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.
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.