Dual Feedback PLL Charge Pump Calibration
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Solution Overview
Problem
Mismatch in charge pump currents due to differences in transconductances of PMOS and NMOS transistors, caused by PVT variations, leads to reference spur and jitter in PLLs, as these mismatches affect the control voltage of voltage-controlled oscillators.
Innovation Solution
A dual feedback control mechanism is implemented in PLL integrated circuits, utilizing a conventional phase locking scheme and an automatic frequency calibrator to calibrate the charge pump, with a current mirror and multi-bit calibration signals to adjust bias voltages and currents, ensuring matching currents and reducing mismatches.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional charge pump is used in PLL, then the circuit structure is simple, but mismatch between up and down currents occurs due to PVT variations, causing reference spur and jitter
Solution Approach 1:
The patent applies preliminary action by performing charge pump calibration before normal PLL operation. The automatic frequency calibrator measures the VCO output frequency and adjusts the charge pump bias currents in advance to compensate for PVT variations, ensuring current matching accuracy is established before the system operates, thereby preventing reference spur and jitter issues.
2Reliability
If charge pump calibration is performed to eliminate current mismatch, then current matching accuracy is improved, but the device complexity increases due to additional calibration circuits
Solution Approach 1:
The patent implements feedback by using the automatic frequency calibrator to continuously monitor the VCO output frequency and feed this information back to adjust the charge pump bias currents. The frequency measurement result is used to generate calibration signals that modify the bias currents, creating a closed-loop system that automatically compensates for PVT variations without requiring complex external calibration equipment.
Solution Approach 2:
The patent applies self-service by designing the calibration system to be self-contained within the PLL circuitry. The automatic frequency calibrator uses internal frequency measurement and comparison circuits to generate its own calibration signals, allowing the charge pump to self-correct its current mismatch without external intervention, thereby reducing the need for additional complex calibration equipment.
3Reliability
If dual feedback control is implemented with automatic frequency calibrator, then reference spur and jitter are reduced, but the device complexity and power consumption increase
Solution Approach 1:
The patent applies segmentation by dividing the feedback control into two distinct paths: the conventional phase-locked loop feedback path and the separate automatic frequency calibration path. The frequency calibrator operates as an independent module that measures VCO frequency and generates calibration signals, while the main PFD and charge pump handle phase locking. This segmentation allows each subsystem to be optimized independently and simplifies the overall control architecture.
4Measurement precision
If calibration signals are applied to adjust charge pump bias currents, then frequency calibration accuracy is improved, but the manufacturing precision requirements increase
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the bias current parameters of the charge pump based on measured frequency deviations. Instead of relying on fixed manufacturing precision, the system changes the bias current parameters in real-time based on PVT conditions and measured frequency errors, allowing the PLL to adapt to manufacturing variations and maintain high calibration accuracy without requiring extremely tight transistor matching.
Data Source
AI summary
Phase-locked loop (PLL) integrated circuits according to embodiments of the invention provide dual feedback control. The first feedback control utilizes a conventional phase locking scheme that passes a feedback clock signal to an input of a phase-frequency detector (PFD). The second feedback control utilizes an automatic frequency calibrator that evaluates a frequency of an output of a voltage-controlled oscillator (VCO) relative to a locked frequency detected during calibration and provides separate calibration control to a charge pump.