Level-Shifted Capacitor Switching Circuit for Low-Stress VCO Tuning
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Solution Overview
Problem
Conventional tunable oscillators face limitations in frequency tuning range and reliability due to parasitic capacitances in capacitor switching circuits, which also lead to excessive voltage stress on transistor switches, particularly in nanometer-scale designs, affecting their durability and phase noise performance.
Innovation Solution
The solution involves stacking multiple transistors with level shifting of control voltages to reduce voltage stress on individual transistor switches, using thin oxide transistors with low OFF-state capacitance, and configuring resistors to manage voltage potentials, thereby reducing parasitic capacitances and enhancing reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single transistor switch is used to control capacitor switching in the oscillator tank, then the circuit complexity is low, but the transistor experiences excessive voltage stress (up to 2×VDD) that reduces reliability and durability
Solution Approach 1:
The patent divides a single high-voltage transistor switch into multiple lower-voltage transistor switches connected in series. Each transistor in the stack experiences only a fraction of the total voltage swing (e.g., VDD instead of 2×VDD), reducing voltage stress and improving reliability. The segmentation of the switching function across multiple devices resolves the contradiction between reliability and complexity.
2Reliability
If thick oxide transistors are used to withstand high voltage stress, then transistor reliability improves, but parasitic capacitance increases which limits frequency tuning range
Solution Approach 1:
By segmenting the voltage stress across multiple thin-oxide transistors in series, each transistor operates within its safe voltage limits without requiring thick oxide. This maintains low parasitic capacitance in each transistor, preserving the wide frequency tuning range while achieving the desired reliability through distributed voltage handling.
Solution Approach 2:
The patent changes the voltage parameter distribution across the transistor stack through level-shifted control signals. Each transistor experiences a controlled voltage range appropriate for thin-oxide devices, allowing the use of low-capacitance transistors while maintaining reliability through proper voltage parameter management.
3Reliability
If large voltage swings (3V peak-to-peak) are applied across the LC tank to meet phase noise specifications, then phase noise performance improves, but transistor devices experience excessive stress that reduces lifetime and causes failures
Solution Approach 1:
The patent segments the high-voltage swing across the LC tank into multiple lower-voltage stages by using a stack of transistors. Each transistor handles a portion of the total voltage swing, preventing any single device from experiencing excessive stress that would reduce lifetime or cause failure, while the overall tank still achieves the required 3V peak-to-peak swing for phase noise performance.
Data Source
AI summary
A capacitance switching element includes first and second capacitors connected in series by transistors. The gates of the transistors are biased by a first signal through one set of resistors, and the sources and drains are biased by a second signal through a second set of resistors. The signals are level-shifted and may be complimentary. To turn the element ON, the first signal may be set to VDD and the second signal may be set to zero. To turn the element OFF, the first signal may be set to a multiple of VDD/2 and the second signal may be set to the multiple plus one of VDD/2. When the element is used in an oscillator tuning circuit, the voltage stress on the transistors is reduced and the transistors may be fabricated with thin oxide. The oscillator may be used in a transceiver of a cellular access terminal.