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We propose and demonstrate an all-fiber scheme to generate a precisely tunable microwave signal by heterodyning the two cascaded single-frequency Brillouin fiber lasers (BFLs). The generated microwave frequency is equivalent to the offset between the first- and the second-order Stokes frequencies. The ~10 GHz generated signal exhibits high stability with a linewidth of 2.8 KHz. By controlling elastic strain to the fiber in the BFL, the microwave frequency can be tuned up to 270 MHz. Owing to frequency pulling effect (FPE) in BFL, the signal frequency can be continuously tuned in a range of 9.6 MHz within one free-spectral range (FSR) of 19.8 MHz, and the tuning accuracy is 100 KHz.