Variable capacitor | EPFL Graph Search

A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio (therefore it is sometimes called a tuning capacitor or tuning condenser), or as a variable reactance, e.g. for impedance matching in antenna tuners. Mechanically controlled capacitance In mechanically controlled variable capacitors, the distance between the plates, or the amount of plate surface area which overlaps, can be changed. The most common form arranges a group of semicircular metal plates on a rotary axis ("rotor") that are positioned in the gaps between a set of stationary plates ("stator") so that the area of overlap can be changed by rotating the axis. Air or plastic foils can be used as dielectric material. By choosing the shape of the rotary plates, various functions of capacitance vs. angle can be created, e.g. to obtain a

Design of novel radiating elements for SATCOM phased arrays in Ku-Band

Baptiste Fernand Hornecker

The present thesis deals with the design of a planar antenna array for the communication between a civil aircraft (Dassault's Falcon) and an Inmarsat satellite. The final goal is to provide high speed Internet access in X/Ku-Band (8-12GHz / 12-18GHz), and possibly even in K/Ka- Band (18-27GHz / 17-40GHz). The study starts with the Ku-Band only, then evolves towards the transmission band and finishes with their Ka-Band counterparts. The main constraint of the thesis is that the final result should be a low profile antenna that will be easy to integrate below the fuselage of an aircraft. At the same time, the antenna has to provide a beam steering mechanism in order to track the position of the targetted satellite while the aircraft is moving. The usual way to achieve it is to implement some dedicated electronic chips below each radiating element of array. Dassault Aviation has its own alternate and more mechanical oriented solutions. Since the communication system is intended for top-notch business jets produced in small series, cost is not here a decisive criterion. This thesis assumes the existence of the beam-steering system and concentrates on a proof of concept and careful design of the antenna elements that will constitute the final array. They have to satisfy tight constraints in terms of size, return loss, mutual coupling with neighbors, efficiency, radiation pattern and dual circular polarization quality. Printed multilayer antennas have been retained as the best candidates, and themain parts of the thesis have been dedicated to their study. The thesis concludes with the presentation of an apparently completely unrelated and independant topic, namely the simulation of nanoscale 2D planar structures based on Graphene. The rationale for this last chapter is to provide a prospective study of the use of Graphene to create a variable capacitance. This component is always needed in the beamforming network of reconfigurable and scanning arrays. Our laboratory is developing a deep knowledge of the electromagnetic properties of Graphene-based devices and these varicaps could lead to very interesting applications in high frequencies, once the Graphene technology has definitely progressed. Thanks to electrostatics Green's functions, a variable capacitor based on Graphene has been simulated, and a Quantumeffect -the Quantum capacitance of Graphene- has been successfully tackled from a numerical point of view.

EPFL2016

A single-chip array of NMR receivers

Jens Anders, Giovanni Boero, Paul Sangiorgio

We present the first single-chip array of integrated NMR receivers for parallel spectroscopy and imaging. The array, optimized for operation at 300 MHz, is composed of eight separate channels, with each channel consisting of a detection coil, a tuning capacitor, a low noise amplifier and a 50 Omega buffer. As all the integrated electronics are placed underneath the reception coils, the array is densely packed. Each single-channel reception coil has a diameter of 500 mu m, resulting in a total active area of 1 mm by 2 mm for the array. The H-1 time-domain spin sensitivity of a single channel is approximately 1 X 10(15) spins/root Hz. (C) 2009 Elsevier Inc. All rights reserved.

2009

A fully integrated IQ-receiver for NMR microscopy

Jens Anders, Giovanni Boero, Paul Sangiorgio

We present a fully integrated CMOS receiver for micro-magnetic resonance imaging together with a custom-made micro-gradient system. The receiver is designed for an operating frequency of 300 MHz. The chip consists of an on-chip detection coil and tuning capacitor as well as a low-noise amplifier and a quadrature downconversion mixer with corresponding low-frequency amplification stages. The design is realized in a 0.13 mu m CMOS technology, it occupies a chip area of 950 x 800 mu m(2) and it draws 50 mA from a supply voltage of 1.8 V. The achieved time-domain spin sensitivity is 5 x 10(14)spins/root Hz. Images of phantoms obtained in our custom-made gradient system with 8 mu m isotropic resolution are reported. (C) 2010 Elsevier Inc. All rights reserved.

2011

Design of novel radiating elements for SATCOM phased arrays in Ku-Band

Baptiste Fernand Hornecker

EPFL2016