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Creating a time-domain plot from a frequency domain study

Posted 25 févr. 2015, 02:55 UTC−5 7 Replies

Stephen Holland
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I am doing frequency-domain analysis and want to generate a synthetic time-domain plot from the result.

i.e. I do a (single frequency) analysis and get a resulting (in general complex) displacement amplitude A from a probe.

Now I want to generate a time-domain plot of A*exp(i*2*pi*freq*t) based on the study result. When I try to do this, since the study is frequency domain, t is undefined.

I've tried creating a second, time-domain study based on a different physics and referencing the frequency domain solution. That creates a 't' I can plot on the horizontal axis, and imports the dependent variables (i.e. A) from the frequency -dependent study. But now 'freq' is undefined, because while the dependent variables were imported from the frequency dependent study I didn't see an option to import the independent variable (freq).

Maybe I'm going about this all wrong. I could obviously plot this up easily in Matlab, but it would be nice if the COMSOL model were self-contained.

(Have an .mph file to attach but I get a 'File Extension Error' dialog when I try to attach it to this posting).
7 Replies Last Post 4 août 2016, 15:18 UTC−4
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Hello Stephen Holland

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H. Mohamed
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Posted: 9 years ago 17 juil. 2015, 11:48 UTC−4
Hi Stephen, Did you find a solution to this problem or not. I face a similar problem here.
I am just the opposite case. I am having a time dependent function and I need the frequency domain response.

Please advise.

Hossam
Hi Stephen, Did you find a solution to this problem or not. I face a similar problem here. I am just the opposite case. I am having a time dependent function and I need the frequency domain response. Please advise. Hossam
Edgar J. Kaiser Certified Consultant
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Posted: 9 years ago 17 juil. 2015, 12:55 UTC−4

Hi,

in Stephen's case with a single frequency: he can use the phase as x-axis parameter and scale accordingly.
In the inverse case, Hossam, you can choose the frequency spectrum instead of time in the x-axis data selection.

Cheers
Edgar

--
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Hi, in Stephen's case with a single frequency: he can use the phase as x-axis parameter and scale accordingly. In the inverse case, Hossam, you can choose the frequency spectrum instead of time in the x-axis data selection. Cheers Edgar -- Edgar J. Kaiser emPhys Physical Technology http://www.emphys.com
Nadia Aloui
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Posted: 9 years ago 24 nov. 2015, 04:53 UTC−5
Dear Edgar,

Can you please provide me more details about the transformation form frequency plot to time plot. In fact, it is not clear to me how using the phase as x-axis generates a time domain plot. It is equivalent to multiply the solution by exp(j*phase*pi/180); phase [rad]. Isn't it?
We should not multiply the solution by exp(j*2*pi*frequency) to generate a time-domain plot.
Thank you in advance.

Best regards,
Nadia
Dear Edgar, Can you please provide me more details about the transformation form frequency plot to time plot. In fact, it is not clear to me how using the phase as x-axis generates a time domain plot. It is equivalent to multiply the solution by exp(j*phase*pi/180); phase [rad]. Isn't it? We should not multiply the solution by exp(j*2*pi*frequency) to generate a time-domain plot. Thank you in advance. Best regards, Nadia
Henrik Sönnerlind COMSOL Employee

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Posted: 9 years ago 24 nov. 2015, 08:13 UTC−5
Hi,

By using a 'Frequency to Time FFT' study step you can generate a time history from frequency domain results, even for several frequencies.

Look at the third study in this example:

www.comsol.com/model/vibration-analysis-of-a-thick-beam-20301

A plot from that model is attached.

Regards,
Henrik
Hi, By using a 'Frequency to Time FFT' study step you can generate a time history from frequency domain results, even for several frequencies. Look at the third study in this example: http://www.comsol.com/model/vibration-analysis-of-a-thick-beam-20301 A plot from that model is attached. Regards, Henrik

Attachments:

Nadia Aloui
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Posted: 9 years ago 26 nov. 2015, 06:11 UTC−5

Thank you for your reply.

Regards,
Nadia
Thank you for your reply. Regards, Nadia
M.A.
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Posted: 8 years ago 24 juin 2016, 19:09 UTC−4
Dear Edgar,

I am relatively new to comsol, and it sounds like doing this should be trivial, but I cannot figure out how to plot some result in frequency domain (AC/DC module in my case, but it should be general post-processing tool) with the "phase" in the x-axis.

Is there a direct simple way to make the phase the variable in the x-axis of the plot of a frequency domain result, without resorting to FFT study step, especially that I'm interested in a single frequency analysis?

And is there an example in the application library to do it for a frequency domain study?

Best regards,
Majd

Dear Edgar, I am relatively new to comsol, and it sounds like doing this should be trivial, but I cannot figure out how to plot some result in frequency domain (AC/DC module in my case, but it should be general post-processing tool) with the "phase" in the x-axis. Is there a direct simple way to make the phase the variable in the x-axis of the plot of a frequency domain result, without resorting to FFT study step, especially that I'm interested in a single frequency analysis? And is there an example in the application library to do it for a frequency domain study? Best regards, Majd
Rishad Arfin
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Posted: 8 years ago 4 août 2016, 15:18 UTC−4
Hello,

I am kind of new in Comsol, so I am facing difficulties in solving this problem. It might be a very silly problem though.

Problem: I have a defined antenna structure in the attachment fig.1 . I have simulated the e-field distribution on the surface. However, now I would like to see how the e-field changes with time along that path (red line) in the structure? So, it is basically a 1D plot of time-varying e-field along that path (red line).

My study type is frequency domain in this case. Please help me with this.
Hello, I am kind of new in Comsol, so I am facing difficulties in solving this problem. It might be a very silly problem though. Problem: I have a defined antenna structure in the attachment fig.1 . I have simulated the e-field distribution on the surface. However, now I would like to see how the e-field changes with time along that path (red line) in the structure? So, it is basically a 1D plot of time-varying e-field along that path (red line). My study type is frequency domain in this case. Please help me with this.

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