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    why real part of input impedance is negative in my CST simulation?

    I am simulating in terahertz (THz) range which source resistance is high (>10 Kohms), I want to obtain S-parameters and input impedance but i saw that in some frequencies the amount of S-parameters are larger than 0 dB and real part of input impedance is also negative.

    i think they are not reasonable, can anyone explain what is wrong in my simulation?

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    Re: why real part of input impedance is negative in my CST simulation?

    Such a ripple in the frequency domain response from time domain solver can result from too much residual energy at the end of the transient simulation. Use more timesteps and see if the ripple decreases.



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    Re: why real part of input impedance is negative in my CST simulation?

    thanks @volker for your helpful comment

    yes, actually the residual energy is high, but how can decrease this?

    i start simulation with increasing number of pulses to 100 and time to 70, but still there is much residual energy in the structure ?



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    Re: why real part of input impedance is negative in my CST simulation?

    Do you get some errors or warning? Is the energy in your simulation domain decaying with time and met the "accuracy" setting in your td-setup? Also check 1d results - port signals.



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    Re: why real part of input impedance is negative in my CST simulation?

    I've seen this sort of behaviour before as a result of the "AR" (auto regression) filter which seems to auto-enable itself in some situations/scenarios...

    [I haven't identified the pattern yet, maybe it's related to the template used? Either way, whenever I come across screwy results with frequency domain 'ringing' like yours, I go hunting for it in the simulation settings dialog]

    The filter intends to speed things up by truncating the time-domain simulation and inferring the corresponding frequency domain. I'm not sure when it's meant to be useful (or what I'm doing wrong) as it always seems to give me results like yours. Turn it off (if it's enabled) and try again :)



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    Re: why real part of input impedance is negative in my CST simulation?

    Quote Originally Posted by johnjoe View Post
    Do you get some errors or warning? Is the energy in your simulation domain decaying with time and met the "accuracy" setting in your td-setup? Also check 1d results - port signals.
    thanks @johnjoe for your help
    it was related to excitation pulse and a material of structure and i solved it,
    but for your question , no there was no error or warning in the messages box but the residual energy remaining in the end of simulation was high (it wasn't below -10 dB)

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    Quote Originally Posted by thylacine1975 View Post
    I've seen this sort of behaviour before as a result of the "AR" (auto regression) filter which seems to auto-enable itself in some situations/scenarios...

    [I haven't identified the pattern yet, maybe it's related to the template used? Either way, whenever I come across screwy results with frequency domain 'ringing' like yours, I go hunting for it in the simulation settings dialog]

    The filter intends to speed things up by truncating the time-domain simulation and inferring the corresponding frequency domain. I'm not sure when it's meant to be useful (or what I'm doing wrong) as it always seems to give me results like yours. Turn it off (if it's enabled) and try again :)
    I do appreciate @thylacine for your comment
    yes i done my simulation with and without AR filter, but the results didn't differ very much and were unacceptable, but as I said in previous comment as frequency of simulation is high (THz), the results are very depend on the materials used and also I must defined the new excitation pulse and new frequency range to let the energy in the structure decayed to the reasonable level.



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