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We present a fully stabilized Yb-fiber frequency comb locked to a microwave standard and an optical reference separately. The carrier-envelope offset frequency is generated by a standard f-2 f interferometer with 40 dB signal-to-noise ratio. The offset frequency and the repetition rate are stabilized simultaneously to the radio frequency reference for more than 30 hours, and the fractional Allan deviation of the comb is the same as the microwave standard of 10?12 at 1 s. Alteatively, the comb is locked to an ultra-stable optical reference at 972 nm using an intracavity electro-optic modulator, exhibiting a residual integrated phase noise of 458 mrad (1 Hz-10 MHz) and an in-loop tracking stability of 1.77× 10?18 at 1 s, which is significantly raised by six orders comparing to the case locked to the microwave frequency standard.