![]() ![]() I may have not answered your question properly. You can make the output stable, but your waveform won't be something you want to show your kids. You can make the waveform cleaner with a lower frequency filter but it will make your output unstable. This is a clear case of what one of my friends calls the "Conservation of Misery Principle". Now here's the really nasty part of your LC filter: It's so close to the fundamental frequency (less than an octave), that it will be boosting the fundamental (peaking) instead of having a flat characteristics. ![]() A double-pole filter like the LC reduces the amplitude by a factor of 4 for every octave, so your filter has to have a corner frequency of slightly less than 1.5*fundamental frequency. To reduce the third harmonic to 3% of the fundamental (for a THD of something like 5%, which is about as ugly a waveform as most sensitive circuits can stand) you need a filter that reduces the third harmonic by a factor of 5 (3%/15%). In the Crowley & Lueng paper hypertexted above, figure 9 shows a third harmonic equal to about 15% of the fundamental (160V for the fundamental, 25V for the third harmonic). People normally only look at the first one-to-three large, low order harmonics since usually the high order harmonics are smaller amplitude and also easier to filter. The purpose of the filter is to reduce the amplitude of the harmonics to a specified maximum level. However, for a first pass, I found an example where someone calculated the spectra for a 100% modulation sinewave. This is a very complex task to provide a complete and accurate simulation for various modulation indices, switching frequencies, and load impedances. ![]()
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