh quality factor silicon microring resonator. Most performance aspects of the MPF are well-balanced. A full width at half-maximum bandwidth of 170 MHz is achieved thanks to the ultrahigh total quality factor as 1.14 × 10^6 of the microring resonator, and the average …