Adaptive Continuous-Time Filter Tuning With Digital PLL Control
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Solution Overview
Problem
Continuous-time filters in RF receivers, particularly in CDMA applications, face challenges due to manufacturing process variations and temperature changes, which affect the cut-off frequency, requiring a tuning scheme that can accurately adjust the filter characteristics without degrading performance and within the constraints of component mismatch and dynamic range.
Innovation Solution
A controller generates a multiple-bit control signal to adjust the RC-time constant of a continuous-time filter, using a digital PLL and switched resistor/capacitor networks to iteratively match the filter's response to a reference word, effectively compensating for process variations and maintaining filter performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a master VCO and PLL are used to tune the continuous-time filter, then the cut-off frequency accuracy is improved, but the circuit area increases due to additional VCO and large components
Solution Approach 1:
The patent extracts the essential tuning function from the complex master VCO-PLL system and implements it using a simple RC time constant measurement circuit. By taking out only the necessary frequency measurement capability and implementing it through RC charging/discharging cycles, the solution achieves accurate filter tuning without requiring the large master VCO and associated PLL components, thus resolving the contradiction between accuracy and circuit area
Solution Approach 2:
The patent creates a simplified copy of the frequency measurement function using RC time constant measurement instead of the original master VCO. By copying the essential tuning control capability through a different, more compact implementation (RC networks with switches), the system achieves the same tuning accuracy without the area overhead of the master VCO and large capacitors
2Ease of operation
If RC charging method is used to estimate RC-time constant, then the tuning simplicity is improved, but the sensitivity to component mismatch and amplifier offsets increases in high-frequency applications
Solution Approach 1:
The patent transforms the static RC time constant measurement into a dynamic process by using switched capacitor networks that can be controlled during operation. The dynamic switching allows the system to measure and adjust the RC time constant in real-time, making the tuning adaptive to component variations and reducing sensitivity to mismatch and offsets while maintaining simplicity
Solution Approach 2:
The patent implements feedback by measuring the actual RC time constant through the charging/discharging cycles and using this information to control the switched capacitor networks. This closed-loop feedback mechanism compensates for component mismatch and amplifier offsets by continuously adjusting the effective RC time constant to achieve the desired filter response
3Manufacturing precision
If discrete switching of resistors and capacitors is used in the feedback path, then the manufacturing precision is improved, but the device complexity increases
Solution Approach 1:
The patent segments the continuous RC time constant adjustment into discrete steps by using switched capacitor networks controlled by digital bits. This segmentation allows precise control of the filter tuning through binary-weighted capacitor switching, achieving manufacturing precision while keeping the circuit complexity manageable through systematic segmentation of the adjustment range
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution ensures accurate filter tuning, reducing the impact of process variations on the cut-off frequency, maintaining filter performance across semiconductor manufacturing variations and temperature changes, and does so without increasing circuit area or sensitivity to component mismatch.
Implementation Method 1
An RC-time constant replicator generates a time-varying output signal having a frequency determined by an RC time constant and a ratio of resistors
Implementation Method 2
The astable multivibrator includes an operational amplifier with a positive feedback loop and a negative feedback loop
Data Source
AI summary
A direct conversion radio-frequency (RF) receiver includes a controller and an adaptive continuous-time filter. The adaptive continuous-time filter receives a multiple-bit control signal generated by the controller to adjust a characteristic of the continuous-time filter. The controller generates the multiple-bit control signal in response to process variation in the semiconductor material used to implement the controller and the adaptive continuous-time filter. A method for tuning an adaptive continuous-time filter comprises determining a RC time constant, converting the RC time constant to a digital word, comparing a select bit of the digital word to a respective bit of a predetermined reference word to generate a control bit, applying the control bit to an adjustable element to modify the RC time constant, repeating the determining, converting, comparing and applying steps until the control bits generate an output word and providing the output word to the adaptive continuous-time filter.