Tawna Wilsey
Junior Member
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Posts: 11
Snohomish, WA
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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
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