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PWM Distortion

A

Andrew Holme

Jan 1, 1970
0
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:

http://www.holmea.demon.co.uk/Misc/PWM/1.gif

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.
 
M

martin griffith

Jan 1, 1970
0
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:

http://www.holmea.demon.co.uk/Misc/PWM/1.gif

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


martin
 
Andrew said:
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:

http://www.holmea.demon.co.uk/Misc/PWM/1.gif

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.

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.
 
M

Mark

Jan 1, 1970
0
I suggest you ask this question over at

comp.dsp

Mark
 
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