[SOLVED] pulse width modulation linearity

[FvM]

I'm unable to detect a systematical nonlinearity in this data. A second order polynomial approximation is nothing better than a linear one. It's rather a case of measurement uncertainty.

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[kishore3677]

Hi FvM,

So what is your conclusion. Where things are leading to?

gvk

 

[FvM]

I have no particular conclusions. I don't know the accuracy of the involved measurements, so it's not clear to me if the results show an actual circuit nonlinearity.

 

[kishore3677]

Hi FvM,

One interesting observation. I used multisim, used AD711 op-amp as comparator. Fed Tri and DC voltage to inputs and measured output widths in virtual scope. The widths are not proportional. This is amazing. All non linearities are ruled out in simulation, but the results are shocking.

gvk

 

[FvM]

With an OP used as comparator, slew rates can affect linearity even at 1 kHz PWM frequency, depending on the setup.

But the advantage of simulation is that you can identify the exact mechanismn how non-linearity is brought up.

 

[godfreyl]

All non linearities are ruled out in simulation, but the results are shocking.

If the minimum timestep is too large, you'll get random errors.

 

[ZekeR]

I'm unable to detect a systematical nonlinearity in this data. A second order polynomial approximation is nothing better than a linear one. It's rather a case of measurement uncertainty.

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This is linear, so what's the deal?

There's going to be noise riding on top of your measurement, but that should be expected. This noise can come from:
a) Your measurement method
b) Noise riding on the triangle
c) Noise riding on the DC signal
d) Variations in propagation delay over the input range
e) Noise internal to the circuit
f) Electromagnetic interference from surrounding circuitry
g) Anything else I'm not mentioning at the moment

Just because your measurements are noisy doesn't mean they're nonlinear.

 

[lalgpt]

Your DC Level value and Ramp height should match. Check these figures.