To put some frequency domain numbers on this. A 25mil alumina substrate is seldom used above about 10GHz in the microwave world because of the dispersion in Zo. The Zo curve looks like a hocky stick with the inflection point right around 10GHz. Dan Dan Swanson EMAIL: d.swanson@xxxxxxxx Andrew Corp. PHONE: 978-834-4085 37 South Hunt Road FAX: 978-388-7077 Amesbury, MA 01913 > -----Original Message----- > From: norhan@xxxxxxxxxxxx [mailto:norhan@xxxxxxxxxxxx] > Sent: Thursday, September 25, 2003 1:27 AM > To: si-list@xxxxxxxxxxxxx > Cc: Raj Raghuram > Subject: [SI-LIST] Re: TEM Approximation > > > Hello Steve, > I should have known that my post will generate questions :-). > > The shape of the distorted Gaussian impulse that you saw is very > typical for microwave substrates. It is all over microwave and > guided wave literature. There are several reasons for the > large dispersion and you correctly pointed out one of them: > > 1. Large dielectric constant of the microstrip substrate (Alumina). > 2. Relatively large dimensions (h=0.635 mm, w=0.5 mm, l=123 mm). > 3. Shape and width of the exciting impulse (Gauss, sigma=19 ps). > > These three reasons give the Gaussian impulse the characteristic > negative tail with some ripples. In addition to these reasons, > the impulse is also influenced by > > 4. Point source excitation (multi mode excitation). > 5. Material losses. > 6. Other reasons that I better not mention. > > The last three visibly influence the impulse, but the changes > that they make are not really important here. > > The main reason why you do not see the characteristic dispersion > tails in your typical PCB TDR measurements is that you do not > have 1-3: your dielectric constant is significantly lower, your > cross sectional dimensions are smaller and your signal waveform > has different characteristics. The last reason really includes > two effects and it requires more elaboration. > While the signal in this example is not particularly fast, it is > slightly faster than the signal out of your TDR and it is an > impulse (not a step). Therefore we have to take the derivative > of your step waveforms to make a comparison. First, the shape of > the resulting impulses (derivatives) is important. Different > impulse shapes are affected by dispersion differently. The TDR > outputs one impulse and the example uses another (Gaussian). > Second, the response to a Gaussian signal is an impulse with > high frequency ripples after it. Similarly the response to the > TDR's input impulse (derivative of the step) is a step with > accentuated high frequency components (j*w*step_response(w)). > But these high frequency components are attenuated by the TDR > because of the limited bandwidth of the TDR system. > The end result is that the useful resolution of the TDR is > reduced when you look at derivatives so impulses do not have > high frequency tails even when they should. > The world looks smoother through a TDR than it really is > (this is not a bad thing :-)). > > I think that the losses in the measurements are a secondary > effect in this type of example. > > Best regards, > Neven > --- > Neven Orhanovic > Applied Simulation Technology > neven@xxxxxxxxxxxxx > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > > > *************************************************************************** Allen Telecom Inc. and Grayson Wireless were acquired by Andrew Corporation (http://www.andrew.com/ <http://www.andrew.com/> ) Please update your email address book to reflect the change of address reflected in the From: portion of this email. **************************************************************************** ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu