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1486 results in Circuits and systems

Page 19 of 75

  • 7 - Digital Hearing Aids

  • Andrew G. Webb
  • Book:
    Principles of Biomedical Instrumentation
    Published online:
    11 August 2019
    Print publication:
    11 January 2018, pp 196-234

  • Summary

    Introduction

    Roughly 10% of the world's population suffers from some form of hearing loss, with over 5% or 360 million individuals suffering from hearing impairment that is considered to be disabling by World Health Organization (WHO) standards. The most common cause of hearing loss is damage to the cochlea, which results in reduced frequency selectivity and associated blurring of complex sounds. According to statistics compiled by the National Institute on Deafness and Other Communication Disorders (NIDCD) approximately 2 to 3 out of every 1000 children in the United States are born with a detectable level of hearing loss in one or both ears. One in eight people in the United States aged 12 years or older has hearing loss in both ears. Approximately 2% of adults aged 45 to 54, 8.5% aged 55 to 64, 25% aged 65 to 74, and 50% of those 75 and older have disabling hearing loss. Figure 7.1 shows the frequency and temporal characteristics of human speech, as well as the loss in different frequency ranges and classification of hearing loss from mild to profound.

    Among people aged 70 and older who could benefit from hearing aids, only ~30% have ever used them, and the percentage for younger adults is significantly lower. The main reasons are the physical visibility of the device (related to social issues), the need for manual adjustments and problems in sub-optimal conditions such as strong winds and distracting background sounds (technical issues). In order to address the societal issue hearing aids are continually being designed to be smaller, with many of the miniaturization techniques outlined throughout this book enabling the reduction in size. Technical issues are being addressed via new developments in both hardware design as well as more powerful signal processing software. Each of these developments presents design challenges in terms of the requirement to package an ever-greater number of electronics in a more limited space, and to ensure that these electronics can operate in a power-efficient manner so as to maintain long battery life.

    This chapter outlines the basic design of digital hearing aids, and the principles behind these designs [1]. The first sections in this chapter consider the physiology and acoustic properties of the human auditory system, the causes of hearing loss and the basics of hearing-aid design.

Principles of Biomedical Instrumentation
  • Andrew G. Webb
  • Published online:
    11 August 2019
    Print publication:
    11 January 2018
  • This accessible yet in-depth textbook describes the step-by-step processes involved in biomedical device design. Integrating microfabrication techniques, sensors and digital signal processing with key clinical applications, it covers: the measurement, amplification and digitization of physiological signals, and the removal of interfering signals; the transmission of signals from implanted sensors through the body, and the issues surrounding the powering of these sensors; networks for transferring sensitive patient data to hospitals for continuous home-monitoring systems; tests for ensuring patient safety; the cost-benefit and technological trade-offs involved in device design; and current challenges in biomedical device design. With dedicated chapters on electrocardiography, digital hearing aids and mobile health, and including numerous end-of-chapter homework problems, online solutions and additional references for extended learning, it is the ideal resource for senior undergraduate students taking courses in biomedical instrumentation and clinical technology.

  • 3 - CAD Techniques

  • Giovanni Ghione, Politecnico di Torino, Marco Pirola, Politecnico di Torino
  • Book:
    Microwave Electronics
    Published online:
    10 November 2017
    Print publication:
    09 November 2017, pp 87-128

  • Summary

    Modeling of Linear and Nonlinear Blocks

    In an integrated microwave circuit, passive devices (such as resistors, inductors, capacitors, transformers, simple and coupled transmission lines) are linear blocks, and can therefore be characterized by linear models. Transistors (field effect or bipolar) and diodes are, instead, nonlinear components also in DC or quasi-static conditions, although they can behave in an approximately linear way under small-signal operation. Linear passive reactive components (such as linear inductors and capacitors) are with memory in the (v, i) port variables; nonlinear reactive elements, like the capacitances of pn or metal-semiconductor junctions, on the other hand are also associated with active components, such as transistors. Thus, MMICs generally include linear and nonlinear elements, memoryless or with memory.

    The small-signal approximation, where the transistor characteristics are linearized in the neighborhood of the DC operating point, is well suited to the design of high-gain or low-noise amplifiers; in small signal conditions the active device is approximated as a linear block with memory. The simulation of distortion and power saturation occurring in power amplifiers (but also in low-noise and high-gain amplifiers in the upper limit of their spurious free dynamic range, see Sec. 8.2.3) require instead a nonlinear transistor model, often including linear and nonlinear memory effects.

    In analog circuits, linear n-ports are conveniently modeled in the frequency domain, selecting conventional variables (such as voltages and currents) as the component input and output, or exploiting circuit variables that are more suited to the analysis and characterization of microwave circuits, such as the power waves, see Sec. 3.2. A linear n-port can be represented through a linear relationship between input and output variables; if we identify the component status in terms of the Fourier transforms of port voltages and currents, according to the choice of the input and output variable sets we obtain wellknown frequency-domain models such as the series (or current-driven, or impedance) representation and the parallel (or voltage-driven, or admittance) representation [1].

    Although a linear element is completely described by a proper set of parameters conveniently sampled over the frequency band of interest, in many cases an equivalent circuit approach is preferred. The equivalent circuit is an approximate model of the component, whose topology and element parameters have to be fitted on measured (or physically simulated) characteristics.

  • 10 - CAD Projects

  • Giovanni Ghione, Politecnico di Torino, Marco Pirola, Politecnico di Torino
  • Book:
    Microwave Electronics
    Published online:
    10 November 2017
    Print publication:
    09 November 2017, pp 522-565

  • Summary

    Introduction

    The present chapter describes a set of simulation or design exercises meant to apply, within a microwave Computer-Aided Design CAD environment, the concepts developed in the rest of the book. Each example is, potentially, a CAD laboratory trace. To allow the reader to replicate the examples, additional material is made available online in the form of a project in its specific CAD environment. We chose to develop the project on two well-known and widely used platforms, Microwave Office (MWO) [1] and ADS (Advanced Design Systems) [2]. To keep the presentation within reasonable limits (and also to avoid fast obsolescence due to the evolution of the CAD tool interfaces), for each example only a synthesis is provided with the main results in graphical form.

    The examples closely follow the evolution of the text and are intended as short and tutorial in nature. Rather than reporting on full real-word designs, leading to a ready-togo layout, they concentrate on specific aspects that are dealt with in the text on a more theoretical equation-based level. All figures are directly copied (sometimes with minor changes and a conversion to black and white) from the CAD tool user interface, their graphical format being in a way a part of the design experience of the user. Some of the CAD projects actually provide a CAD implementation of numerical examples presented throughout the text.

    Microstrip Line and Stub Matching of a Complex Load

    The project proposes a CAD implementation of Example 2.5. A line plus stub section (the stub in short circuit) is set up to match to the impedance. The design frequency is chosen as 10 GHz. The electrical line length at 10 GHz is initially set equal to the approximate values found in Example 2.5, and a further optimization is carried out, obtaining slightly different values. Fig. 10.1 shows the resulting input impedance compared to the goal,.

  • 2 - Passive Elements and Circuit Layout

  • Giovanni Ghione, Politecnico di Torino, Marco Pirola, Politecnico di Torino
  • Book:
    Microwave Electronics
    Published online:
    10 November 2017
    Print publication:
    09 November 2017, pp 28-86

  • Summary

    Transmission Lines

    Transmission lines (TXLs), simple or multiconductor, are a key distributed element in microwave circuits. They operate as signal transducers between components but are also the building block for passive distributed elements such as couplers, filters, matching sections, power dividers. In hybrid and monolithic microwave circuits the preferred guiding structures are the so-called TEM or quasi-TEM lines, characterized by broadband behavior and by the absence of a cutoff frequency, that is found in metal waveguides.

    From a theoretical standpoint, N metal conductors with a ground plane support N TEM or quasi-TEM propagation modes. In TEM (Transverse ElectroMagnetic) modes, the electric and magnetic fields are transverse with respect to the propagation direction, i.e., orthogonal to the line axis. A purely TEM mode propagates in a line without conductor losses and with a homogeneous cross section, while lossy metal lines and lines with a non-homogeneous cross section support quasi-TEM modes with small longitudinal field components. An example of quasi-TEM line is the microstrip, where the cross section is partly filled by a dielectric, and partly by air. TEM and quasi-TEM lines may also support upper propagation modes with a cutoff frequency; however, the excitation of those modes is to be avoided because they contribute to radiation losses and coupling.

    Transmission Line Theory

    Transmission lines are a simple but convenient model for one-dimensional wave propagation, serving as a bridge between circuit theory and electromagnetics. Suppose first that the line is made of two lossless parallel conductors, one acting as the signal conductor, the other as the return or ground conductor, surrounded by a homogeneous, lossless medium. Such a structure supports a TEM propagation mode in which the electric and the magnetic fields lie in the line cross section and are orthogonal to the line axis and wave propagation direction; see Fig. 2.1 (a). In a TEM mode the transverse electric field can be derived from a potential function satisfying, in the line cross section, the Laplace equation. This is uniquely determined by the potential of the signal line v(z, t) with respect to the ground line, where the coordinate z is parallel to the line axis and propagation direction. The transverse magnetic field is in turn related to the total current i(z, t) flowing in the signal conductor; −i(z, t) flows instead in the return conductor.

  • 1 - A System Introduction to Microwave Electronics

  • Giovanni Ghione, Politecnico di Torino, Marco Pirola, Politecnico di Torino
  • Book:
    Microwave Electronics
    Published online:
    10 November 2017
    Print publication:
    09 November 2017, pp 1-27

  • Summary

    Radio Frequencies, Microwaves, Millimeter Waves

    The meaning of high-speed and high-frequency electronics has changed in a century of technological evolution; see [1]. Radio-frequencies (RF) were at first introduced within the framework of radio broadcasting and point-to-point transmission at the beginning of the twentieth century; however, new applications (such as TV broadcasting and above all the radar around 1940), fostered in turn by the availability of RF signal generators at increasing frequency, caused a gradual explosion of new application fields in the RF, microwave and millimeter wave frequency bands. Before 1970, high-frequency systems were based on vacuum tube generators; in the following decades, solid-state semiconductor devices able to operate above 1 GHz, based both on Silicon and on compound semiconductor technologies, were gradually introduced, leading to a new paradigm, the hybrid and then monolithic Microwave Integrated Circuit, (M)MIC, see, e.g., [1, Ch. 16]. Integration and the resulting downsizing of RF and microwave systems ultimately made microwave electronics, at least in high-volume applications, a low-cost commodity well suited to the consumer market.

    For the sake of clarity, let us focus on the meaning of the terms RF, microwaves and millimeter waves and on some of the significant applications in each frequency band:

  • • The RF band includes frequencies between a few MHz and 1 GHz, with free space wavelengths of the order of 1 m (30 cm at 1 GHz); the applications include analog radio broadcasting through frequency modulation (FM), and also many other systems, like TV broadcasting, point-to-point communications (for instance, the lowest band in European GSM systems), long-range radar systems, industrial applications like RF heating and RF drying.

  • • The microwave frequency band includes frequencies between 1 GHz and 30 GHz, with free space wavelengths between 30 cm and 1 cm. In the microwave range, several applications exist: below 10 GHz we have all mobile phone systems up to the LTE (Long-Term Evolution, commonly known as 4G LTE) standard, many Wireless Local Area Network (WLAN) applications, terrestrial and satellite radio links, radar systems, positioning systems. Also the electronic part of long-haul optical communication systems operates in this band, although in this case many of the exploited circuit architectures are digital rather than analog.

  • 9 - Microwave Measurements

  • Giovanni Ghione, Politecnico di Torino, Marco Pirola, Politecnico di Torino
  • Book:
    Microwave Electronics
    Published online:
    10 November 2017
    Print publication:
    09 November 2017, pp 489-521

  • Summary

    Introduction

    This chapter aims at providing an overview of the experimental characterization techniques of linear and nonlinear one- and two-ports. After a short review of some basic instrumentation tools (the power meter, the vectorial voltmeter, the spectrum analyzer) in Sec. 9.2, the characterization of linear one- and two-ports, such as passive circuits and linear amplifiers, based on the measurement of power waves (see Sec. 3.2.2) and the related circuit-oriented parameters are discussed. As a first example, one-port measurements are introduced, whose fundamental tool is the so-called reflectometer (Sec. 9.3). The analysis of the reflectometer already allows the main issues involved in microwave measurement set-ups to be discussed, such as: the use of directional couplers to separate incident and reflected waves at the port where the Device Under Test (DUT) is connected; the need to perform measurements at a suitably low Intermediate Frequency (IF) rather than at RF to improve accuracy; the need to calibrate the measurement system to correct for systematic errors in the instrumentation.

    In Sec. 9.4 the discussion is extended to the characterization of two-port linear devices adopting the two-port Vector Network Analyzer (VNA), the key instrument for high-frequency characterization first introduced by Hewlett Packard, around 1970 [2, 3]. Downconversion approaches are discussed in Sec. 9.4.1, while a number of calibration strategies are reported in Sec. 9.4.2.

    Large-signal rather than linear-only characterizations are fundamental in the experimental assessment of power amplifiers. In Sec. 9.5 the most important aspects related to load-pull measurements are discussed. Sec. 9.6 is devoted to system-level characterization of RF components under modulated signal excitation, with particular attention to power amplifiers. Finally, Sec. 9.7 presents an overview of noise measurement techniques.

    Basic Microwave Instrumentation Tools

    Microwave measurements exploit a number of tools that are also commonly found in low-frequency electronic or electrical characterizations. Their function and principle of operation will be briefly recalled here.

    Power meters are instruments able to measure the power dissipated by a resistive load being part of the meter.

  • 6 - Microwave Linear Amplifiers

  • Giovanni Ghione, Politecnico di Torino, Marco Pirola, Politecnico di Torino
  • Book:
    Microwave Electronics
    Published online:
    10 November 2017
    Print publication:
    09 November 2017, pp 261-351

  • Summary

    Introduction

    The task of RF amplifiers is to transfer the input signal to the load with an increased power, without overly corrupting the signal through added noise and linear or nonlinear distortion (harmonic or intermodulation). From the energy standpoint, the amplifier converts DC power from the power supply into RF power to the load with a certain efficiency. The output signal should be as faithful as possible a replica of the input signal, or, in a digital context, should replicate with an acceptable error the symbols in the input signal. This requires constant gain over the frequency bandwidth and the input signal dynamic range, linear phase relationship between input and output, and, in general, low enough nonlinear distortion. Such requirements should be satisfied for the whole amplifier frequency bandwidth and for a range of the input signal amplitude defining the amplifier dynamics or Spurious Free Dynamic Range (SFDR); see Sec. 8.2.3. Because of such requirements, amplifiers should behave, within the SFDR, as linear or quasi-linear components, i.e., nonlinearity is a factor that adversely affects the amplifier performance.

    As already discussed, three main amplifier classes can be found in transceivers, the Low-Noise Amplifier (LNA) whose main purpose is to amplify weak input signals to the receiver chain with an acceptable compromise between gain and noise; the highgain amplifier (present in the RX and TX stages, often in the IF section), where noise is not the main concern but the primary purpose of design is to maximize the amplifier gain; the Power Amplifier (PA), whose main purpose is to deliver an output signal of the TX chain with a power level adequate for the specific system. The PA maximum power is limited by signal distortion and, ultimately, by power saturation. The maximum power that a transistor can deliver is in fact approximately proportional to the product of the maximum output current and of the breakdown voltage, and distortion increases with increasing output power. Indeed, increasing the transistor periphery or area, thus increasing the saturation power, and keeping the output signal level low with respect to the saturation power (the so-called output backoff, see Chapter 8) is a way to reduce (in class A amplifiers) distortion, and therefore to increase the amplifier dynamics.

Page 19 of 75