[SI-LIST] Re: From spectrum of i(t) to spectrum of d(i(t))/dt

Junfeng,

I haven't seen replies to your questions, and you still seem reluctant to
believe that your calculations are correct.

You got the spectrum for i(t).  It is a simple mathematical operation to go
from there to the spectrum of di/dt, without having to first calculate di/dt
and then perform an FFT on that.  You are doing the right thing already.  It
is not inaccurate.

In regards to your last question (comparing spectra of different waveforms),
the spectrum of d(i(t))/dt does not necessarily scale with the spectrum of
i(t), because the relative magnitudes of the frequency components that make
up those spectra likely differ.  If the two waveforms being compared had
exactly the same shape, or the same relative frequency content, then i(t)
and d(i(t))/dt would be proportional; otherwise, they aren't.

For example, if you find that the 2nd harmonic is five times greater, but
the 21st harmonic happens to be 30 times greater, then one would expect that
the spectrum of di/dt has a much greater difference than the spectrum of
i(t).

It's also not good to compare a spectrum with a waveform's peak value, again
unless the waveshape is a constant and doesn't change.

Regards,
Andy


>     For my case, let's take an example, suppose I have two waveforms, one is
> 100k gates, the other is 1000k gates. The rest is the same.
>     In time domain, I find that the peak di/dt is increased by let's say 5
> x. In frequency domain (current spectrum), the peak of the 2nd harmonic is
>     increased by 10 dB uV, So how is the increase in the real di/dt spectrum
> ? still 10 dB uV  ?  that is my question.


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