PLL Phase Modulation for Down-Spread Clock Generation
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Solution Overview
Problem
Existing spread spectrum clock generation (SSCG) methods face limitations in reducing electromagnetic interference (EMI) due to narrow frequency spectrum, high costs, and difficulty in implementing down spread or up spread modulation profiles, especially with phase modulation (PM) methods which tend to produce noisy outputs and require complex circuitry.
Innovation Solution
A phase modulation method that injects a phase modulation profile pulse after the phase frequency detector (PFD) in a phase-locked loop (PLL) using pulse density modulation (PDM) or pulse width modulation (PWM), allowing for down spread implementation equivalent to center spread modulation, with a PLL loop that tracks the inserted pulse to adjust oscillation frequency and filter the modulated signal for better output clock performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If phase modulation (PM) method is used for spread spectrum clock generation, then circuit complexity is reduced compared to frequency modulation, but down spread implementation is not available and output noise increases
Solution Approach 1:
The patent embeds a phase modulation stage within the PLL loop and a frequency modulation stage outside the PLL loop, creating a nested structure where the PM provides the base spread spectrum function with reduced complexity, and the FM stage adds the down spread capability. This nested arrangement allows the system to achieve both PM's circuit simplicity and FM's down spread functionality.
Solution Approach 2:
The patent combines phase modulation and frequency modulation techniques into a single hybrid SSCG system. The phase modulation occurs within the PLL loop through a phase modulator, while frequency modulation is applied outside the loop through a frequency modulator. This merging of PM and FM approaches allows the system to achieve down spread capability (from FM) while maintaining relatively simple circuitry (from PM).
2Device complexity
If phase modulation (PM) method is used for spread spectrum clock generation, then circuit complexity is reduced, but jitter performance deteriorates due to noisy output
Solution Approach 1:
The patent introduces a low-pass filter as an intermediary component between the phase modulator and the output. This filter mediates the noisy output from the phase modulation process by attenuating high-frequency noise components while preserving the desired spread spectrum signal. The filter acts as a buffer that cleans up the PM output before it becomes the final clock signal, thereby improving jitter performance without adding significant circuit complexity.
3Adaptability or versatility
If frequency modulation (FM) with triangular wave is used for spread spectrum clock generation, then down spread capability is achieved, but circuit complexity and cost increase
Solution Approach 1:
The patent divides the frequency modulation function into two segments: a coarse frequency modulation stage within the PLL loop that handles the majority of the frequency variation, and a fine frequency modulation stage outside the loop that provides the down spread capability. This segmentation allows each stage to be optimized for its specific function, reducing the overall circuit complexity compared to implementing full down spread capability in a single FM stage.
4Object-generated harmful factors
If spread spectrum clock generation is implemented to reduce electromagnetic interference, then EMI emissions are reduced, but the total energy radiated remains unchanged requiring larger frequency band distribution
Solution Approach 1:
The patent employs dynamic modulation techniques where both the phase and frequency of the clock signal are continuously varied according to the spread spectrum coding sequence. This dynamic modulation causes the signal energy to be redistributed across a wide frequency band in a time-varying manner, effectively spreading the energy density across multiple frequency channels. The hybrid PM-FM approach optimizes this energy distribution by leveraging the complementary strengths of both modulation types.
Data Source
AI summary
Spread spectrum clock generation (SSCG) using phase modulation. A first clock signal having a first frequency spectrum may be modulated using phase modulation to produce a second clock signal. The phase modulation may include providing a phase modulation profile corresponding to the integrated frequency modulation profile, to adjust a scaling factor used in obtaining the second clock signal. The phase modulation profile may be provided in the form of a pulse or pulses, which may be injected through pulse density modulation or pulse width modulation at the output of a phase frequency detector comprised in a phase locked loop circuit used in generating the second clock signal. This modified phase modulation technique removes the down spread limitation present in traditional PM implementations, and also provides better jitter performance and lower cost than traditional PM implementations.