Medium-to long-term frequency stability of high-performance CW double-resonance Rb standard

We present the short-term noise budget and metro-logical studies on the medium-to long-term stability of our Rb standard. Our clock exhibits a short-term stability of ~ 1.4×10^-13 τ^-1/2. The dependance of microwave power shift on light intensity, and the possibility to nullify the microwave power shift is demonstrated. The perturbing vapor cell geometric effect on medium-to long-term time scales, limiting the stability around 6×10^-14 level is identified. Further required improvements are discussed.