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Modeling >> Transmission Lines and Other Distributed Devices >> Modeling 50 ohm transmission line in Spectre
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Message started by Sunny on Jul 29^th, 2004, 1:47pm

Title: Modeling 50 ohm transmission line in Spectre
Post by Sunny on Jul 29^th, 2004, 1:47pm

I've tried using the "tline" element in Spectre's analoglib to model 50 ohm board traces, but it's not very clear what properties need to be specified for this element, and what values I should use. Tried looking up help in Spectre, but didn't help much

I'm trying to model the board traces so that they can be included in a 50 ohm differential driver design.
What properties need to be defined?
I'm currently using:
charac. impedance: 50
delay time (for a 1" trace on FR4): 145pS
Frequency: ?? (100Mhz to 1GHz signals)
(Is it frequency of the signal I'm not sure why this is needed)
normalized length: ?
v1 ??
i1 ??
v2 ??
i2 ??
propagation velocity normalized ??
Normalized length : The documentation says "Normalized electrical length in wavelengths at 'f'
of a lossless line. What is 'f'? It's not frequency (that is freq).
What is reference frequency?
Loss resistance and conductance per unit length (not sure about the numbers for these for
50 ohm microstrip on FR4)

Thanks!

Title: Re: Modeling 50 ohm transmission line in Spectre
Post by Mighty Mouse on Jul 29^th, 2004, 5:25pm

There are three different ways of specifying the electrical length of a tline. You can simply specify it in seconds using td. Or you can specify it in wavelengths, using nl and f, in which case td=nl/f. Or you can specify it in meters using len and vel, in which case td=len/(vel*c).

For your case, it is sufficient to use "z0=50 nl=145pS" to model a lossless line.

- MM -

Title: Re: Modeling 50 ohm transmission line in Spectre
Post by Mighty Mouse on Jul 29^th, 2004, 5:37pm

By the way, the tline is best suited for modeling coax lines. You would be better off using LMG (line model generator) to produce an mtline model. LMG allows you to describe the line in terms of its physical dimensions and properties of the materials.

- MM -