Abstract

The authors report on the experimental test of a new concept for reducing the limitation on the short-term frequency stability of passive frequency standards due to local oscillator phase noise. Systems that use sine-wave modulation to interrogate a stable resonance are limited in short-term frequency stability by phase noise at the second harmonic of the modulation, f/sub m/. The new concept uses notch filters at /spl plusmn/2f/sub m/ from the carrier to reduce this effect. Tests on a modified passive rubidium standard demonstrate an improvement of approximately 18 in /spl sigma//sub y/(/spl tau/). The dual notch filters proved to be feasible and were obtained commercially. Measurements suggest that ultimate performances of approximately 2 /spl times/ 10/sup -14/ /spl tau//sup -1/2/ are possible if the atomic resonance has sufficient quality. >

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