Dual N-Path Tracking Filters for RF Front-End Isolation
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Solution Overview
Problem
Current RF front-end circuits for multi-band radios, particularly in advanced cellular radio access technologies like LTE, face challenges with bulky, costly, and inefficient surface acoustic wave (SAW) filters that fail to provide sufficient isolation and out-of-band rejection, impacting receiver sensitivity and noise figure.
Innovation Solution
A dual N-path filter configuration with two sets of filters operating on the same frequency but with different on-time durations and phases, where each filter has multiple paths with switches controlled by clock signals, allowing for additive combination of their outputs to achieve enhanced rejection characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If SAW filters are used for transmit-receive isolation, then isolation performance is improved, but device size, cost, and insertion loss worsen
Solution Approach 1:
The patent replaces mechanical SAW filters with an electronic N-path filter system that uses switched capacitors and clock signals to achieve frequency-selective filtering. This substitution eliminates the need for bulky SAW filters while maintaining isolation performance through digital control of capacitor switching at different phases of the clock cycle.
Solution Approach 2:
The patent changes the filtering approach from fixed-frequency SAW filters to dynamically controllable N-path filters where the effective filtering parameters are changed through clock signal frequency and phase adjustments. This allows the same hardware to adapt to different frequency bands and isolation requirements without physical filter changes.
2Reliability
If SAW filters are used for transmit-receive isolation, then isolation performance is improved, but insertion loss and noise figure worsen
Solution Approach 1:
The patent replaces passive SAW filters with an active N-path filter system that uses electronic switching to achieve filtering. This substitution reduces insertion loss because the switched capacitor implementation has lower inherent losses compared to SAW filter propagation losses, directly improving receiver sensitivity and noise figure.
3Reliability
If SAW filters are used for frequency isolation, then isolation performance is improved, but device complexity and cost worsen
Solution Approach 1:
The patent implements a universal N-path filter architecture that can serve multiple functions: transmit-receive isolation, frequency selection, and bandpass filtering. The same switched capacitor circuitry with phase-distributed capacitors can be configured for different frequency bands and isolation requirements, reducing overall device complexity compared to having separate dedicated filters for each function.
Solution Approach 2:
The patent merges the isolation function with the filtering function in a single N-path filter structure. The phase-distributed capacitor network simultaneously provides frequency-selective filtering and transmit-receive isolation, eliminating the need for separate filter components and reducing overall device complexity.
4Adaptability or versatility
If multi-band radios operate over wide frequency ranges, then radio capability is improved, but RF front-end circuit tunability worsens
Solution Approach 1:
The patent implements dynamic tunability in the RF front-end through the N-path filter's clock signal control. By changing the clock frequency and phase relationships, the filter's center frequency and bandwidth can be dynamically adjusted to match different radio frequency bands, enabling wide frequency range operation without complex mechanical or manual tuning mechanisms.
Data Source
AI summary
A tracking circuit has first and second filters controlled by clock signals and a combiner. Each filter has N paths in parallel between an input and an output, each path comprising a respective first/second sub-circuit and a switch (N is an integer >1). The clock signals selectively control each of the N switches of the first and second filters at a same frequency, and in variously described embodiments the first and second on-time durations may be different, the first and second sub-circuits may be different, or both first/second on-time durations and first/second sub-circuits may be different. Signals output from the first and second filter are added at a combiner. In certain examples each path in the first and second filter is controlled by same-phase clock signals, and every path on either filter are controlled with different-phase clock signals.