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Implement ternary modulation for Class-D amps (Part 2)

04 Sep 2014  | Anthony Nowicki, Ron Jesme, Chris Roed

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Part 1 of this series discussed the methodology behind Class-D amplifiers and the ternary modulation scheme.

In order to perform this harmonic reduction, a pure sine wave must first be applied to the Class-D amplifier system. Using the ternary modulated bitstreams described earlier, two pulse density modulated bitstreams were applied to the dual H-bridge. The intended result was the pure sine wave generated in figure 6a.

Figure 6a: Resultant pure sine wave constructed from the differential pulse density modulated bitstreams.

However, the output was monitored differentially and the result was slightly distorted and a Fast Fourier Transform (FFT) revealed several harmonics present in the output signal. The output signal was captured and analysed in Matlab for the amplitude and phase angle of the harmonic content. This harmonic information was then used to alter the input signal, pre-distorting it before the H-bridge of the amplifier. The subtraction of the harmonic content was quite small relative to the overall sine wave generated; figure 6b and 6c show exaggerated examples of the harmonics and their pre-distorted resultant signal respectively.

Figure 6b: Simultaneous plots of the pure sine wave, an exaggerated 3rd harmonic sine wave, and the resultant distorted sine wave.

Figure 6c: Resultant exaggerated distorted sine wave constructed from the differential pulse density modulated bitstreams, using the addition of the exaggerated 3rd harmonic in 6b.

In order to perform the harmonic reduction, the PDM bitstream had to be applied to the modified Class-D amplifier with the desired sine wave as the output. When looking at the output of the Class-D amplifier differentially, a sine wave is generated and the oscilloscope images are shown in figure 7 where the red plot is the differential voltage reading off of the output of the amplifier and the purple plot is the current signal through the load antenna.

Figure 7: Oscilloscope plot of Class-D amplifier output using PWD generated bitstreams. Red trace is voltage viewed differentially, Purple is current through load antenna.

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