Designing a Noise-Minimized Microwave Frequency Synthesizer for Mercury Ion Clock

We report on the investigation of both absolute and residual phase noises in a 40.507-GHz microwave frequency synthesizer, which is utilized as the local oscillator (LO) in a mercury ion microwave clock. The synthesizer utilizes direct digital synthesis (DDS), in-loop mixing phaselocking, and direct frequency multiplication techniques to achieve a frequency resolution of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.8 \mu\text{Hz}$</tex>. The single-sideband phase noise power spectral densities at Fourier-frequencies of 1 Hz, 10Hz, 100Hz, and 1 kHz are measured as −42 dBc · Hz<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup>, 52 dBc · Hz<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup>, −75 dBc · Hz<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup>, and −99 dBc · Hz<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup>, respectively. In our experiment, the stability of the mercury ion clock achieves <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.7 E-14 \tau^{\wedge}(-1/2)$</tex> using the low-noise synthesizer. Finally, the synthesizer lays a foundation for optimizing system parameters and improving performance in the closed-loop operation of the mercury ion microwave clock.