Reducing the effect of local oscillator phase noise on the frequency stability of passive frequency standards

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_m. The new concept uses notch filters at ±2f_m from the carrier to reduce this effect. Tests on a modified passive rubidium standard demonstrate an improvement of approximately 18 in σ_y(τ). The dual notch filters proved to be feasible and were obtained commercially. Measurements suggest that ultimate performances of approximately 2 × 10^-14 τ^-1/2 are possible if the atomic resonance has sufficient quality