DDS Hybrid Phase-Lock Loop for Low-Jitter Clock Synchronization
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Solution Overview
Problem
Digital audio systems face challenges in generating a stable, low-jitter clock signal due to high jitter in timing references and limitations in existing synchronization circuits, such as those using analog phase-lock loops, which restrict jitter-reduction performance.
Innovation Solution
A synchronization circuit employing a direct digital synthesis (DDS) hybrid phase-lock loop that uses a phase integrator, lookup table, or CORDIC algorithm to generate a low-jitter clock output, with feedback mechanisms and digital filtering to control the DDS circuit, allowing for a stable clock output synchronized with a jittery timing reference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an analog phase-lock loop (APLL) with numerically-controlled oscillator is used for clock synchronization, then the circuit can generate a synchronized clock signal, but the intermediate clock signal frequency limit and single-bit information update rate restrict the jitter-reduction performance and loop bandwidth optimization
Solution Approach 1:
The patent replaces the traditional analog phase-lock loop with a direct digital synthesis (DDS) based system. The DDS circuit generates clock signals digitally, eliminating the mechanical and analog limitations of the APLL. The phase detector operates entirely in the digital domain, comparing phases of reference and feedback signals to generate error signals that control the DDS, thereby achieving superior jitter reduction without intermediate frequency constraints
Solution Approach 2:
The patent changes the fundamental operating parameters by moving from analog voltage-controlled oscillation to digital direct synthesis. The DDS circuit allows for precise digital control of output frequency and phase, enabling the loop bandwidth to be optimized independently of intermediate clock frequencies. The system can achieve high jitter reduction performance by digitally adjusting the loop filter characteristics and DDS control parameters
2Measurement precision
If the loop bandwidth of the APLL is lowered to remove jitter from the intermediate clock signal, then jitter is reduced, but the inherent noise of the APLL's oscillator cannot be removed effectively
Solution Approach 1:
The patent eliminates the noisy analog oscillator by using a DDS circuit that generates clock signals through digital computation. The DDS uses a phase accumulator and lookup table to generate precise sinusoidal or square wave outputs without the thermal noise and phase noise inherent in analog oscillators. The reference signal is processed digitally, allowing jitter filtering while maintaining signal purity
Solution Approach 2:
The patent introduces a digital loop filter as an intermediary between the phase detector and the DDS control input. This digital filter selectively attenuates jitter components at specific frequencies while preserving the fundamental clock signal and reducing oscillator noise. The digital filter can be designed with precise transfer functions to optimize both jitter removal and noise suppression independently
3Measurement precision
If a DDS circuit is used to generate clock output, then higher clock frequencies and better jitter performance are achieved, but the circuit requires complex digital processing including phase integrators, lookup tables, or CORDIC algorithms
Solution Approach 1:
The patent uses a lookup table that stores pre-computed sinusoidal values, effectively copying the mathematical function of sine wave generation into a memory structure. This allows the DDS circuit to generate accurate clock signals at high frequencies without performing complex real-time trigonometric calculations, significantly reducing the processing burden while maintaining high jitter performance
Solution Approach 2:
The patent pre-computes and stores sinusoidal values in a lookup table before they are needed for clock generation. This preliminary action of creating the lookup table offline allows the runtime operation to simply retrieve pre-calculated values through simple address generation and memory access, avoiding complex real-time computation and enabling high-frequency operation with minimal processing complexity
Data Source
AI summary
A direct digital synthesis (DDS) hybrid phase-lock loop for low-jitter synchronization provides a mechanism for generating a low-jitter clock from a timing reference that has a high jitter level. A DDS circuit provides a clock output and has an input for receiving a rational number. The rational number represents a ratio between the frequency of the clock output and the frequency of another stable clock provided to the circuit. In one implementation, a phase output of the DDS circuit is compared to a phase determined from an incoming timing reference and in another implementation, the low-jitter clock output is utilized to generate a phase number via a counter that is clocked by the clock output and captured by the timing reference.