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vivkr

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how does Spectre simulate transmission lines?
Oct 10^th, 2007, 3:03am

I was wondering how Spectre (or another simulator) simulates transmission lines and such distributed circuits.

Generally, simulators work by solving network equations (KCL/KVL) on every point of a lumped circuit. In
distributed circuits, one cannot apply KCL/KVL as these do not hold, and it is also not possible to represent the
circuit in a lumped format.

How does it work then? Could someone enlighten me please?

Thanks
Vivek

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didac

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There's a million
ways to see the
things in life

Posts: 247
manresa,spain

Re: how does Spectre simulate transmission lines?
Reply #1 - Oct 10^th, 2007, 6:34am

Hi Vivek,
I think that the method of simulating transmission lines varies from simulator to simulator and in which domain you are working(time or frequency). I think that what spectre does for pss analysis is given frequency data(generated by touchstone or mtline) it makes a ifft(or what it has built in) and make a time convolution, for transient simulation I think that what it generates is a RLCG lumped model(that has a BW of limitation) and apply nodal analysis over it.
I've found a master thesis that discuss this issue:http://www.aplac.hut.fi/publications/dt-anu/main.html and gives insight about PSPICE,ELDO and HSPICE as well as general considerations.
Hope it helps,

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ali.m

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Re: how does Spectre simulate transmission lines?
Reply #2 - Oct 15^th, 2007, 9:43pm

vivkr wrote on Oct 10^th, 2007, 3:03am:

I was wondering how Spectre (or another simulator) simulates transmission lines and such distributed circuits.

Generally, simulators work by solving network equations (KCL/KVL) on every point of a lumped circuit. In
distributed circuits, one cannot apply KCL/KVL as these do not hold, and it is also not possible to represent the
circuit in a lumped format.

How does it work then? Could someone enlighten me please?

Thanks
Vivek

Hi Vivek:

If you go to the �EDA Tools� and select Tools>> RF, you might see some options showing the possibility of Transmission line modeling by Cadence. But in fact these are not going to simulate what you are really searching for.

You would need to model you TL in some other softwares like HFSS, ADS to simulate the TL. Take the simulated file from these softwares back to Cadence and put it into a �Black Box�. Now you are able to simulate you circuit with TL.

The other way is to draw the schematic of you TL model, and substitute all the parasitic parameters and then run the simulation.

Cheers
Ali

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vivkr

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Posts: 780

Re: how does Spectre simulate transmission lines?
Reply #3 - Oct 16^th, 2007, 4:53am

Hi didac,

Thanks very much for the explanation. This answers my question quite well. So, a transmission line is mapped to a lumped network
with a finite bandwidth. Depending on the mapping resolution, the bandwidth over which the approximation is valid is determined,
and so is the time needed for simulation. Thanks for the links as well.

I wonder if it is possible to use a different method of simulating these that would be faster.

Hi Ali,

Thanks for your feedback. I am not trying to simulate a transmission line, but was interested in the underlying theory of simulating these
with simulators that enforce KCL, KVL.

Regards
Vivek

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Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: how does Spectre simulate transmission lines? Reply #4 - Oct 16^th, 2007, 12:14pm Spectre maps transmission lines to lumped approximations only when it is doing an RF simulation. For conventional SPICE analyses, such as transient & AC, it uses a distributed representation. -Ken
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