Charge-Rotating IIR Filter for High Stopband Rejection
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Solution Overview
Problem
Conventional IIR filters face challenges in achieving high stop band rejection, especially in advanced communication systems like 5G, due to high power consumption and noise, while also requiring complex calibration mechanisms and sensitive components that are difficult to scale down in CMOS technology.
Innovation Solution
A high-order discrete-time charge rotating IIR filter with second-order antialiasing filtering is developed, utilizing capacitors and an optional gm-cell, which improves stop band rejection by more than 30 dB, achieving low power consumption, ultra-low noise, and excellent linearity, and is scalable with Moore's law, suitable for 28 nm CMOS technology.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional IIR filters are used to achieve stop band rejection, then filtering function is provided, but power consumption is high and noise performance is poor
Solution Approach 1:
The patent replaces conventional operational amplifier-based filtering mechanisms with a charge-rotating switched-capacitor system. This substitution eliminates the need for high-power active components and uses passive capacitor charging/discharging cycles to achieve filtering, dramatically reducing power consumption while maintaining stop band rejection performance.
Solution Approach 2:
The filter employs periodic charge sampling and rotation through multiple non-overlapping clock phases (PH1-PH4). The switched-capacitor network periodically transfers charge between capacitors in a cyclic manner, enabling continuous filtering operation with ultra-low power consumption suitable for battery-powered 5G devices.
2Reliability
If higher order filtering is implemented to improve stop band rejection, then filtering performance improves, but device complexity and calibration requirements increase
Solution Approach 1:
The high-order filter is segmented into multiple identical second-order building blocks (biquads). Each biquad uses the same switched-capacitor topology with capacitors C1-C4 and switches S1-S4, arranged in cascaded stages. This modular segmentation allows systematic achievement of high-order filtering (6th order or higher) without proportionally increasing complexity, as each stage follows the same simple pattern.
Solution Approach 2:
The patent employs universal second-order biquad sections that can be cascaded to achieve various filter orders and characteristics. The same basic circuit topology serves multiple functions: filtering, impedance matching, and gain control, reducing overall system complexity compared to using different specialized circuits for each function.
3Reliability
If conventional filtering components are used, then filtering is achieved, but components are sensitive to PVT variation and difficult to scale down
Solution Approach 1:
The filter performance parameters (cut-off frequency, Q-factor, gain) are controlled by capacitor ratios rather than absolute capacitor values. Since capacitor ratios are much less sensitive to PVT variations than absolute values, this approach provides stable filtering performance across process, voltage, and temperature ranges while enabling scaling to advanced nanoscale CMOS technologies.
Solution Approach 2:
The patent replaces voltage-mode operational amplifiers with current-mode charge-rotating switched-capacitor circuits. This substitution eliminates the PVT-sensitive transconductance parameters of op-amps and uses only passive capacitors and switches, which are much more robust to process variations and can be easily scaled to smaller technology nodes.
4Reliability
If additional Gm cells are added to achieve second order antialiasing filtering, then stop band rejection improves, but power consumption increases
Solution Approach 1:
The patent replaces the need for additional Gm cells with a charge-rotating switched-capacitor network that inherently provides second-order antialiasing filtering. The periodic charge sampling and rotation through multiple capacitor stages creates the necessary filtering effect without requiring extra active amplifying components, thus avoiding additional power consumption.
Solution Approach 2:
The switched-capacitor network performs multiple functions simultaneously: it provides filtering, impedance transformation, and signal routing all through the same charge rotation mechanism. This self-service approach eliminates the need for separate Gm cells that would otherwise be required to achieve the same filtering performance.
Data Source
AI summary
A novel and useful high-order discrete-time charge rotating (CR) infinite impulse response (IIR) low pass filter is presented. The filter utilizes history capacitor arrays incorporating banks of capacitors. A linear interpolation technique is used in the IIR filter with second order antialiasing filtering, whose transfer function is sinc(x)2 per stage. It also uses a gm cell, rather than operational amplifiers, and is thus compatible with digital nanoscale technology. A 7th-order charge-sampling discrete time filter is disclosed. The order of the filter is easily extendable to higher orders. The charge rotating filter is process scalable with Moore's law and amenable to digital nanoscale CMOS technology. Bandwidth of the filter is precise and robust to PVT variation. The filter exhibits very low power consumption per filter pole, low input-referred noise, wide tuning range, excellent linearity and low area per minimum bandwidth and filter pole.