DDS Distortion Correction With Notch Filtering for Spur Reduction

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Solution Overview

Problem

Direct digital synthesizers (DDS) generate spurious frequency components in analog waveforms, which are difficult to remove using traditional filtering techniques, affecting the accuracy of signal transmission to circuits-under-test.

Innovation Solution

A method and apparatus that generate a digital sinusoidal waveform, combine it with an analog distortion correction waveform, and use notch filter circuitry to filter out spurious components, generating a filtered composite output waveform, which is then amplified and converted back to digital form to analyze and correct distortion, including the use of Discrete Fourier Transform to determine harmonic amplitudes and phases for calibration.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If traditional low-distortion sinusoidal generators are used to remove spurious components, then signal quality improves, but device complexity increases significantly

Engineering Contradiction:

Improvesignal qualityVSAvoidfiltering technique complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent changes the operating parameters of the DDS device by introducing correction signals that modify the phase and amplitude of the output waveform. The system measures distortion components and applies compensating signals with specific phase and amplitude parameters to cancel spurious components, thereby improving signal quality without requiring complex filtering hardware

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent implements a feedback mechanism where the output signal from the DDS is measured to detect spurious components, and this measurement is used to generate correction signals that are fed back to cancel the distortion. The system continuously monitors and adjusts the correction signals based on the measured distortion levels, achieving low distortion through closed-loop control rather than complex open-loop filtering

Inventive Principle:

Principle #23Feedback

2Productivity

If DDS devices operate at high speeds with fast switching, then productivity improves, but spurious frequency components increase

Engineering Contradiction:

Improveswitching speedVSAvoidspurious frequency components

Core Design Contradiction:

ProductivityVSObject-generated harmful factors

Solution Approach 1:

The patent converts the harmful spurious components generated by fast DDS switching into a measurable signal that can be used to create correction signals. The system measures the spurious components and uses this information to generate compensating signals that cancel the distortion, thereby transforming the harmful effect into a useful measurement that enables automatic correction

Inventive Principle:

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

Solution Approach 2:

The patent applies preliminary anti-action by generating correction signals in advance that are designed to cancel the expected spurious components. The system pre-calculates the required correction based on measured distortion characteristics and applies these correction signals before they can cause significant degradation, enabling fast switching while maintaining signal quality

Inventive Principle:

Principle #9Preliminary anti-action

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

Effectively reduces spurious frequency components in DDS-generated waveforms, improving signal quality and accuracy for testing circuits-under-test by using a combination of digital-to-analog and analog-to-digital conversion with notch filtering and distortion correction.

Implementation Method 1

notch filter circuitry configured to receive the composite output waveform and to filter the composite output waveform to generate a filtered composite output waveform

Methodology Applied

Scientific EffectNotch filtering: Filter (electronic)

Implementation Method 2

generating a Discrete Fourier Transform (DFT) of the amplified digital signal to provide at least one of an amplitude and a phase of a non-fundamental component of the analog sinusoidal waveform

Methodology Applied

Scientific EffectDiscrete Fourier Transform:

Data Source

PatentUS8269480B2Method and apparatus for identifying and reducing spurious frequency components

Publication Date: 2012.09.18 XCERRA CORP

AI summary

A method for identifying and reducing spurious frequency components is provided. A method in accordance with at least one embodiment of the present disclosure may include generating a digital sinusoidal waveform at a direct digital synthesizer (DDS) and receiving the digital sinusoidal waveform at an audio digital-to-analog converter. The method may further include converting the digital sinusoidal waveform to an analog sinusoidal waveform containing spurious frequency components, combining the analog sinusoidal waveform with an analog distortion correction waveform to generate a composite output waveform and receiving the composite output waveform at notch filter circuitry. The method may also include filtering the composite output waveform to generate a filtered composite output waveform and amplifying a difference between the filtered composite output waveform and a signal from a circuit-under-test (CUT) to generate an amplified analog signal. The method may also include converting the amplified analog signal to an amplified digital signal. Of course, additional implementations are also within the scope of the present disclosure.