SerDes CDR Gain Adjustment for VCO Nonlinearity and PVT Drift
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Solution Overview
Problem
Voltage-controlled oscillator (VCO)-based clock and data recovery (CDR) systems in serializer-deserializer (SerDes) devices face significant challenges due to large variations in data lock time and jitter tolerance caused by non-linearity, which are exacerbated by process, voltage, and temperature (PVT) variations, making it difficult to maintain calibration within acceptable linear regions.
Innovation Solution
The solution involves a phase difference detection mechanism combined with gear shifting control signals to generate a first control signal for the VCO, which is integrated with a second control signal, and the output is calibrated using a frequency calibration module to create a look-up table for fine calibration values, allowing for adjustments in gear shifting gains to stabilize the CDR loop gain, thereby reducing lock time and jitter tolerance variability and making it independent from PVT variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional VCO-based CDR is used, then the system can recover clock and data signals, but large variations in data lock time and jitter tolerance occur due to VCO non-linearity
Solution Approach 1:
The system performs preliminary calibration of the VCO before normal operation. A calibration mode is activated where the VCO is tuned across its frequency range and calibration values are stored in a lookup table. During normal operation, these pre-calibrated values are used to compensate for non-linearity, eliminating the need for real-time correction and ensuring consistent lock time and jitter tolerance.
Solution Approach 2:
The system implements feedback through a calibration process where the VCO output is monitored and compared against reference values. Calibration values are adjusted based on measured performance, and these corrected values are stored for use during operation. This feedback loop ensures that non-linearity effects are compensated, maintaining reliable clock and data recovery across varying conditions.
2Stability of the object's composition
If VCO calibration is limited to linear regions, then non-linearity effects are reduced, but it becomes difficult to maintain calibration over large PVT variations
Solution Approach 1:
The VCO frequency range is divided into multiple segments or regions, each with its own calibration values stored in a lookup table. Instead of attempting to maintain a single linear calibration across the entire range, the system segments the operating range and applies appropriate calibration values for each segment. This allows the system to handle large PVT variations by selecting the appropriate calibration segment based on current operating conditions.
Solution Approach 2:
The calibration approach transitions from a static, fixed calibration to a dynamic system that adapts to PVT variations. The system includes logic to detect current operating conditions and select appropriate calibration values from the lookup table. This dynamic adaptation allows the VCO to maintain optimal performance across wide process, voltage, and temperature ranges while preserving linearity in each operating region.
3Reliability
If gear shifting control is added to adjust VCO gain, then lock time and jitter tolerance variability is reduced, but circuit complexity increases
Solution Approach 1:
Gear shifting control values are pre-calculated and stored in the lookup table during the calibration phase. Instead of implementing complex real-time calculations to determine appropriate gear shifting amounts, the system uses pre-computed values that are simply retrieved during operation. This preliminary computation approach maintains CDR loop gain stability while avoiding the need for complex real-time control circuitry.
Solution Approach 2:
The lookup table acts as an intermediary between the calibration process and the VCO control. Rather than directly implementing complex gear shifting logic, the system uses the lookup table to translate calibration measurements into appropriate control values. This intermediary structure simplifies the control circuitry while maintaining the ability to adjust VCO gain for optimal performance.
Data Source
AI summary
In described embodiments, a VCO based CDR for a SerDes device includes a phase detector, a VCO responsive to a first control signal and a second control signal and generating an output signal, a frequency calibration module configured to calibrate the frequency of the output signal by performing a coarse calibration and a subsequent fine calibration, a gear shifting control module controlling a gain change of the first and second control signals in time, and a look-up table created by fine calibration values generated from the frequency calibration module, wherein the programmed variable gain of the gear shifting control module is calculated by a calculation circuit employing the fine calibration values stored in the look-up table, the calculation of the calculation circuit adjusts gear shifting down, and adjusts a gear shifting gain, and adjusting an overall CDR gain over a VCO control curve.