Receiver Mixer DC Offset Injection for IMD2 Cancellation

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Solution Overview

Problem

Digital communication systems face impairments such as second-order intermodulation distortion (IMD2) and direct current (DC) offset, which degrade receiver performance and dynamic range, particularly due to local oscillator leakage and undesired DC components in RF receiver circuits.

Innovation Solution

Injecting a controlled DC offset at the mixer output to counteract local oscillator leakage and using DC estimation engines to determine and correct DC offsets, thereby improving the input-referred second-order intercept point (IIP2) performance and removing DC components before analog-to-digital conversion.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If local oscillator leakage is present in RF receiver circuits, then the receiver can operate with standard mixing architecture, but second-order intermodulation distortion performance degrades and IIP2 decreases

Engineering Contradiction:

Improvemixing architectureVSAvoidIIP2 performance

Core Design Contradiction:

Ease of manufactureVSManufacturing precision

Solution Approach 1:

The patent applies preliminary anti-action by injecting a DC offset signal at the mixer output before the harmful LO leakage can completely degrade the IIP2 performance. This DC offset creates a counteracting effect that pre-compensates for the second-order intermodulation distortion, thereby improving the overall distortion performance while maintaining the standard mixing architecture

Inventive Principle:

Principle #9Preliminary anti-action

Solution Approach 2:

The patent converts the harmful effect of LO leakage into a benefit by utilizing the same mixing mechanism that causes second-order distortion to also upconvert the injected DC offset. This creates an anti-LO leakage signal that cancels the harmful leakage, transforming the mixer's vulnerability into a solution for improving IIP2 performance

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

2Ease of operation

If DC offset is present at mixer output, then the receiver can process signals through standard conversion paths, but dynamic range of analog-to-digital converters decreases and noise performance degrades

Engineering Contradiction:

Improvesignal processing pathVSAvoiddynamic range

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The patent extracts and removes the harmful DC offset component from the signal path using DC offset cancellers positioned after the mixer. By separating the DC removal function from the main signal processing path, the system maintains ease of operation while protecting the analog-to-digital converters from DC-induced dynamic range degradation and noise artifacts

Inventive Principle:

Principle #2Taking out (Extraction)

3Reliability

If DC offset cancellation is implemented to improve dynamic range, then analog-to-digital converter performance improves, but device complexity increases

Engineering Contradiction:

Improvedynamic rangeVSAvoidcircuit architecture

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent introduces DC offset cancellers as intermediary components between the mixer and analog-to-digital converter. These cancellers act as mediators that selectively remove DC offsets without affecting the main signal path, thereby improving dynamic range while adding minimal complexity through dedicated DC correction stages rather than redesigning the entire receiver architecture

Inventive Principle:

Principle #24Intermediary (Mediator)

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

Enhances distortion performance and reduces DC-related noise, improving signal detection and dynamic range by effectively canceling DC offsets and mitigating the impact of local oscillator leakage on receiver IIP2 performance.

Implementation Method 1

downconverting mixers

Methodology Applied

Scientific EffectFrequency mixing:

Implementation Method 2

local oscillator (LO) leakage to the inputs of various RF receiver circuits, including downconverting mixers

Methodology Applied

Scientific EffectLO leakage:

Implementation Method 3

DC offset cancellation in receivers

Methodology Applied

Scientific EffectDC offset cancellation:

Data Source

PatentUS8213555B2Methods and apparatus to improve distortion performance and direct current (DC) offset cancellation in receivers

Publication Date: 2012.07.03 TEXAS INSTRUMENTS INC

AI summary

Methods, apparatus, and articles of manufacture are described for improving distortion performance and for direct current offset cancellation in receivers. In one example, a method of improving distortion in a receiver includes determining a direct current (DC) amplitude of an offset signal, generating the offset signal, and providing the offset signal to an output of a mixer to improve the distortion performance of the mixer.