The optical fiber used as a microwave delay line exhibits high stability andlow noise and makes accessible a long delay (>100 microseconds) in a widebandwidth (about 40 GHz, limited by the optronic components). Hence, it findsapplications as the frequency reference in microwave oscillators and as thereference discriminator for the measurement of phase noise. The fiber issuitable to measure the oscillator stability with a sensitivity of parts in1E-12. Enhanced sensitivity is obtained with two independent delay lines, aftercorrelating and averaging. Short-term stability of parts in 1E-12 is achievedinserting the delay line in an oscillator. The frequency can be set in stepsmultiple of the inverse delay, which is in the 10-100 kHz region. This article adds to the available references a considerable amount ofengineering and practical knowledge, the understanding of 1/f noise,calibration, the analysis of the cross-spectrum technique to reduce theinstrument background, the phase-noise model of the oscillator, and theexperimental test of the oscillator model.
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