Successive Requantizer for Fractional-N PLL Spurious Tone Control
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Solution Overview
Problem
Fractional-N phase-locked loops (PLLs) in wireless communication systems face challenges with spurious tones introduced by digital delta-sigma modulators, which are exacerbated by nonlinear analog circuit behavior, leading to increased power consumption, circuit area, and limited design options.
Innovation Solution
A successive requantizer is introduced to replace the delta-sigma modulator, performing digital quantization one bit at a time to engineer quantization noise with desirable properties such as non-linearity robustness, thereby reducing spurious tones and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a digital delta-sigma modulator is used to coarsely quantize the digital sequence in a fractional-N PLL, then the output frequency can be set to a fractional value (N+α)fref, but spurious tones are introduced due to quantization noise being subjected to nonlinear distortion in analog circuitry
Solution Approach 1:
The quantization process is divided into multiple stages: a first quantizer performs coarse quantization, and a second quantizer performs fine quantization on the error signal. This segmentation allows the system to achieve fractional frequency output while reducing spurious tones by distributing quantization noise across multiple stages rather than relying on a single high-resolution quantizer that would be more susceptible to nonlinear distortion.
Solution Approach 2:
An error signal is introduced as an intermediary between the first and second quantizers. The error signal represents the difference between the desired fractional frequency and the coarse quantization output, and it serves as the input to the second quantizer. This intermediary approach allows the system to achieve precise fractional frequency control while keeping each quantization stage operating in a more linear regime, thereby reducing spurious tone generation.
2Object-generated harmful factors
If highly linear analog circuitry is used to suppress spurious tones, then spurious tone levels are reduced, but power consumption and circuit area increase
Solution Approach 1:
The patent replaces the need for highly linear analog circuitry with a digital solution. By using multiple quantization stages with an error signal, the system achieves spurious tone suppression through digital processing rather than relying on linear analog circuitry. This substitution of digital for analog approaches reduces power consumption and circuit area while maintaining or improving spurious tone suppression performance.
3Object-generated harmful factors
If highly linear analog circuitry is used to suppress spurious tones, then spurious tone levels are reduced, but circuit area increases
Solution Approach 1:
The patent replaces the need for highly linear analog circuitry with a digital solution. By using multiple quantization stages with an error signal, the system achieves spurious tone suppression through digital processing rather than relying on linear analog circuitry. This substitution of digital for analog approaches reduces power consumption and circuit area while maintaining or improving spurious tone suppression performance.
Solution Approach 2:
The patent changes the operating parameters of the quantization process by introducing multiple stages with different resolution levels. The first quantizer operates at lower resolution and the second at higher resolution, with the error signal bridging them. This parameter change allows the system to achieve the same spurious tone suppression that would otherwise require high-resolution single-stage quantization or highly linear analog circuitry, thereby reducing circuit area.
Data Source
AI summary
A successive requantizer, which serves as a replacement for a ΔΣ modulator in a fractional-N PLL or a DAC, and avoids spurious tone problems, thereby circumventing the tradeoffs that result from reliance on the common approach of making highly linear analog circuitry to avoid spurious tones. A successive requantizer fractional-N PLL of the invention has the potential to reduce power consumption and the cost of commercial communication devices. The successive requantizer performs digital quantization one bit at a time in such a way that the quantization noise can he engineered to have desirable properties such as non-linearity robustness. The successive requantizer is applicable to most high-performance digital communication systems, such as cellular telephone handsets and wireless local and metropolitan area network transceivers.