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Problem with FDTD (Extraction of S Parameters, problem with input and output waveform

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mevadapratik

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Hi,

I am simulating simple waveguide model using FDTD Method for S Parameter extraction. But, I am unable to simulate it successfully. Output waveform always goes unbounded....and input waveform also does not start properly. It starts with ripples. Is there any solution for this???? If it is necessary, i can upload the code also......

Please reply asap........
 

Hi,

I am simulating simple waveguide model using FDTD Method for S Parameter extraction. But, I am unable to simulate it successfully.

Some details would be nice (e.g. How are you trying to compute the S parameters)

Output waveform always goes unbounded

That can be anything from a bug in your code, inappropriate ABCs,...

....and input waveform also does not start properly.

How to you insert the wave?

It starts with ripples. Is there any solution for this???? If it is necessary, i can upload the code also......

Please reply asap........
 

Hi.....

I have taken waveguide having cutoff at 3 GHz. I calculated the dimensions of waveguide using the cutoff frequency. I modeled it.

I use dt=dt=1/(c*((1/cell_x)^2+(1/cell_y)^2+(1/cell_x)^2)^0.5);

I sample geometry such that 100 samples can be accommodated in one wavelength................

I use PEC BC condition like this...

ex(1: xsize,1: ysize,:)=0;
ey(1: xsize,1: ysize,:)=0;
ez(1: xsize,1: ysize,:)=0;%if wave propagation is in z direction......this apply for all four faces of waveguide......

For excitation, ez(xsize-2,ysize-2,round(ksize/2))=ez(xsize-2,ysize-2,round(ksize/2))...
+ exp(-(t-0.5e-12).^2/1e-25)*5.*exp(i*5e15*t); %I give excitation in between structure

For two faces on direction of wave propagation, I define ABC boundary.........

I try to extract time domain signal at middle axis of waveguide geometry......but, and separate the incident and reflected signal........but, not able to do it since output waveform is going infinite......

Can you please tell me what could be the problem???? if you need, i will upload the code also......

Thank you....
 

Hi.....

I have taken waveguide having cutoff at 3 GHz. I calculated the dimensions of waveguide using the cutoff frequency. I modeled it.

I use dt=dt=1/(c*((1/cell_x)^2+(1/cell_y)^2+(1/cell_x)^2)^0.5);
You are using the maximal stable time step dt_max (actually the stability is only guaranteed for dt < dt_max). If you are using floats then this can be a problem

I sample geometry such that 100 samples can be accommodated in one wavelength................

Since you are using a Gaussian pulse for the excitation I am not quite sure which wavelength you mean here

I use PEC BC condition like this...

ex(1: xsize,1: ysize,:)=0;
ey(1: xsize,1: ysize,:)=0;
ez(1: xsize,1: ysize,:)=0;%if wave propagation is in z direction......this apply for all four faces of waveguide......

For excitation, ez(xsize-2,ysize-2,round(ksize/2))=ez(xsize-2,ysize-2,round(ksize/2))...
+ exp(-(t-0.5e-12).^2/1e-25)*5.*exp(i*5e15*t); %I give excitation in between structure

I am a bit unclear what the 'i' in exp(i*5e15*t) should be.
Are you using complex numbers?

What cell size are you using?

For two faces on direction of wave propagation, I define ABC boundary.........

What ABC are you using?

I try to extract time domain signal at middle axis of waveguide geometry......but, and separate the incident and reflected signal........but, not able to do it since output waveform is going infinite......

Can you please tell me what could be the problem???? if you need, i will upload the code also......

I hope that looking at the code will not be necessary (what language are you using?)

Thank you....
 

Hi
dt plays the Major Role in stability, reducing it from that magic value increases the stability while increasing simulation duration. Thus at first step reduce dt by 10% and more and more until it get stable.
 

Hi
dt plays the Major Role in stability, reducing it from that magic value increases the stability while increasing simulation duration. Thus at first step reduce dt by 10% and more and more until it get stable.

if dt satisfies CFL then the simulation is stable. If dt > CFL limit then except for exceptional cases the simulation will get unstable. But once dt < CFL limit you cannot 'increase' stability. Also the more you decrease dt the less accurate your simulation gets (e.g. numerical dispersion increases).

Another question is what materials (e.g. metamaterials) are used and how they are modelled.
 

hi.......thank you for your replies....

As you said, i will try with dt<dt_max(10%dt_max)......

I am taking 100 samples in one wavelength...wavelength considered here is the minimum wavelength of operation......

For truncation at two faces, I am using Mur's First order ABC.......

In excitation, please remove that exp(i*5e15*t) term......i am using cos term in place of it......I apologies for my mistake in writing......

And about material, I am using vaccum having Er=1 Mu=1 for waveguide cavity.........

Is all these thing ok...........????????????

I am also not able to understand how can I get scattered field which is coherent with Input Waveform??? I read Allen Teflove's Book....but still have the confusion.....can you please give some ideas????

Thank you.............
 

hi.......thank you for your replies....

As you said, i will try with dt<dt_max(10%dt_max)......

I almost always use dt_max for doubles and 0.9999 * dt_max for floats.
The smaller dt/dt_max gets the larger the error

I am taking 100 samples in one wavelength...wavelength considered here is the minimum wavelength of operation......

I was actually interested in concrete numbers.

For truncation at two faces, I am using Mur's First order ABC.......

This may be the reason for your problems. I occasionally used 2nd order Mur which does have
stability problems. I think I read somewhere that 1st order Mur has less problems but I am not
sure.

I think you should dump the signals at several locations, especially close to the boundary and check where the signals start growing. If it is at the boundary you should consider implementing
other ABCs (or check for bugs)

And about material, I am using vaccum having Er=1 Mu=1 for waveguide cavity.........

you are using only vacuum and PEC?

Is all these thing ok...........????????????

I am also not able to understand how can I get scattered field which is coherent with Input Waveform??? I read Allen Teflove's Book....but still have the confusion.....can you please give some ideas????

Thank you.............

I would first try to deal with the instability. But the simplest method is to simply wait until the input has sufficiently decayed and then use the remaining bit as the reflected/scattered signal. This works if your input pulse is relative short and you measure far enough from the scatterer
 

Hi...

I tried all the things.....but i am not able to find S Parameters of waveguide......

I have attached the MATLAB file of my code.....can please anybody correct it? I am not able to solve problem........

Thank you...................
 

Attachments

  • fdtd_3D_waveduide.rar
    2 KB · Views: 107

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