Spread Spectrum Clock Recovery Using Predictive Modulation Playback
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Solution Overview
Problem
Conventional clock recovery circuits face challenges in reliably recovering spread spectrum clock signals with good noise resistance at high data rates, particularly due to the difficulties in separating periodic frequency variations caused by spread spectrum modulation from phase noises, which leads to increased jitter and inter-symbol interference.
Innovation Solution
The proposed solution involves a circuit with an error detector to compare spread spectrum signals and recovered clock signals, producing error signals for modulation and phase differences, a record and playback unit to predict periodic modulation, and a signal generator to produce the recovered clock signal, allowing for narrowband filtering of phase noise while accurately tracking frequency changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional CDR circuits are used to recover clock signals, then the circuit design is simple, but the noise resistance deteriorates at high data rates due to inability to separate periodic frequency variations from phase noises
Solution Approach 1:
The patent segments the error signal into two distinct components: a wideband component containing periodic frequency variations from spread spectrum modulation, and a narrowband component containing phase noises. This segmentation allows each component to be processed separately through dedicated filtering paths, improving noise resistance without requiring a complete redesign of the entire CDR circuit.
Solution Approach 2:
The patent introduces an intermediary processing stage that separates and filters different frequency components of the error signal before they are used for clock recovery. By inserting this intermediary filtering mechanism, the system can selectively remove periodic frequency variations while preserving phase noise information, thereby improving noise resistance without excessive circuit complexity.
2Object-affected harmful factors
If spread spectrum clock generation is used to reduce EMI, then electromagnetic interference is reduced, but clock recovery becomes more difficult due to periodic frequency variations
Solution Approach 1:
The patent extracts the periodic frequency variation component from the total error signal through wideband filtering. By taking out this specific component that causes clock recovery difficulties, the system can process the remaining narrowband phase noise separately, thereby maintaining the EMI reduction benefits of spread spectrum while simplifying the clock recovery process.
Solution Approach 2:
The patent performs preliminary filtering of the error signal to remove periodic frequency variations before the clock recovery process. By preparing and cleaning the error signal in advance through separate wideband and narrowband filtering stages, the subsequent clock recovery operation becomes easier and more reliable, addressing the difficulty caused by spread spectrum modulation.
3Speed
If the loop filter bandwidth is increased to track frequency variations, then frequency tracking improves, but phase noise filtering deteriorates
Solution Approach 1:
The patent segments the error signal processing into two parallel filtering paths: a wideband path for frequency tracking and a narrowband path for phase noise filtering. This segmentation allows each path to be optimized independently - the wideband path can track frequency variations quickly while the narrowband path effectively filters phase noise, resolving the trade-off between tracking speed and noise filtering.
Solution Approach 2:
The patent transitions from a single-dimensional filtering approach to a two-dimensional filtering approach by processing the error signal through both wideband and narrowband filters simultaneously. This dimensional expansion allows the system to achieve both fast frequency tracking and effective phase noise filtering by operating in multiple frequency domains at once.
Data Source
AI summary
Circuits, methods, apparatus, and systems for recovering a clock from a spread spectrum signal having a periodic modulation profile. The circuits generally include an error detector circuit configured to compare the spread spectrum signal and a recovered clock signal, and to produce a first error signal corresponding to the periodic modulation profile and a second error signal corresponding to phase differences other than the spread spectrum modulation, a record and playback unit configured to record a value to a frequency memory, said value based on the first error signal and produce a third error signal corresponding to a predicted periodic modulation based at least in part on the recorded value, and a signal generator configured to produce the recovered clock signal in response to the second and third error signals.