Digital Pre-Distortion for DAC Harmonic and Image Signal Cancellation
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Solution Overview
Problem
Digital-to-analog converters (DACs) introduce non-linear distortions such as harmonic signals and image signals, which are difficult to address, especially in digital media cable communication systems, where existing standards become less stringent as the number of channels increases, leading to signal distortions that affect the Spurious-Free-Dynamic-Range (SFDR) and Adjacent-Channel-Leakage-Ratio (ACLR).
Innovation Solution
A pre-distortion system that generates and applies correction signals to the input signal based on predetermined frequencies and distortion characteristics of the DAC, using signal generators, controllers, and correction modules to cancel or attenuate harmonic and image signals before digital-to-analog conversion, thereby reducing distortions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If digital signals are processed through conventional mixing and modulation to generate multi-channel output, then channel count increases and system versatility improves, but non-linear distortions including harmonic and image signals increase, worsening signal quality
Solution Approach 1:
The patent applies pre-distortion processing to the digital signal before it enters the DAC. Correction coefficients are calculated based on the expected non-linear characteristics of the DAC, and these coefficients are used to pre-modify the digital signal in advance. This preliminary action compensates for the non-linear distortions that will occur during conversion, allowing the system to maintain high channel counts while suppressing harmonic and image signal distortions.
2Measurement precision
If equalizers are used at the receiver to attenuate linear distortions, then signal quality improves, but non-linear distortions such as harmonic and image signals remain difficult to address
Solution Approach 1:
Instead of attempting to correct non-linear distortions at the receiver end, the patent applies pre-distortion at the transmitter side. The system calculates correction coefficients that represent the inverse of the expected non-linear characteristics, and applies these coefficients to the digital signal before conversion. This preliminary anti-action causes the non-linear distortions to cancel out during DAC conversion, making them unnecessary to detect or correct at the receiver.
3Ease of manufacture
If no pre-distortion is applied, then device complexity remains low and ease of manufacture is maintained, but SFDR and ACLR performance deteriorate due to uncorrected harmonic and image signals
Solution Approach 1:
The patent replaces complex analog filtering and correction hardware with digital signal processing. Correction coefficients are calculated using digital algorithms based on the DAC's transfer characteristics, and these coefficients are applied through digital multiplication and addition operations. This substitution of digital processing for analog correction mechanisms maintains relatively simple hardware architecture while achieving improved SFDR and ACLR performance.
Data Source
AI summary
A pre-distortion circuit that may introduce a pre-distortion signal in a communication channel by determining a harmonic signal of the signal to be output. One or more image correction signals of the signal to be output may be determined. The one or more image correction signals may be complex conjugate signal variations of the signal to be output. The harmonic signal, the one or more image correction signals and the signal to be output may be combined into a combined output signal. The combined output signal may be transmitted to a digital-to-analog converter. The predistortion circuit may be implemented in a FPGA, an ASIC, a digital-to-analog converter, and/or a separate IC package.