The frequency spectrum of pulse width modulated signals

A complete digital audio amplifier has been developed, implemented and tested. The process is entirely computational, and the output load and filter are the only analog components in the system. The process makes use of digital signal processing and a switching power stage to provide both high fidelity and high efficiency, beginning with a digital audio data stream. The advantages of naturally-sampled pulse-width modulation (PWM) are discussed in depth, including spectral analysis and comparisons to uniformly-sampled PWM. It is shown that natural PWM does not introduce audible distortion at switching frequencies consistent with power electronics practice. Interpolation methods for sample data conversion to natural PWM are discussed, and error analysis is presented based on Lagrange's Expansion Theorem. Noise-shaping processes are used to support high fidelity with practical values of time resolution. A counter conversion process enforces switching dead time in the inverter gate signals. The experimental full-bridge inverter implementation demonstrates that miniaturization is possible. A complete test system delivered more than 50 W into an 8 /spl Omega/ load with an efficiency of 80% and total harmonic distortion plus noise of 0.02%.

High-fidelity PWM inverter for digital audio amplification: Spectral analysis, real-time DSP implementation, and results

Presents the development of a vibration-induced power generator with total volume of /spl sim/1cm/sup 3/ that uses laser-micromachined springs to convert mechanical energy into useful electrical power. The goal of this project is to create a minimally sized electric power generator capable of producing enough voltage to drive low-power ICs and/or micro sensors for applications where mechanical vibrations are present. Thus far, we have developed a generator capable of producing 2V DC with 64Hz to 120Hz input frequency at /spl sim/250/spl mu/m vibration amplitude. We have also demonstrated that this generator has enough power to drive an IR transmitter to send 140ms pulse trains with /spl sim/60sec power generation time.

Infrared signal transmission by a laser-micromachined, vibration-induced power generator

An analytical technique for calculating the harmonic characteristics of the output voltage of an H-bridge inverter with dead time is presented. The analysis is based on a three-dimensional (3-D) model derived for generating the pulsewidth-modulated (PWM) pulse train. By applying double Fourier analysis, a generalized and elegant mathematical function for describing the harmonic components of the output voltage is formulated. The function can be divided into two parts: an ideal part representing the PWM signal without dead time and a correction part representing the dead-time effect. The function provides detailed composition of the fundamental component, signal harmonics, carrier harmonics, and cross-modulated harmonics. The proposed technique has been verified using examples and the theoretical predictions are confirmed with the results obtained from simulations using PSpice.

Analytical technique for calculating the output harmonics of an H-bridge inverter with dead time

A digital Class-D amplifier comprises a pulsewidth modulator (PWM) and an output stage. In this paper, we simplify the time-domain expression for the algorithmic PWM linear interpolation (LI) sampling process and analytically derive its double Fourier series expression. By means of our derivation, we show that the nonlinearities of the LI process are very low, especially given its modest computation complexity and low sampling frequency. In particular, the total-harmonic distortion (THD) /spl ap/0.02% and foldback distortion is -98.4 dB (averaged from modulation indexes M=0.1 to 0.9) for the 4-kHz voiceband bandwidth @1-kHz input, 48-kHz sampling. We also describe a simple hardware for realizing the LI process. We propose a frequency doubler (with small overheads) for the pulse generator for the PWM, thereby reducing the counter clock rate by 2, leading to a substantial /spl sim/47% power dissipation reduction for the Class-D amplifier. By means of computer simulations and on the basis of experimental measurements, we verify our double Fourier series derivation and show the attractive attributes of a Class-D amplifier embodying our simplified LI sampling expression and reduced clock rate pulse generator. We show that our Class-D amplifier design is micropower (/spl sim/60 /spl mu/W @1.1 V and 48-kHz sampling rate, and THD /spl ap/0.03%) and is suitable for practical power-critical portable audio devices, including digital hearing aids.

/pdf/a-micropower-low-distortion-digital-class-d-amplifier-based-3o0pr8zj6u.pdf

A micropower low-distortion digital class-D amplifier based on an algorithmic pulsewidth modulator

An enhanced sampling process which can be used to reduce spectral distortion in the output of digital pulse width modulated class D amplifiers is presented. This process arises from a generalisation of the concepts of natural and uniform sampling and can be adjusted such that the spectra produced by the process can take a number of forms for the same signal. Analytical expressions for the spectra produced by the sampling process are derived for both single and double sided modulation. The performance of the process is assessed by case studies on full bandwidth and bass to midrange audio class D amplifiers. It is shown that at carrier frequencies around 88 kHz in a full bandwidth amplifier and 44 kHz in a bass to midrange amplifier the new sampling process will result in significantly lower distortion when compared with either natural or uniform sampling.

Reduction of spectral distortion in class D amplifiers by an enhanced pulse width modulation sampling process

A pulse-width modulation sampling process has been derived which is suitable for implementation in an entirely digital system, but has a performance comparable with that of natural sampling. The new process introduces negligible distortion to audio bandwidth signals at pulse carrier frequencies of 176 kHz or above. As a result it should be possible to realise a high efficiency, high performance class D power amplifier.

http://ieeexplore.ieee.org/iel3/1685/4608/00181627.pdf

Improved sampling process for a digital, pulse-width modulated, class D power amplifier

Digital audio class D amplifiers require a digital process to convert the sampled audio signal into a pulse width modulated (PWM) signal. The present methods of generating PWM are either analogue (natural sampling) and are unsuitable for digital systems or use a digital process (uniform sampling) which introduces excessive distortion. An alternative sampling process is described which is suitable for digital implementation yet introduces the low levels of distortion usually associated with natural sampling.

Digital sampling process for audio class D, pulse width modulated power amplifiers

Distortion analysis and reduction in a completely digital PWM class D power amplifier

The use of digital techniques for pulse width modulation conversion offers greater flexibility than traditional analogue methods. A technique is described that allows the PWM sampling process to be varied over a range of possibilities which offer different compromises between the type of distortion produced by natural and uniform sampling. With optimisation of a parameter, use of this process can lead to an overall reduction in distortion compared with conventional analogue techniques.

S.P. Leigh

Papers

Reduction of spectral distortion in class D amplifiers by an enhanced pulse width modulation sampling process

Improved sampling process for a digital, pulse-width modulated, class D power amplifier

Digital sampling process for audio class D, pulse width modulated power amplifiers

Distortion analysis and reduction in a completely digital PWM class D power amplifier

Distortion minimisation in pulse width modulated systems using a digital sampling process