Delay Line Circuit Fine-Tuning for Low-Jitter Clock Timing
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Solution Overview
Problem
In high-speed DDR systems, inconsistent delay step variations in delay lines introduce clock jitter, which increases noise in output signals due to significant variations in delay steps, particularly in the 6-8 picosecond range.
Innovation Solution
A delay line circuit with a digitally controlled delay line controller and phase interpolator, comprising multiple delay units and inverters, allows for tunable delays across a wide frequency range, with fine-tuning capabilities that reduce step delay variations by compensating for phase variations and process corners using a speed control unit like a capacitor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a delay line uses a chain of inverters to achieve wide delay adjustments, then the delay range is improved, but the delay step variations increase causing clock jitter
Solution Approach 1:
The delay line is divided into multiple delay units, each containing a specific number of inverters (e.g., two inverters per unit). These units are selectively enabled or disabled to achieve coarse delay adjustment. This segmentation allows the system to cover a wide delay range while maintaining consistent delay steps within each unit, thereby reducing clock jitter.
Solution Approach 2:
The delay line incorporates dynamic control mechanisms including a delay line controller that selectively enables/disables delay units based on desired delay values, and a phase interpolator that provides fine-grained phase adjustment. This dynamic control allows precise delay tuning while maintaining stable delay steps, resolving the contradiction between wide delay range and low jitter.
2Reliability
If delay steps are made small (2-3 picosecond range) to reduce jitter, then clock jitter is reduced, but the delay adjustment range becomes limited
Solution Approach 1:
The delay line uses multiple delay units with small delay steps (2-3 picoseconds each) to reduce jitter. By combining many such small-step units and selectively enabling them, the system achieves both low jitter and wide delay range. The coarse delay is obtained by selecting different numbers of delay units, while fine delay is achieved by the phase interpolator.
Solution Approach 2:
The delay line employs a nested structure where delay units are arranged in series and can be selectively activated. The phase interpolator provides fine-tuning nested within the coarser delay unit selection. This nested approach allows the system to achieve wide delay range through multiple selectable units while maintaining small delay steps for low jitter.
3Measurement precision
If a chain of inverters is used for fine pulse delay control, then precise phase control is achieved, but process corner variations cause inconsistent delay steps
Solution Approach 1:
The delay line incorporates a process sensing circuit that detects process corner variations and provides feedback to the delay line controller. Based on this feedback, the controller adjusts the delay unit selections and phase interpolator settings to compensate for process variations. This feedback mechanism maintains consistent delay steps across different process corners while preserving fine phase control precision.
Solution Approach 2:
The system dynamically changes operating parameters including the number of enabled delay units and the phase interpolator settings based on detected process corners. By adjusting these parameters in response to process variations, the system maintains consistent delay steps and precise phase control across different manufacturing processes and operating conditions.
Data Source
AI summary
A delay line circuit including: a coarse-tuning arrangement, including delay units, the coarse-tuning arrangement being configured to coarsely-tune an input signal by transferring the input signal through a selected number of the delay units and thereby producing a first output signal; and a fine-tuning arrangement configured to receive the first output signal at a beginning of a signal path which includes at least three serially-connected inverters, finely-tune the first output signal along the signal path, and produce a second output signal at an end of the signal path; the fine-tuning arrangement including: a speed control unit which is selectively-connectable, and a switching circuit to selectively connect the speed control unit to the signal path based on a process-corner signal.