DDS Signal Phase Variation to Suppress Radar Frequency Spurs
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Solution Overview
Problem
Radar systems using Direct Digital Synthesizers (DDS) face issues with frequency spurs, which increase the noise floor and make it difficult to detect smaller targets due to the modulation of unwanted signals, particularly in CW radar systems.
Innovation Solution
The system employs a DDS to generate a primary output frequency characteristic that remains consistent across multiple signals within an integration period, with changes in the input clock source frequency or starting phase between each signal, ensuring that frequency spurs have different characteristics, thereby reducing their accumulation during coherent integration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a DDS device is used to generate modulated signals in CW radar, then signal generation flexibility and precise control are improved, but frequency spurs increase the noise floor and reduce detection capability
Solution Approach 1:
The patent applies periodic action by systematically varying the phase or frequency of the DDS output signal across multiple transmitted signals. This periodic variation causes the frequency spurs to shift positions in the spectrum, so that when signals are coherently integrated, the spurs do not accumulate at the same frequency locations, thereby reducing the overall noise floor while maintaining the flexibility benefits of DDS signal generation.
Solution Approach 2:
The patent implements parameter changes by modifying the phase or frequency parameters of the DDS output signal between successive transmitted signals. These parameter variations cause the spurious signals to appear at different frequency positions during integration, preventing their coherent accumulation and thus reducing the noise floor while preserving the precise control capability of the DDS device.
2Measurement precision
If multiple signals are coherently integrated to improve signal to noise ratio, then detection capability is improved, but frequency spurs from DDS accumulate and increase the noise floor
Solution Approach 1:
The patent uses periodic action by applying systematic phase or frequency variations to the DDS output across the multiple signals being integrated. This causes the frequency spurs to periodically shift their spectral positions, ensuring that during coherent integration, the spurs do not maintain consistent phase relationships and therefore do not accumulate constructively, while the desired signal maintains its coherence.
Solution Approach 2:
The patent applies parameter changes by modifying the phase or frequency parameters of the transmitted signals generated by the DDS. These parameter changes cause the spurious components to appear at different frequency locations during the integration process, preventing their coherent accumulation and thus allowing multiple signals to be integrated without significant noise floor elevation.
3Object-affected harmful factors
If the DDS output phase is changed between signals to reduce spur accumulation, then noise floor is reduced, but signal coherence for integration may be affected
Solution Approach 1:
The patent applies local quality by making a selective distinction between the desired signal component and the spurious signal components. By carefully controlling which parameters are varied (phase or frequency) and by what amount, the invention ensures that the desired signal maintains its coherence for integration while the spurious components, which have different spectral characteristics, are caused to shift positions and thus do not accumulate. This localized differentiation in treatment resolves the contradiction between reducing noise floor and maintaining signal coherence.
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
This approach effectively reduces the impact of spurs on the noise floor, enhancing the detection of smaller targets by smearing out spur phases during integration, resulting in a lower noise level and improved signal-to-noise ratio.
Implementation Method 1
a mixer for mixing the received signal with a delayed version of itself to derive an intermediate frequency (IF) signal
Implementation Method 2
an integrator for coherently integrating the derived IF signals from the first and subsequent modulated signals
Data Source
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AI summary
A DDS based system, such as a radar, includes means for generating a plurality of transmission signals using a DDS, and means for integrating signals derived therefrom, such as received signals. The system further includes means for varying the relative starting phase of the plurality of transmission signals, or adjusting the DDS input clock whilst maintaining similar primary output frequency characteristics of the transmission signals. The approach has the effect of changing the location of unwanted frequency spurs in each of the transmission signals, and hence the effects of these are decreased in the integration process. An improvement in the sensitivity of the system results. Although primarily suited to radar applications the invention may find utility in other systems such as sonar or lidar systems.