Fractional-N PLL Spread Spectrum Logic for Conflict-Free Phase Steps
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Solution Overview
Problem
The integration of spread spectrum clocking (SSC) and fractional-N phase locked loop (PLL) circuits poses conflicts due to simultaneous phase step requests in the same or opposite directions, leading to potential failures in the feedback divider and increased complexity in phase shifting circuits, which are difficult to manage effectively.
Innovation Solution
A combined spread spectrum and fractional-N phase locked loop circuit is designed with a logic interface circuit that combines directional and phase step control signals from both SSC and fractional-N logic, ensuring that phase steps are managed appropriately to avoid conflicts, allowing for single or no phase steps as needed, thereby preventing huge phase jumps and simplifying the phase shifting process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If spread spectrum clocking and fractional-N PLL are integrated to reduce EMI, then electromagnetic interference is counteracted, but conflicts arise when both logic circuits request phase steps simultaneously
Solution Approach 1:
A logic interface circuit is introduced as an intermediary between the spread spectrum logic and fractional-N logic. This mediator receives phase step requests from both logic circuits, resolves conflicts by serializing simultaneous requests, and generates appropriate control signals for the phase selector. The intermediary prevents feedback divider failure by ensuring that phase steps are applied sequentially rather than simultaneously, while still allowing both EMI-reducing functions to operate.
Solution Approach 2:
The logic interface circuit implements a feedback mechanism where it monitors phase step requests from both spread spectrum and fractional-N logic, detects conflicts when requests occur simultaneously, and adjusts the timing of phase step application. The feedback loop ensures that phase steps are serialized to prevent feedback divider failure while maintaining the EMI reduction benefits of both techniques.
2Adaptability or versatility
If a phase shifting circuit handles both phase steps simultaneously, then both phase adjustments are applied, but the circuit complexity increases significantly
Solution Approach 1:
The complex phase shifting functionality is extracted from the main phase selector and delegated to the logic interface circuit. The logic interface circuit handles the complexity of resolving simultaneous phase step requests from spread spectrum and fractional-N logic, while the phase selector itself remains relatively simple. This extraction of complexity to a dedicated control circuit maintains adaptability while managing device complexity.
3Object-affected harmful factors
If electromagnetic shielding is used to counteract EMI, then electromagnetic interference is reduced, but hardware investment increases considerably
Solution Approach 1:
Instead of changing the physical structure of the system through electromagnetic shielding, the patent changes the operational parameters of the PLL by implementing spread spectrum clocking. The center frequency is modulated in accordance with an appropriate pattern, spreading the spectral energy and reducing EMI. This parameter-based approach achieves EMI reduction through signal processing rather than physical shielding, avoiding increased hardware investment.
Data Source
AI summary
In applications that use fractional-N phase locked loops (PLLs), the use of spread spectrum clocking (SSC) to reduced electromagnetic interference (EMI) would be desirable, but conflicts can occur. Here, a circuit is provided that includes both fractional logic circuitry and spread spectrum logic circuitry. This logic circuitry operates in combination with a phase selector to generally ensure that the likelihood of conflicts (which can occur in conventional circuit) are reduced.