>> Not if the line is dissipative -- in a non dissipative line you could
>> do that, though.
>
>
> sure, I didn't say.. non dissipative is my case.
I was seeing "dispersive" and making it into "dissipative". I think
that looking at the volts/position relationship is still not valid in a
dispersive line, however.
>
>> If you're determined to do it using an FFT then all I can suggest is
>> that you make your simulation long enough [...]
>
>
> thanx for your tips, Tim! now implementation time ;)
>
> bye
> Not if the line is dissipative -- in a non dissipative line you could do
> that, though.
sure, I didn't say.. non dissipative is my case.
> If you're determined to do it using an FFT then all I can suggest is
> that you make your simulation long enough [...]
thanx for your tips, Tim! now implementation time ;)
bye
Reply by Tim Wescott●May 4, 20062006-05-04
Gianguido wrote:
>> The time dispersal is part of the system behavior, you don't want to
>> throw it away.
>
>
> yes, that's the point: maybe I'll try a fft of the whole signal vector
> in space at the desired time T. This way I should get the effective H(f)
Not if the line is dissipative -- in a non dissipative line you could do
that, though.
>
>> Why don't you use a sinusoidal input, and look at the magnitude and
>> phase of the sinusoidal output? That will give you H(f) directly.
>
>
> Such a way is good, but to get a wideband H(f) I should run a lot of
> simulations, one for every frequency of the observation band. That's why
> I wanted to use a wideband pulse such gaussian, to get all information
> in one step
>
If you're determined to do it using an FFT then all I can suggest is
that you make your simulation long enough to capture a good long bit of
the tail as the thing settles out, and window the data with something
that prevents the very end from getting chopped off -- I'd use a
constant that transitions to a raised cosine at the very end of the time
sample.
No matter what you do you're going to find inaccuracies at the
high-frequency end where there's not much signal content from the cable
and lots of windowing effects from the finite measurement interval.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
Reply by Gianguido●May 4, 20062006-05-04
> The time dispersal is part of the system behavior, you don't want to throw
> it away.
yes, that's the point: maybe I'll try a fft of the whole signal vector in
space at the desired time T. This way I should get the effective H(f)
> Why don't you use a sinusoidal input, and look at the magnitude and phase
> of the sinusoidal output? That will give you H(f) directly.
Such a way is good, but to get a wideband H(f) I should run a lot of
simulations, one for every frequency of the observation band. That's why I
wanted to use a wideband pulse such gaussian, to get all information in one
step
thanx
Reply by Tim Wescott●May 3, 20062006-05-03
Gianguido wrote:
> Hallo,
> I'm simulating a transmission line with FD-TD. I have to compute the
> numerical transfer function of the simulated line, which is dispersive,
> with a gaussian pulse stimulus. To do that I compute the fft of gaussian
> pulse before puttin it into line (easy: I exactly know how long it is in
> time), then after some propagation, and then compute H(f)=V(f,out)/V(f,in).
> My question is: after propagating a bit, the pulse is wider in time..
> how can I take into account this when computing fft? is there a sort of
> threshold I can apply?
>
> thanx in advance
The time dispersal is part of the system behavior, you don't want to
throw it away. Why don't you use a sinusoidal input, and look at the
magnitude and phase of the sinusoidal output? That will give you H(f)
directly.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
Reply by Gianguido●May 3, 20062006-05-03
Hallo,
I'm simulating a transmission line with FD-TD. I have to compute the
numerical transfer function of the simulated line, which is dispersive, with
a gaussian pulse stimulus. To do that I compute the fft of gaussian pulse
before puttin it into line (easy: I exactly know how long it is in time),
then after some propagation, and then compute H(f)=V(f,out)/V(f,in).
My question is: after propagating a bit, the pulse is wider in time.. how
can I take into account this when computing fft? is there a sort of
threshold I can apply?
thanx in advance