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An optical source of microwaves with very low phase noise for communications and radar systems is realized and tested. It is obtained by dual-frequency operation of a diode pumped Er,Yb:glass laser. An electrooptic crystal inserted inside the resonator permits both to tune the frequency difference between orthogonally polarized eigenstates, and to turn the laser into a voltage controlled oscillator. An optical phase-locked loop is then implemented in the GHz range, resulting in a measured instrument limited 3 dB-linewidth of 10 Hz. The phase noise is shown to be -100 dBc/Hz at 10-kHz offset.