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We report on the experimental test of a new concept for reducing limitation on short-term frequency stability of passive frequency standards due to local oscillator phase noise. Systems that use sinewave modulation to interrogate a stable resonance are limited in short-term frequency stability by phase noise at the second harmonic of the modulation, f, . This effect limits the fractional frequency stability to approximately uy(7) = 0.9 fm/vo (S,(2f,))2, where uo is the carrier frequency and S,(2f,) is the phase noise at twice the modulation frequency. This new concept uses notch filters at If: 2f, from the carrier to reduce this effect. Tests on a modified passive rubidium standard demonstrate an improvement of approximately 18 in ay(7). The dual notch filters proved to be feasible and were obtained commercially. Measurements suggest that ultimate performances of approximately 2 x 10-l4i% are possible if the atomic resonance has sufficient quality. Additional refinements may reduce this limitation even further.

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