Improved PWM-Based Sinewave Generation: Example of the Impedance Measurement

Using the pulse-width modulation (PWM) is a very convenient and efficient way to generate the approximation of the AC, in the simplest case sinewaves in the embedded test & measurement solutions. Partly, because the on-chip PWM modulator is available in most of the microcontroller and DSP chips. In the current paper improved (compared with conventional PWM) implementation of the binary waveforms is proposed. The idea is based first on the conversion of the desired AC (e.g., sinewave) to quantize in the amplitude domain waveform, to be further converted to the binary pulses with a duty cycle, corresponding to each quantization level. Such an approach is described and analyzed, with several examples of the implementation and application for impedance measurement solutions. The results show the significant improvements of the approximated binary signals both in the frequency and time domain, by optional simple low- or bandpass filtering of the binary signal, over classical PWM, while still preserving simplicity (e.g., number of needed binary signal segments) of the generated waveforms (e.g., compared with delta-sigma bitstreams). The beneficial usage of the proposed approach for the impedance measurement applications is described, as well as to generate excitation waveforms, as well as to be used as reference waveforms for synchronous demodulation of the response signal. It is shown, that for the frequency range of 10 to 70 kHz total harmonic distortion below 2.5% can be achieved by the proposed improved PWM solution.