Mkaragou,

  Since no one has answered this, I will take a shot. The n-port is
best approach for modeling cables. The mtline is more appropriate
for package and board applications. When extracting parameters
for the cable, you will need data points at low frequency to assure
a good extrapolation to dc. For the twisted pair using mixed-mode
s-parameters is better.  

                                                           Best Regards,

                                                               Sheldon

One more piece of information that may (or may not!) be useful.....

To model coax and cat5 cables, you have a couple of options available.

The simplest approach to model a single cat5/coax cable is by specifying a
constant characteristic impedance Zo and time delay td parameter in
mtline(tline).  For shorter cables or lower frequencies this may be a
reasonable approximation, but for longer lossy lines, you may want to add skin
and dielectric loss, fc/alphac and fd/alphad.  This approach overall may have
limited accuracy.  We suggest that you make comparisons with measurements if
possible.

Another approach would be the use mtline with r, l, g and c data values along
with rskin and gdloss to model the frequency effects.  This data may be
obtained from the manufacturer or by time-domain and frequency-domain
measurements.  See "EDA Development for 10 Gigabit Ethernet Measurement Based
Hspice Model Library", Mayrand & DiMinico, DesignCon 2003.

Coax lines have well known equations to obtain R, L, G and C which can be used
with mtline.  These expressions take into consideration the geometry and
electrical characteristics of the cable.  But, for increased accuracy you may
need to use a field solver in order to obtain the frequency dependent R(f) L(f)
G(f) and C(f) parameters.

The above approaches rely on the mtline element.  It should be noted that
mtline assumes that a quasi-tem approximation is valid.  This assumption breaks
down for long lines and above some cut-off frequency where higher-order modes
begin to propagate.  We strongly recommend that you compare mtline results with
measurements if possible.

An alternative approach would be the use of NPORT with measured S-parameter
data.  Since the lines are likely to be long, you must provide a small (fmin)
sampling frequency in order to ensure adequate impulse response modeling up to
tmax, where tmax is the impulse response time including delay, i.e., fmin <
(1 / tmax).  If the cable is unusually long, then care must be taken to ensure
an appropriate value of fmin sampling since the delay may be quite large.

Again... for these types of lines, measurement results would be a useful
resource to validate any of the approaches described above.

Best regards,

Tawna