Drift-compensated Low-noise Frequency Synthesis Based on a cryoCSO for the KRISS-F1

TL;DR

This paper presents a drift-compensated, low-noise frequency synthesizer based on a cryogenic sapphire oscillator, achieving exceptional short-term stability and minimal drift, suitable for atomic fountain clock interrogation.

Contribution

The paper introduces a novel drift-compensated frequency synthesizer using cryoCSO with improved stability and drift correction for atomic clock applications.

Findings

Short-term stability better than 5×10⁻¹⁵ at 1 s

Synthesizer stability surpasses quantum projection noise limit

Fountain clock stability reaches 2.5×10⁻¹⁴ at high atom number

Abstract

In this paper we report on the implementation and stability analysis of a drift-compensated frequency synthesizer from a cryogenic sapphire oscillator (CSO) designed for a Cs/Rb atomic fountain clock. The synthesizer has two microwave outputs of 7 GHz and 9 GHz for Rb and Cs atom interrogation, respectively. The short-term stability of these microwave signals, measured using an optical frequency comb locked to an ultra-stable laser, is better than $5\times10^{-15}$ at an averaging time of 1 s. We demonstrate that the short-term stability of the synthesizer is lower than the quantum projection noise limit of the Cs fountain clock, KRISS-F1(Cs) by measuring the short-term stability of the fountain with varying trapped atom number. The stability of the fountain at 1-s averaging time reaches $2.5\times10^{-14}$ at the highest atom number in the experiment when the synthesizer is used as an interrogation oscillator of the fountain. In order to compensate the frequency drift of the CSO, the output frequency of a waveform generator in the synthesis chain is ramped linearly. By doing this, the stability of the synthesizer at an average time of one hour reaches a level of $10^{-16}$ which is measured with the fountain clock.