[SI-LIST] Re: From spectrum of i(t) to spectrum of d(i(t))/dt
- From: "jun feng" <junfeng.zhou@xxxxxxxxx>
- To: a.ingraham@xxxxxxxx
- Date: Sun, 20 May 2007 21:03:30 +0200
As it is well known that the fast changing current casues the di/dt , which
is proporational to Electromagnetic Emission. We could say the faster
and higher the di/dt , the larger EMI. However are there any quantative
relationship between these two ? I mean the spctrum of di/dt and the EMI.
The output of current probe is in voltage, which is proporatioanl to the
current with a sensitivity 5mV/mA in my case. With the scope and spectrum
analyzer, I can have the time domain di/dt( in Matlab, the time domain is
just a snap of the long waveform) and the current spectrum. Then, I would
like to compare how these two are correlated ? For instance, if 10 times
larger in time domain di/dt , what happens in frequency domain ? If I
simply compare the di/dt and current spectrum, then if first order, they are
proporatioanl to each other. But the spectrum I got is for the current, not
the spectrum of di/dt , so maybe the results would be not accurate, or maybe
totaly wrong. This is why I want to know exactly how can I get the sepectrum
of di/dt from the spectrum of current.
On 5/19/07, Andrew Ingraham <a.ingraham@xxxxxxxx> wrote:
>
> > The differentiation in amperes is not amperes but not unitless,
> > either.
>
> Correct.
>
> > The time derivative of the current simply results in "voltage"
> > -after a correction factor with the inductance of the medium.
>
> Another note for the original questioner: Taking derivatives does indeed
> tend to result in lots of high frequency "noise," and it is necessary to
> limit the bandwidth at some point, so that the noise doesn't become
> enormous. Take this into consideration, especially with actual
> measurements. In analog circuits, there is a reason (this being one of
> them) why you often find integrator circuits but rarely find
> differentiator
> circuits.
I agree that taking derivatives would results in lots of high frequency
component, that is for sure since integrator is low pass and differentiator
is simply high pass.
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.
regards,
Junfeng
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