Hybrid Oscillator Frequency Control With Digital Centering in PLLs
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Solution Overview
Problem
Existing digital phase-locked loops (PLLs) face challenges in achieving high-resolution, low-noise frequency control of oscillators due to complexity, power, and area requirements, with previous approaches either requiring high-resolution DACs or large numbers of varactors to manage varactor control and noise degradation.
Innovation Solution
A hybrid analog/digital implementation that combines a digital control path with an analog control path to control varactors within a tank circuit, where the digital path uses dithering and a delta-sigma modulator, and the analog path centers the digital control signal, reducing the number of capacitors and complexity while maintaining low noise performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a high-resolution DAC is used to control an analog varactor, then frequency control resolution is improved, but device complexity and power consumption increase
Solution Approach 1:
The patent divides the frequency control into two separate paths: a digital control path that handles coarse frequency adjustments and an analog control path that handles fine frequency adjustments. This segmentation allows each path to use simpler, lower-resolution components while achieving the overall high frequency control resolution. The digital path uses a simple digital-to-analog converter controlling a first plurality of varactors, while the analog path uses a continuous control signal for a second plurality of varactors.
2Device complexity
If dithering is used to avoid DAC, then device complexity is reduced, but quantization noise degrades oscillator phase noise
Solution Approach 1:
The patent segments the varactor control into two groups: a first plurality of varactors controlled by the digital path with dithering, and a second plurality of varactors controlled by the analog path. The analog control path continuously adjusts the frequency, which suppresses the quantization noise generated by the digital dithering path, thereby maintaining low phase noise while keeping the overall system simpler than using a high-resolution DAC.
3Adaptability or versatility
If a large number of varactors are used to achieve wide frequency tuning range, then frequency range is improved, but device area and complexity increase
Solution Approach 1:
The patent divides the varactor array into two groups: a first plurality of varactors for coarse frequency tuning and a second plurality of varactors for fine frequency tuning. This segmentation allows the system to achieve a wide frequency tuning range without requiring a single large array of varactors, thereby reducing the overall device area while maintaining frequency versatility.
4Manufacturing precision
If purely analog control path is used with DAC complement, then linearity is improved, but power and area requirements increase
Solution Approach 1:
The patent implements a hybrid control system where the digital control path handles the majority of the frequency tuning range with simple switching, and the analog control path provides continuous fine adjustment for linearity. By segmenting the control functions, the system achieves good frequency control linearity without requiring a fully analog implementation, thereby reducing power consumption and area requirements compared to traditional purely analog approaches.
Data Source
AI summary
A hybrid analog/digital control approach for a digitally controlled oscillator augments a digital control path with an analog control path that acts to center the digital control path control signal within its range. The digital control path controls a first group of varactors within an oscillator tank circuit using a digital filter and a delta sigma modulator, which generates a dithered control signal for at least one of the first group of varactors. The analog control path controls a second group of varactors in the tank circuit but actively tunes only one varactor at a time. The analog control path performs relatively low bandwidth centering of the digital control signal resulting in negligible impact on PLL bandwidth, stability, and noise performance. Instead, the digital control path dominates in setting the PLL dynamic and noise behavior, and has reduced range requirements due to the centering action.