PWM Distortion

I don't know to what extent you are simulating a real circuit, but the even order products wouldn't necessarily have to be produced by "distortion" - whatever that might be in a switching circuit. I'd expect them to arise if the mark-to-space ration of the circuit moved away from exactly 50%.

The obvious mechanism to generate th asymmetry is different propagation delays for the raisng and falling edges - which can be seen in real digital parts (and their data sheets).

Because this differential delay is absolute, and pretty much independent of frequency, you'd expect the mark-to-space raito to get steadily worse as you raise the clock frequency.

--------- Bill Sloman, Nijmegen.

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I'm investigating the intrinsic distortion of uniformly-sampled PWM as it affects the PFD output in a fractional-N synthesizer. One thing I've tried is to replace the complex MASH-generated divide-by-N sequence with a much simpler square wave. This is what the resulting PFD output spectrum looks like:

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The PWM sampling rate (reference/comparision frequency) was 512 and the square wave frequency was 64. You can see the fundamental and 3rd harmonics of the square wave, but there are also tones at the even-order harmonics which I think might be distortion products.

What interests me is the way the amplitude of the even-order tones seems to increase with frequency. I thought the amplitude of an IMD product could only be proportional to a power of the input amplitude. I don't understand how it could also be proportional to frequency.

When I run my simulation using the real MASH sequence, I get the PFD output spectrum I would expect, but there is a noise signal rising at 20 dB per decade superimposed on top of it: i.e. distortion with amplitude proportional to frequency.

I don't know for sure if these tones are PWM distortion - maybe they're due to a loss of floating-point precision in my simulation; but I'm using the lcc-win32 C compiler because it has a "long double" type which supports the full 80-bit precision of the Intel FPU.

Any pointers would, as ever, be appreciated ...

TIA Andrew.

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Andrew Holme

No pointers, sorry. I was just wondering how Boki would have phrased you query :-)


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martin griffith

I suggest you ask this question over at



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