Phase Interpolator Slew Rate Measurement via Duty Cycle Distortion
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Solution Overview
Problem
Current integrating phase interpolators face challenges in accurately measuring slew rate over a wide frequency range due to limitations in controlling the voltage swing and maintaining current sources in saturation, which affects phase noise and headroom.
Innovation Solution
A circuit is developed that adjusts the bias current and common-mode voltage to estimate the slew rate by measuring duty cycle distortion across different common-mode thresholds, using AC-coupled inverters and a digital-to-analog converter to calibrate the programmable bias current, ensuring optimal voltage swing and minimizing phase noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If voltage swing is increased to improve slew rate measurement accuracy, then measurement precision improves, but current sources leave saturation region causing phase noise increase
Solution Approach 1:
The patent changes the common-mode voltage parameter to enable accurate slew rate measurement without requiring large voltage swings that would push current sources out of saturation. By adjusting the common-mode level, the measurement can be performed with small differential swings that maintain current source saturation, thus avoiding phase noise while achieving measurement accuracy.
2Object-affected harmful factors
If voltage swing is reduced to maintain current sources in saturation, then phase noise decreases, but slew rate measurement accuracy deteriorates
Solution Approach 1:
The patent introduces common-mode voltage control as an intermediary mechanism that decouples the relationship between differential voltage swing and current source saturation. This allows independent optimization: small differential swings maintain saturation (low phase noise) while the common-mode adjustment enables accurate measurement of the slew rate.
3Productivity
If bias current is increased to improve slew rate, then productivity improves, but headroom decreases affecting circuit operation
Solution Approach 1:
The patent makes the bias current programmable and dynamically adjustable, allowing optimization for different operating conditions. The bias current can be programmed to specific values to achieve desired slew rates while maintaining adequate headroom, and can be adjusted based on operating frequency and load conditions.
4Measurement precision
If common-mode voltage is adjusted to optimize measurement, then measurement precision improves, but device complexity increases
Solution Approach 1:
The patent implements common-mode voltage control that serves multiple functions: it enables accurate slew rate measurement, maintains current source saturation, and allows programmable bias current optimization. This multi-functional approach achieves measurement precision without proportionally increasing device complexity.
Data Source
AI summary
An apparatus is described and includes a current integrating phase interpolator core having a programmable bias current; an inverter circuit coupled to an output of the current integrating phase interpolator core for receiving a signal comprising a periodic sawtooth waveform therefrom; a digital-to-analog (D/A) converter for setting an input common mode voltage of the inverter circuit; a duty cycle measurement (DCM) circuit for measuring a duty cycle distortion (DCD) of a clock signal output from the inverter circuit; and a circuit for computing a difference between a first state of the DCD of the clock signal corresponding to the input common mode voltage of the inverter circuit being set to a high voltage and a second state of the DCD of the clock signal corresponding to the input common mode voltage of the inverter circuit being set to a low voltage.