DDS Filter Network for High-Frequency Spurious Suppression
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Solution Overview
Problem
Signal processing systems face challenges in achieving high spurious free dynamic range (SFDR) performance, particularly at high frequencies, due to degraded spurious emission performance in direct digital synthesizers (DDS), which limits their use in devices like RF receivers.
Innovation Solution
A system comprising a direct digital synthesizer (DDS) with an anti-aliasing filter and an output filter network, including switches and filters, is configured to provide a wideband and narrowband spurious free dynamic range of −40 dBc or better, utilizing a frequency plan with clean frequencies that are sub-multiples of the DDS reference frequency, and a clean frequency step multiple determined by the DDS Nyquist frequency, to enhance SFDR performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a direct digital synthesizer (DDS) is used for signal generation, then frequency resolution and tuning flexibility are improved, but spurious emission performance degrades at high frequencies
Solution Approach 1:
The output spectrum is segmented into multiple bands using a bank of band-pass filters, with each filter targeting a specific frequency range. This segmentation allows spurious emissions to be selectively rejected in different spectral regions, improving overall SFDR performance while maintaining DDS flexibility.
Solution Approach 2:
An anti-aliasing filter is introduced as an intermediary component between the DDS output and the output filter network. This filter serves as a preliminary stage to attenuate aliasing components and spurious emissions before the signal reaches the main filtering stage, thereby improving spurious emission performance.
2Speed
If the DDS input clock frequency is increased to achieve higher output frequencies, then the frequency range is expanded, but spurious free dynamic range performance degrades
Solution Approach 1:
The anti-aliasing filter performs preliminary filtering of spurious emissions and aliasing components before the signal is further processed. By addressing spurious emissions early in the signal chain, the system maintains better SFDR performance even at higher output frequencies generated by increased clock frequencies.
Solution Approach 2:
The problem of spurious emissions at high frequencies is addressed by moving to a different dimensional approach - using multiple band-pass filters that operate in parallel across different frequency bands. This allows simultaneous coverage of a wide frequency range while maintaining high SFDR performance in each individual band.
3Measurement precision
If additional filtering components are added to improve SFDR performance, then spurious emission rejection is enhanced, but device complexity increases
Solution Approach 1:
The band-pass filter bank serves multiple functions simultaneously: it provides frequency selection, spurious emission rejection, and anti-aliasing. By combining these functions into a single filter network structure, the system achieves high SFDR performance without proportionally increasing device complexity.
Solution Approach 2:
The system optimizes filter parameters such as center frequencies, bandwidths, and Q-factors to achieve maximum spurious emission rejection with minimal filter stages. By carefully selecting filter parameters based on the specific DDS operating conditions, high SFDR performance is achieved with a relatively compact filter network.
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 significantly improves narrowband and wideband SFDR performance to −70 dBc or better, allowing for improved spurious emission reduction and filtering, enabling the use of DDS technology in high-frequency applications like RF receivers with enhanced sensitivity and reduced interference.
Implementation Method 1
an anti-aliasing filter (AAF) operatively coupled with the DDS
Implementation Method 2
an output filter network configured to receive the amplified third output signal and to provide a filtered fourth output signal having a wideband and narrowband spurious free dynamic range (SFDR) of about −70 dBc or better
Data Source
AI summary
Techniques and architecture are disclosed for improving spurious performance in a signal generator/system. The disclosed techniques/architecture can be used, for example, to enhance/improve the wideband and/or narrowband spurious free dynamic range (SFDR) between a given carrier signal and spurious signals. In some example instances, wideband and/or narrowband SFDR may be improved to about −40 dBc or better. In some other example instances, wideband and/or narrowband SFDR may be improved to about −70 dBc or better. The disclosed techniques/architecture can be implemented in a wide variety of signal generators/systems, such as a direct digital synthesizer (DDS)-based system, and over a wide range of input clock frequencies (e.g., in the range of about 10 MHz to 40 GHz, or higher).