LC Oscillator Fine Tuning With Varactor Banks and Sigma-Delta Control
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Solution Overview
Problem
Existing digitally controlled oscillators (DCOs) used in digital phase locked loops, such as those in Bluetooth radios, face challenges in achieving the required resolution for Gaussian frequency shift keying (GFSK) applications, particularly in maintaining low quantization error phase noise, especially at a carrier frequency of 2.4 GHz, where the capacitance for tuning needs to be on the order of 4.4 attofarads for the least significant bit.
Innovation Solution
A digitally controlled oscillator (DCO) design incorporating an LC tank circuit with a fine tuning mechanism that includes an RC filter, a low frequency tuning bank of varactors, and a high frequency tuning bank controlled by a sigma delta modulator, utilizing a binary control word with integer and fractional parts to adjust varactor switches and minimize noise contributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional LC tank oscillator is used for GFSK applications, then the device structure is simple, but the frequency resolution is insufficient (cannot achieve less than 2.4 KHz resolution at 2.4 GHz carrier frequency)
Solution Approach 1:
The oscillator is divided into multiple functional blocks: LC tank circuit for frequency generation, varactor tuning circuits for frequency adjustment, sigma-delta modulator for high-resolution control, and digital filter for noise reduction. Each block performs a specific function to collectively achieve the required frequency resolution while managing complexity through modular design.
Solution Approach 2:
The patent transitions from direct digital control of capacitance to a multi-dimensional control approach using sigma-delta modulation. The control word is processed through a sigma-delta modulator that operates at a higher sampling rate, effectively adding a time-domain dimension to the frequency control mechanism and achieving sub-2.4 KHz resolution.
2Measurement precision
If the capacitance for tuning is reduced to achieve the required resolution (4.4 attofarads for LSB), then the frequency resolution improves, but the quantization error phase noise increases
Solution Approach 1:
The patent implements a feedback mechanism where the output of the sigma-delta modulator is fed into a digital filter that processes the control signal before applying it to the varactor tuning circuit. This feedback loop helps reduce quantization errors by filtering out high-frequency noise components and smoothing the control signal, thereby reducing phase noise while maintaining frequency resolution.
Solution Approach 2:
The patent changes the parameters of the control system by using a sigma-delta modulator operating at a high sampling rate (e.g., 10x the output frequency) and employing a digital filter with specific cutoff frequencies. These parameter changes allow the system to achieve fine frequency resolution while suppressing quantization noise through careful selection of modulation and filtering parameters.
3Measurement precision
If a high resolution digitally controlled oscillator is used for GFSK with 250 KHz peak modulation depth, then the modulation accuracy improves, but the device complexity increases
Solution Approach 1:
The patent designs a multi-functional DCO structure where the same core circuitry serves multiple purposes: frequency generation, frequency modulation, and high-resolution control. The varactor tuning circuits and sigma-delta modulator work together to provide both coarse and fine frequency adjustment, eliminating the need for separate circuits for different modulation depths and improving overall system efficiency.
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 proposed DCO design achieves the necessary resolution for GFSK applications with reduced quantization noise, ensuring the output signal is locked to the reference signal in phase and frequency, effectively addressing the limitations of previous designs by using a combination of varactor banks and sigma delta modulation for precise frequency control.
Implementation Method 1
a low frequency tuning bank coupled to a common node, the low frequency tuning bank comprised of a first plurality of varactors each having switches respectively associated therewith
Implementation Method 2
an RC filter
Implementation Method 3
a sigma delta modulator receiving the fractional portion of the digital control word as input and controlling the switches of the high frequency tuning bank based upon a voltage represented by the fractional portion
Data Source
AI summary
Disclosed herein is a fine capacitance tuning circuit for a digitally controlled oscillator. The tuning circuit has low and high frequency tuning banks formed by varactors that have their top plates connected to one another. A controller initially sets states of switches selectively connecting the bottom plates of the varactors of the low frequency bank to a low voltage, a high voltage, or to an RC filter, in response to an integer portion of a control word. A sigma-delta modulator initially sets the states of switches selectively connecting the bottom plates of the varactors of the high frequency bank to either the low voltage or the high voltage, in response to a fractional portion of the control word. The controller modifies the states of the switches of the tuning banks in a complementary fashion, based upon comparisons between the fractional portion of the control word and a series of thresholds.