Abstract
The demand for more accurate and precise time standards and clocks has persisted for several decades and there has been an increment in the necessity to enhance the accuracy of the clock. The rapid increase of emerging applications such as Virtual Reality (VR), advanced positioning, navigation and radar systems, and high-resolution spectroscopy have accelerated research on stable and precise clock. In addition to long-term stability which leads to clock fluctuation over time or Allen deviation, the short-term stability and phase noise of the signal also influence on determining the overall performance of a communication system. The demand for more accurate and precise time standards and clocks has persisted for several decades and there has been an increment in the necessity to enhance the accuracy of the clock. The rapid increase of emerging applications such as Virtual Reality (VR), advanced positioning, navigation, and radar systems, and high-resolution spectroscopy has accelerated research on stable and precise clocks. In addition to long-term stability which leads to clock fluctuation over time or Allen deviation, the short-term stability and phase noise of the signal also influence on determining the overall performance of a communication system. The optical Frequency Division (OFD) technique utilizes a high-quality factor ultra-stable optical reference and optical frequency comb, allowing to coherently divide down the frequency and phase of the pristine optical signal, resulting in a phase noise reduction and low-phase microwave signal generation.
Moreover, many emerging applications such as spectroscopy and advanced 5G and 6G communication systems require large bandwidth and fine-tuning frequency resolution while preserving the additive phase noise of the signal.
OFD technique allows the transfer of low noise and stable optical signal to millimeter wave or microwave signal, nonetheless, their limited frequency tuning resolution posses against their full potential. Thus in order to leverage the benefit of ultra-stable and low-phase noise signal generated through the optical method, a sub-Hz electronic frequency synthesizer is essential. The synthesizer facilitates tuning between selected signals obtained through optical frequency division. Electronic frequency synthesizers based on Direct Digital Synthesizer (DDSs) have shown extraordinary performance over their counterpart conventional Voltage Control Oscillators (VCOs) and Phase Locked Loops (PLLs) in terms of additive phase noise and tuning resolution, however, their output frequency is limited, therefore the need for broadband low phase noise DDS-based frequency synthesizer is vital.
