Switched Variable Capacitor for Linear RF Capacitance Control
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Solution Overview
Problem
Existing variable capacitors in wireless devices face limitations in achieving precise control over capacitance changes and linearity, especially when using MOS varactors, which are limited by parasitic components and require voltage variation, failing to meet the demands of sensitive RF circuits.
Innovation Solution
A variable capacitor design featuring a switch with controlled impedance between terminals, utilizing a transistor with capacitors coupled between nodes, where the capacitance is controlled via a control node, allowing for a maximum capacitance that is at least six times the minimum, and is a substantially linear function of the control word applied, implemented on a single die or in a module.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a MOS varactor is used as a variable capacitor, then voltage control is achieved, but parasitic components degrade performance in sensitive RF circuits
Solution Approach 1:
The variable capacitor is divided into multiple discrete capacitor elements (first capacitor, second capacitor, third capacitor, fourth capacitor) that can be independently switched. This segmentation allows selective connection of capacitor elements to achieve desired capacitance values while avoiding the parasitic issues of continuous varactor structures.
Solution Approach 2:
Multiple capacitor elements are combined in parallel and series configurations through switching networks. The first and second capacitors are coupled in parallel between first and second nodes, while the third and fourth capacitors are coupled in parallel between third and fourth nodes. These combinations enable precise capacitance control without parasitic degradation.
2Ease of operation
If a varactor is used to change capacitance, then voltage variation control is achieved, but the maximum capacitance is limited and may be less than required
Solution Approach 1:
The capacitor configuration dynamically changes based on switch states. Capacitor elements can be connected in parallel to maximize capacitance or in series to minimize capacitance. The switching network enables the variable capacitor to adapt its total capacitance value by selectively connecting capacitor elements C1, C2, C3, and C4 in different configurations, thereby achieving a maximum capacitance that exceeds varactor limitations.
3Quantity of substance
If discrete capacitors and switches are used instead of a varactor, then capacitance ratio is improved, but device complexity increases
Solution Approach 1:
The switching network performs multiple functions: it connects capacitor elements in parallel configurations to maximize capacitance, in series configurations to minimize capacitance, and in various intermediate states to achieve precise capacitance values. The same switching infrastructure handles both capacitance maximization and minimization, as well as intermediate values, reducing overall system complexity despite the multiple capacitor elements.
4Ease of operation
If MOS varactors are used for capacitance change, then voltage control is achieved, but linearity is insufficient for effective RF signal work
Solution Approach 1:
Instead of continuously varying capacitance through voltage changes as in varactors, the invention changes capacitance in discrete, controlled steps by switching between predefined capacitor configurations. The capacitance values are determined by the specific combinations of C1, C2, C3, and C4 connected in parallel or series, providing predictable and linear capacitance changes that improve RF signal processing accuracy.
Data Source
AI summary
A variable capacitor is disclosed, having a variable capacitance between a first node and a second node. The variable capacitor comprises a switch having a first terminal and a second terminal, the impedance between the first terminal and the second terminal being controllable via a control node. The variable capacitor further includes a first capacitor coupled between the first terminal and the first node, and a second capacitor coupled between the second terminal and the second node.