Active Harmonic Trap Filter for Deep RF Notch Rejection

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Solution Overview

Problem

Existing high-pass filters for RF radio front-ends face challenges in efficiently rejecting unwanted frequencies, particularly IEEE 802.11 wireless LAN frequencies, while maintaining low insertion loss and sharp frequency transitions, which are crucial for UWB applications, and are often hindered by the high cost of high-Q components and increased die area with higher order filters.

Innovation Solution

Incorporating a series resonant circuit with a voltage-controlled current source (VCCS) to actively manage current flow through reactive components, enhancing the effectiveness of a series resonant circuit to deepen the notch frequency and reduce unwanted signals without affecting the desired signal, and using a differential circuit with center-tapped inductors for area savings.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If high-Q reactive components are used to optimize insertion loss and frequency transition, then filter performance is improved, but component cost increases

Engineering Contradiction:

Improvefilter performanceVSAvoidcomponent cost

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent changes the Q parameter from a component property to a circuit property by using active components (operational amplifiers, voltage-controlled current sources) to dynamically control the effective Q of the resonant circuits. This allows achieving high-Q performance without relying on expensive high-Q passive components, thereby resolving the contradiction between filter performance and component cost.

Inventive Principle:

Principle #35Parameter changes

2Manufacturing precision

If higher order filters are employed to increase sharpness of frequency transition, then frequency selectivity is improved, but die area increases

Engineering Contradiction:

Improvefrequency transition sharpnessVSAvoiddie area

Core Design Contradiction:

Manufacturing precisionVSArea of stationary object

Solution Approach 1:

The patent employs dynamically controllable resonant circuits with voltage-controlled current sources that can adjust their characteristics in real-time. This dynamic control allows a lower-order filter to achieve the frequency transition sharpness of a higher-order filter, reducing the required die area while maintaining frequency selectivity.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent replaces the traditional mechanical approach of increasing filter order (adding more passive LC stages) with an electronic control mechanism using active components. The operational amplifiers and voltage-controlled current sources electronically enhance the frequency selectivity without requiring additional physical filter stages, thus reducing die area.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Reliability

If thicker metal layers are deposited to improve inductor Q, then inductor performance is improved, but manufacturing time and cost increase

Engineering Contradiction:

Improveinductor QVSAvoidmanufacturing time

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The patent uses standard-thickness metal layers to fabricate inductors with moderate Q values, accepting that these inductors alone would not provide sufficient filter performance. The performance deficiency is compensated by active Q-enhancement circuits, avoiding the need for expensive and time-consuming thick-metal deposition processes while achieving the required filter performance.

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

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

The solution achieves low insertion loss, sharp frequency transitions, and deep notch rejection of unwanted frequencies with reduced component cost and die area, optimizing filter performance for UWB applications.

Implementation Method 1

A typical series resonant circuit is formed from an inductive component (with reactance XL) and a capacitive component (with reactance XC) connected together in series. At the resonant frequency, the reactances of the two components cancel (i.e., XC−XL=0) and the series resonant circuit forms a short that sinks the signal to ground

Methodology Applied

Scientific EffectResonance: Resonance

Implementation Method 2

an active circuit; the active circuit comprising a voltage controlled current source (VCCS) arranged to change the current flow through the second reactive component in dependence on a voltage sensed (or measured) on the signal path

Methodology Applied

Scientific EffectVoltage-controlled current source effect:

Data Source

PatentUS12451868B2Filter

Publication Date: 2025.10.21 NOVELDA AS

AI summary

A filter circuit comprising: a signal path for carrying a signal from an input to an output; the signal path comprising a first reactive component; a first node on the signal path; a first series resonant circuit comprising at least a second reactive component in series with a third reactive component, the first series resonant circuit being connected between the first node and a ground; an active circuit; the active circuit comprising a voltage controlled current source (VCCS) arranged to change the current flow through the second reactive component in dependence on a voltage sensed (or measured) on the signal path. The first series resonant circuit forms a single harmonic trap with a notch frequency defined by the component values of its reactive components. The effectiveness of the series resonant circuit is dependent upon the strength with which it draws current from the signal path at its resonant frequency.