T-Network Capacitor Bank for Fine VCO Capacitance Resolution
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Solution Overview
Problem
Conventional capacitor banks for high frequency voltage controlled oscillators (VCOs) face challenges in providing precise variable capacitance values due to manufacturing variations, especially at high frequencies like 12 GHz, where small capacitance values lead to significant random variations impacting system performance.
Innovation Solution
The use of T-network capacitor cells with MIM capacitors and NMOS transistors, where each T-network capacitor unit is connected in series or parallel to provide a high resolution capacitance value, reducing the impact of manufacturing non-uniformities by utilizing larger capacitance values, thus achieving a capacitance difference six times larger than conventional cells with similar desired change.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitor cells with small capacitance values are used, then the capacitance resolution requirement is met, but manufacturing variations cause large random capacitance variations
Solution Approach 1:
The capacitor cell is segmented into a T-network configuration with three separate MIM capacitors (C1, C2, C3) instead of a single capacitor. This segmentation allows the equivalent capacitance to be determined by a combination of multiple larger capacitors, reducing the impact of manufacturing variations on the total capacitance value while maintaining fine resolution control through digital-to-analog conversion of the capacitor selections.
Solution Approach 2:
The invention changes the parameter approach by using larger individual capacitor values in a T-network configuration rather than small capacitor values. The equivalent capacitance is controlled by changing which capacitors are connected in the network through switch control, allowing fine capacitance resolution to be achieved through combinatorial selection rather than relying on small capacitor values, thereby reducing sensitivity to manufacturing variations.
2Manufacturing precision
If larger capacitance values are used, then manufacturing variations are reduced, but the capacitance resolution becomes too coarse for high frequency applications
Solution Approach 1:
The capacitor cell is segmented into a T-network configuration with three separate MIM capacitors (C1, C2, C3) instead of a single capacitor. This segmentation allows the equivalent capacitance to be determined by a combination of multiple larger capacitors, reducing the impact of manufacturing variations on the total capacitance value while maintaining fine resolution control through digital-to-analog conversion of the capacitor selections.
Solution Approach 2:
The invention adds a dimensional aspect by using a T-network topology with multiple branches instead of a simple series or parallel configuration. This multi-dimensional arrangement allows independent control of different capacitor combinations, enabling fine capacitance resolution to be achieved through combinatorial selection of capacitors in different network branches, effectively adding a control dimension that resolves the contradiction between large capacitor values and fine resolution.
3Device complexity
If conventional capacitor cells are used, then the circuit complexity is low, but the capacitance change is limited by parasitic capacitances
Solution Approach 1:
The capacitor cell is segmented into a T-network configuration with three separate MIM capacitors (C1, C2, C3) instead of a single capacitor. This segmentation allows the equivalent capacitance to be determined by a combination of multiple larger capacitors, reducing the impact of manufacturing variations on the total capacitance value while maintaining fine resolution control through digital-to-analog conversion of the capacitor selections.
Solution Approach 2:
The T-network configuration acts as an intermediary structure between the digital control signals and the analog capacitance value. The network of capacitors and switches serves as a mediator that translates digital bit selections into precise analog capacitance values, improving the reliability of capacitance control by providing a structured transformation path that minimizes the direct impact of parasitic capacitances.
Data Source
AI summary
A capacitor bank adapted to provide a variable capacitance to an electronic circuit is provided. The capacitor bank has at least a most significant bit and a least significant bit and includes a first capacitor cell having a first capacitance value and disposed between a first node and a second node, thereby forming the most significant bit. The capacitor bank also includes a second capacitor cell having a second capacitance value and disposed between the first node and the second node, thereby forming the least significant bit. The second capacitance value is less than the first capacitance value. Additionally, the second capacitor cell includes a first T-network capacitor unit in electrical communication with a source of a transistor and a second T-network capacitor unit in electrical communication with a drain of the transistor.