RF Sampling DAC Dither Generation for Lower Spurious Emissions
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current RF sampling digital-to-analog converters (DACs) in wireless base stations face limitations due to current source mismatches, leading to high Integral Nonlinearity, which impacts spurious performance and spectral emissions, making it difficult to meet stringent spurious emission requirements.
Innovation Solution
A dither generator is introduced to improve spurious performance by adding a dither signal to the RF DAC input, utilizing a noise generator, interpolation filter, and waveform storage circuit to produce a dither signal with a low peak-to-average power ratio, effectively reducing thermal load and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If current steering DACs are implemented with thermometric and binary-weighted current sources, then the DAC can achieve basic conversion function, but current source mismatches limit spurious performance and increase Integral Nonlinearity
Solution Approach 1:
The patent applies preliminary action by adding a dither signal to the digital input of the DAC before the conversion process. This dither signal, generated by the noise generator and shaped by the interpolation filter, is introduced in advance to randomize quantization errors and reduce the impact of current source mismatches on spurious performance and Integral Nonlinearity.
Solution Approach 2:
The patent employs parameter changes by modifying the statistical characteristics of the input signal through dithering. The noise generator produces a random signal that changes the effective input parameters, thereby transforming the deterministic quantization error into a stochastic process that can be filtered and controlled, improving both INL and spurious performance.
2Reliability
If a dither signal is added to the RF DAC input to improve spurious performance, then current source mismatches are reduced, but thermal load and power consumption increase
Solution Approach 1:
The patent applies periodic action through the interpolation filter which shapes the dither signal in a structured, periodic manner. The filter processes the random noise signal through a deterministic sequence of operations that repeat at each sampling interval, creating a controlled periodic pattern that reduces peak power demands while maintaining the beneficial statistical properties of dithering.
Solution Approach 2:
The patent uses copying by generating a dither signal that is a simplified statistical copy of ideal random noise, rather than using full-random high-power noise. The noise generator produces a pseudo-random sequence that replicates the essential statistical properties of true random noise but with reduced power requirements and controlled thermal impact.
3Productivity
If current source mismatches are present in the DAC, then the basic conversion operates, but third harmonic distortion, fifth harmonic distortion, and intermodulation distortion are impacted
Solution Approach 1:
The patent introduces an intermediary element - the dither signal - that mediates between the digital input and the analog output of the DAC. This intermediary randomizes the interaction between the input signal and the mismatched current sources, transforming deterministic distortion products into stochastic noise that can be filtered, thereby reducing harmonic and intermodulation distortion while maintaining conversion functionality.
4Device complexity
If spurious emissions are not controlled, then the transmitter operates without additional filtering, but adjacent channel power ratio deteriorates and filter design becomes complex
Solution Approach 1:
The patent applies preliminary action by pre-processing the signal with dithering before DAC conversion, which reduces spurious emissions at their source. This preliminary treatment minimizes the burden on subsequent filtering stages, allowing for simpler filter designs while achieving the required adjacent channel power ratio performance.
Data Source
AI summary
A circuit includes a noise generator and a delay element. The output of the noise generator couples to the input of the delay element. The output of the delay element is coupled to a first input of a logic circuit, and the output of the noise generator is coupled to a second input of the logic circuit. The output of the logic circuit is coupled to a first control input of a waveform storage circuit. The waveform storage circuit is configured to produce a first digital waveform on its output responsive to a first logic state on the output of the logic circuit and to produce a second digital waveform on its output responsive to a second logic state on the output of the logic circuit. A sequencer has a sequencer output coupled to the second control input of the waveform storage circuit.