A Study of Factors Governing the Performance of Systems Using Pulse-Length Modulation

Abstract

Since many semiconductor devices may be operated as very efficient switches it is desirable to make use of these switching properties as a means of obtaining high efficiency linear power amplification. One method of achieving this is to vary the length of the pulses of a wave train in accordance with the signal to be amplified. The modulated wave train may be demodulated by simply passing the wave train through a low-pass filter. The majority of the published work on this technique is concerned with ideal systems. It is the objective of this research work to establish some of the more important design criteria for pulse-length modulation systems. The analytical work is divided into three distinct sections. The first of these sections deals with distortion arising in the modulation process. The effects of system parameters on the system linearity are analysed on a static basis, where the system input is d.c., and in the frequency domain. In general the static analysis is required before the analysis in the frequency domain can be made. In addition to examining,in detail, the effect of a number of particular system parameters a generalised spectrum analysis has been made for non-linear pulse length modulation. The second section considers the efficiency of switching output stages operating in conjunction with the low-pass filter necessary for demodulation. The power dissipation in the elements of the output stage is analysed in terms of the d.c. characteristics of the active. elements. As as result of the complexity of the problem a quasi-dynamic approach has been used. Expressions are obtained relating the power dissipation to the pulse length. The pulse length is then modulated in a sinusoidal manner and the power dissipation integrated over one cycle of the modulating frequency. It was shown in the first section that nonlinearities in the modulation process given rise to systems with non-linear transfer functions. The final section of the thesis introduces the possibility of producing systems with prescribed non-linear transfer functions by means of pulse-length modulation.