Switched-Capacitor Anti-Aliasing Filter for Sampling-Frequency Notches
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Solution Overview
Problem
Traditional continuous-time filters face challenges in achieving desired low-pass signal transfer functions with well-controlled roll-off and notches at integer multiplications of the sampling frequency, leading to undesired spectrum repetitions and aliasing during analog-to-digital signal conversion.
Innovation Solution
The anti-aliasing filter (AAF) incorporates window-sampling switches and an operational amplifier with closed-loop feedback, allowing for desired filter roll-off and notches at integer multiplications of the sampling frequency, while also improving linearity and dynamic range through a combination of resistors, capacitors, and switches that operate in specific phases to achieve the desired signal transfer function.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If traditional continuous-time filter is used, then simple filter structure is achieved, but desired notches at integer multiplications of sampling frequency cannot be obtained
Solution Approach 1:
The patent implements periodic switching of capacitors between series and parallel configurations synchronized with the sampling frequency. This periodic action creates notches at integer multiplications of the sampling frequency by periodically modifying the filter's impedance characteristics, thereby achieving precise spectral notching without complex continuous-time filter designs.
Solution Approach 2:
The filter transitions from a static continuous-time structure to a dynamic switched structure where capacitor connections change periodically. This dynamic reconfiguration allows the filter to adapt its transfer function in time, creating the desired notches at specific frequency multiples while maintaining a relatively simple overall structure.
2Productivity
If sampling is performed at sampling frequency, then discrete-time signal conversion is achieved, but spectrum repetitions and aliasing occur
Solution Approach 1:
The anti-aliasing filter is applied before sampling to preemptively suppress spectrum repetitions and aliasing. By creating notches at integer multiplications of the sampling frequency in advance, the filter prevents these harmful spectral components from being folded into the baseband during sampling, thereby eliminating aliasing before it occurs.
Solution Approach 2:
The patent exploits the periodic nature of spectrum repetitions caused by sampling and converts this potentially harmful effect into a useful feature. By deliberately designing notches at the exact frequencies where spectrum repetitions occur, the filter uses the sampling-induced spectral structure to its advantage, systematically suppressing aliasing components.
3Ease of manufacture
If open-loop gm-C topology is used, then simple circuit implementation is achieved, but limited dynamic range and linearity are obtained
Solution Approach 1:
The patent introduces feedback mechanisms in the switched capacitor filter structure to improve linearity and dynamic range. The feedback paths allow the circuit to compensate for non-linearities and maintain better signal fidelity across a wider dynamic range, while still using relatively simple switched capacitor building blocks that are easy to implement in integrated circuits.
Data Source
AI summary
The invention provides an anti-aliasing filter (AAF) for discretization at a sampling period. The AAF may include an operational amplifier having an input terminal and an output terminal, a first capacitor coupled between the input terminal and the output terminal, a second capacitor, and a first switch coupled between the first capacitor and the second capacitor. During a first phase, the first switch may conduct the second capacitor to the first capacitor. During a second phase, the first switch may stop conducting the second capacitor to the first capacitor. The first phase may last for one said sampling period.