Spread Spectrum PLL Calibration for Low-Jitter EMI Control
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Solution Overview
Problem
Standard low-frequency periodic modulation in clocking signals for digital cores can exceed jitter limits in microcontrollers, leading to electromagnetic interference (EMI) issues, particularly in applications with tight maximum jitter allowances, and existing spread spectrum clocks may not adequately mitigate these problems.
Innovation Solution
A spread spectrum modulated Phase Locked Loop (PLL) device with a double calibration arrangement, including a cross-correlator to mitigate short-term and long-term jitter, and a bandwidth tuner to maintain electromagnetic compatibility, uses a sequence of random or pseudo-random values to modulate the clock signal, reducing peak EMI energy and maintaining consistent PLL bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If standard low-frequency periodic modulation is used to spread spectrum clock signal, then electromagnetic energy is spread over wider bandwidth, but accumulated jitter exceeds specified jitter limits of data peripherals
Solution Approach 1:
The patent changes the modulation parameters by using random or pseudo-random sequences instead of periodic modulation, with modulation amplitude that varies between 0 and a maximum value. This stochastic modulation approach spreads electromagnetic energy while controlling jitter accumulation, achieving both EMI reduction and jitter compliance for tight maximum jitter allowances
Solution Approach 2:
The patent uses a pseudo-random sequence generator that creates deterministic random-like sequences. These sequences copy the beneficial spreading properties of true random modulation while maintaining reproducibility and controllability, allowing jitter to be managed within specified limits for data peripherals
2Object-affected harmful factors
If modulation amplitude is increased to reduce peak EMI energy, then electromagnetic compatibility improves, but jitter accumulation increases
Solution Approach 1:
The patent employs dynamic modulation amplitude that varies over time according to the random sequence, rather than using constant amplitude modulation. The amplitude dynamically adjusts between 0 and a maximum value, allowing the system to achieve EMI reduction through spreading while controlling peak jitter excursions through the statistical properties of the modulation
Solution Approach 2:
The patent implements calibration loops that measure actual jitter and EMI performance, then provide feedback to adjust modulation parameters. This closed-loop control enables the system to optimize the balance between EMI reduction and jitter control, adapting to specific application requirements for tight maximum jitter allowances
Data Source
AI summary
Representative implementations of devices and techniques provide calibration for a spread spectrum PLL device modulator. A first calibration loop includes a cross correlator to correlate a sequence of random or pseudo-random signals injected into a signal path of the PLL device to a phase error signal of the PLL device. A second calibration loop includes a bandwidth tuner to tune a bandwidth of the PLL device and to reduce a jitter of the PLL device.