Swept Local Oscillator Tracking Filter for Spurious Signal Attenuation

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Solution Overview

Problem

Swept local oscillators using direct digital synthesis (DDS) face challenges in maintaining high spectral purity due to spurious spectral components and phase noise, which worsen as the frequency is swept, affecting the performance of radio-frequency applications.

Innovation Solution

A high spectral purity swept local oscillator system is achieved by using a band-pass filter with an adjustable center frequency, controlled in real-time by a phase comparator and loop filter, ensuring the filter's center frequency tracks the instantaneous frequency of the DDS oscillator, thereby attenuating spurious signals and phase noise.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If a swept DDS oscillator is used to achieve frequency agility, then frequency tuning capability is improved, but spurious spectral components and phase noise increase

Engineering Contradiction:

Improvefrequency agilityVSAvoidspurious spectral components

Core Design Contradiction:

Adaptability or versatilityVSObject-generated harmful factors

Solution Approach 1:

The patent extracts and removes spurious spectral components from the DDS oscillator output by passing the signal through a band-pass filter that selectively attenuates unwanted frequency components while preserving the desired oscillator signal

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent introduces a band-pass filter as an intermediary component between the DDS oscillator and the output, which mediates the signal by allowing the desired frequency range to pass while blocking spurious components

Inventive Principle:

Principle #24Intermediary (Mediator)

2Object-generated harmful factors

If a fixed band-pass filter is used to attenuate spurious signals, then spectral purity is improved at a specific frequency, but tracking capability across frequency sweep is lost

Engineering Contradiction:

Improvespurious spectral componentsVSAvoidfrequency tracking capability

Core Design Contradiction:

Object-generated harmful factorsVSAdaptability or versatility

Solution Approach 1:

The patent makes the band-pass filter dynamic by continuously adjusting its center frequency to track the instantaneous frequency of the swept DDS oscillator, ensuring that the filter remains effective throughout the entire frequency sweep range

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent implements a feedback mechanism where the instantaneous frequency information from the DDS oscillator is used to control the center frequency of the band-pass filter, creating a closed-loop system that automatically tracks frequency changes

Inventive Principle:

Principle #23Feedback

3Object-generated harmful factors

If the center frequency of the band-pass filter is adjusted in real-time to track the oscillator frequency, then spectral purity across frequency sweep is improved, but system complexity increases

Engineering Contradiction:

Improvespurious spectral componentsVSAvoidfilter control circuitry

Core Design Contradiction:

Object-generated harmful factorsVSDevice complexity

Solution Approach 1:

The patent makes the control circuitry multi-functional by using the same instantaneous frequency information for both generating the oscillator output and controlling the filter center frequency, eliminating the need for separate control systems

Inventive Principle:

Principle #6Universality (Multi-functionality)

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 system effectively reduces spurious spectral components and phase noise, maintaining high spectral purity across the frequency sweep, enhancing the performance of radio-frequency applications by ensuring the band-pass filter's center frequency aligns with the instantaneous frequency of the DDS oscillator.

Implementation Method 1

comparing, using a phase comparator, the phase at the output of the band-pass filter to the phase at its input

Methodology Applied

Scientific EffectPhase comparison:

Implementation Method 2

the circuitry includes a loop filter

Methodology Applied

Scientific EffectFiltering: Filter (electronic)

Implementation Method 3

the band-pass filter includes a varactor

Methodology Applied

Scientific EffectVaractor effect:

Data Source

PatentUS9083351B1Phase lock loop with tracking filter for attenuating spurious signals of a swept local oscillator

Publication Date: 2015.07.14 RAYTHEON CO

AI summary

A high spectral purity swept local oscillator including a tracking filter. The output of a swept DDS oscillator may be improved by filtering it with a band-pass filter having an adjustable center frequency, which is adjusted in real time to track the instantaneous frequency of the DDS oscillator. The tracking may be accomplished by comparing, using a phase comparator, the phase at the output of the band-pass filter to the phase at its input, and feeding back to the frequency control input of the band-pass filter a signal corresponding to the phase difference measured by the phase comparator.