transmission line as verilog-a or spectre-HDL

[Svenn Are Bjerkem]

Hi,

I have been using the lmg application and transmission line to model a
channel. I was wondering if it is possible to convert the transmission line
equations into verilog-a or ahdl since I then could do some parametric
simulations without recalculation in lmg.

I have been looking at the telegraph equations for some time now, but
neither google nor my math skills have been able to find an equation that I
can convert to hdl.

Maybe I am missing something about transmission lines....

--
Svenn

 

[Svenn Are Bjerkem]

Hi again,

I forgot to tell that I want to vary the length of the transmission line. I
do not know any RLC or G values of the channel so I would need to calculate
them once with lmg and then only have the hdl block extend this for a
length parameter.

--
Svenn

 

[Andrew Beckett]

On Mon, 09 May 2005 17:22:43 +0200, Svenn Are Bjerkem <svenn.are@bjerkem.de>
wrote:

Hi again,

I forgot to tell that I want to vary the length of the transmission line. I
do not know any RLC or G values of the channel so I would need to calculate
them once with lmg and then only have the hdl block extend this for a
length parameter.

Hi Svenn,

In that case what you want to do is to use lmg to write out a file of
frequency dependent RLGC matrices, rather than a subcircuit model. This should
be more accurate anyway. The advantage of this approach is that the RGLC
matrices are per unit length.

You then use the mtline component (from analogLib) which corresponds to
spectre's mtline component (spectre -h mtline). On this component you can
specify a length, and then this may be swept (provide you're using IC5032
or later - there was a bug in an earlier IC50 subversion).

The one issue with mtline is that if you want to use it with RF analyses,
you need to be using IC5141 or MMSIM60. And even then, it currently is only
supported for driven PSS analysis. Autonomous support is coming soon, if I
remember rightly. Otherwise, you have to use the subcircuit model approach
from lmg, which produces a model with fixed length, and you can't sweep it.

BTW, in lmg, use Options->Output File control, and set it to RLGC. For
complex multi-line situations (especially if the lines are long), not writing
out the subcircuit model can save a _lot_ of time!

Regards,

Andrew.

 

[Richard Griffith]

Svenn Are Bjerkem wrote:

Hi,

I have been using the lmg application and transmission line to model a
channel. I was wondering if it is possible to convert the transmission line
equations into verilog-a or ahdl since I then could do some parametric
simulations without recalculation in lmg.

I have been looking at the telegraph equations for some time now, but
neither google nor my math skills have been able to find an equation that I
can convert to hdl.

Maybe I am missing something about transmission lines....

Doesn't the standard library have a tline element where you can set the
R,L,G,C parameters?


MNA stamp for a transmission line

[Y11 Y12][V1] [I1]
[V21 Y22][V2] = [I2]

Where the currents are into the transmission line. V1,I1 near ends, V2,
I2 far ends.

Using the matrix parameters, R,L,G,C in per unit length values
Z = R+sL
Y = G+sC
Let A=ZY, and get the solution to the eigenvalue problem
LI-A=0
S is the eigen vectors, L the eigenvalues. Let Ki = sqrt(Li), individual
eigen value square roots.
Construct G = diag(Ki). A diagonal matrix of the square root of the
eigen values. Not matrix square root, individual value square root.
R = inv(Z)*S*G
E1 = diagonal( (exp(-2Ki*d)+1)/(1-exp(-2Ki*d)) )
E2 = diagonal( 2/(exp(-Ki*d)-exp(Ki*d)) )

Y11 = Y22 = R * E1 * inv(S)
Y21 = Y12 = R * E2 * inv(S)

Note that the resulting Y stamp is a very complex function of the
complex frequency s. The result is therefore only good in the frequency
domain. So ahdl or other models based on this are only good in the
frequency domain.

In the time domain you can get reasonably good approximations by using
multiple segments of R,L,G,C structures of 1/5 wavelength units
(preferrably 1/10) to make up your total length.


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