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Simulators >> RF Simulators >> TL Modelling
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Message started by unevb on Apr 21^st, 2011, 1:28am

Title: TL Modelling
Post by unevb on Apr 21^st, 2011, 1:28am

Hi,

I am trying to find the S-parameters of a integrated Coplanar Waveguide using Agilent's Momentum tool... The signal line is on the top metal layer of a 9 metal stack and I have 2 GND lines running parallel to the signal conductor on both sides of it.

I define 4 ports :
1. Port 1 : Top of the Signal line
2. Port 2 : Bottom of the Signal line
3. Port 3 : Gnd line on the left
4. Port 4 : Gnd line on the right

I get a *.s4p from after my simulation. In my ADS schematic simulation, I connect Port 3 and Port 4 to the reference ground plane , and use the TL as a 2 port device.

Is there a way to accurately extract the S-parameters at a 2 port (*.s2p) from the EM simulations itself? In other words, is there a way to specify that the conductors on the sides of the signal line are GND lines, in the EM simulation. I need a 2-port S-matrix because I want to convert the S-parameters to ABCD to find out the Zo and gamma of the TL

Thanks!

Title: Re: TL Modelling
Post by pancho_hideboo on Apr 21^st, 2011, 11:47am

Simply resimulate 4port-sp box as 2port by sp-analysis.

Here connect gnd to port3 and port 4.

Title: Re: TL Modelling
Post by unevb on Apr 24^th, 2011, 2:21pm

Thanks, that works well.

For calculating the Zo, I am converting the S-parameters to ABCD parameters

A = cosh( gamma * l)
B = Zo * sinh (gamma * l)
C = sinh (gamma * l) / Zo
D = cosh (gamma * l)

I am using B/C to estimate the Zo of the line. Is this a correct approach?

The Zo varies a lot with frequency (Fig1 from Matlab). In Fig.2 (from ADS), I have terminated the TL with 95-ohm (Zo at 20GHz) , the input impedance is approximately 100-ohms. For higher frequency Zo is much lower though.

In such a case, how come people talk about 100-ohm TL without specifying the exact frequency range over which this is true?