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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
What happens if you build a class D amplifier, put a PLL in on the "audio" in, and then a triangle wave frequency that is a harmonic of a resonant tank as well? The PLL would keep it in resonance, obviously, but then what would the harmonics do?
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Where is the resonant tank you are discussing? A class D amp has a low-pass filter on the output, usually an LC filter, but I wouldn't call it a resonant tank. A PLL has to have some kind of oscillator in it which might have a resonant tank, though common PLL chips have CMOS ring oscillators without a true tank.
Most tank circuits at low frequencies will not exhibit resonance at harmonics of their resonant frequency. Transmission line or distributed tank circuits will, but that only applies to higher frequencies or large structures.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Looking back at what I typed, I apologize for my vague words.
I meant, if it was possible to feed a, say, DRSSTC, a sine wave of it's own resonant frequency using a class D amplifier?
What is the difference between using a square wave for resonance, a triangle wave, and a sine wave for resonance? I understand they would all achieve it, but what would each do voltage/current resonant rise, wise?
Why do the switches in a DRSSTC have to be able to handle so much current if it's at resonance? Would it be possible to give it tinier "spurts" of square waves to maintain resonance, but a much smaller overall current draw?
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
If you look at the fourier series of a square wave and a triangle wave they are both composed of an infinite series of sine waves that have been added together. The sine waves are composed of the harmonics of the fundamental frequency of the wave, in the case of a square wave it is only the odd harmonics (fundamental, 3rd harmonics, 5th, 7th, ...).
For a square wave at 1kHz for example, it is the same as a 1kHz sine wave plus a 3kHz sine wave at a smaller amplitude, plus a 5kHz sine wave at even smaller amplitude, and on to infinity. But you get can get quite close to the final representation with say 5 terms or so. Using fourier analysis one can find the ampltidue of each component sine wave. Take a look at
So when you drive a sharply peaked (high Q) resonant tank with a square wave, the tank will easily pass current at the resonant frequency, but not necessarily at the harmonics.
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An interesting thought...
