Higher-Order N-Path Filters for Flatter Passbands and Steeper Rejection
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Solution Overview
Problem
Conventional N-path filters exhibit a large passband droop and insufficient rejection at TX leakage frequencies, limiting their effectiveness in wireless communication systems due to their first-order nature.
Innovation Solution
The N-path filter design is enhanced by increasing its order to at least two, incorporating a common drain amplifier circuit in each branch, and utilizing feedback capacitors and degeneration circuits to achieve a flatter passband and steeper rejection, while maintaining low power consumption and a low number of active components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional first-order N-path filters are used, then the device complexity is low, but the passband droop is large and out-of-band rejection is insufficient
Solution Approach 1:
The filter is divided into N parallel branches, each with its own switch and impedance element. By selectively connecting these segmented branches to the common node, the filter achieves higher effective order and improved passband flatness without proportionally increasing overall complexity
Solution Approach 2:
Each branch in the N-path filter serves multiple functions: it provides filtering action, contributes to the overall filter order, and enables selective frequency response shaping. The common drain amplifier circuit also serves dual purposes as both an active impedance element and a signal buffering stage
2Manufacturing precision
If the filter order is increased to improve passband flatness and rejection, then the filtering performance improves, but the number of active components and power consumption increase
Solution Approach 1:
Multiple filter branches are merged into a single N-path structure sharing common nodes and control logic. The common drain amplifier circuits in parallel branches are combined to achieve high-order filtering while reusing the same control signal infrastructure, reducing the proportional increase in active components
Solution Approach 2:
The switches in each branch are controlled by periodic non-overlapping clock signals to selectively connect branches to the common node. This periodic switching action enables the filter to achieve high-order response through time-division multiplexing of the parallel branches, improving rejection without requiring all components to be simultaneously active
Data Source
AI summary
Certain aspects of the present disclosure provide N-path filters with wider passbands and steeper rejection than conventional N-path filters with only a single pole in each filter path. These N-path filters also have a flatter passband with decreased passband droop. One example N-path filter generally includes a plurality of branches selectively connected with a common node, each branch of the N-path filter comprising a switch connected in series with an impedance comprising a common drain amplifier circuit. In certain aspects, the amplifier circuit may include a degeneration circuit for stability and/or a poly-phase feedback circuit to reduce in-band peaking.