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This type of cable is most used for single ended data transmission, is familiar to designers and can
consist of any number of conductors. Ribbon cable is very flexible because it is narrow, and can fit in thin
spaces where round multiconductor cable would not. Grounds can be used as "barriers" between
asynchronous signals.
A simple for formula for the characteristic impedance of two parallel wires is
Where d is the diameter of the conductor, and s is the separation between wire centers. The propagation
delay is
For multiconductor (crosstalk) simulations, it is important to obtain the conductor to conductor coupling
parameters LM and CM. Here are suggestions for obtaining these parameters:
Contact cable vendor for mutual capacitance and inductance data.
Measure the capacitance and inductance matrices in the lab.
Use a 2-D field solver, such as the code provided in [5].
Round Multiconductor Cable Considerations
Round multiconductor cable is generally discouraged for high-speed applications because the worst case
amount of crosstalk that may occur varies from cable to cable. Yet, statistical analysis of crosstalk can be
performed for cables meeting tight manufacturing specifications. If a known minimum distance between
two sensitive conductors along with a maximum parallel length can be determined, then the resulting
crosstalk can be studied.
Best bets for obtaining the inductance and capacitances matrices are:
Measure the capacitance and inductance matrices in the lab.
Use a 2-D field solver, such as the wire-separation approximation 2-D code provided in [5].
)
2
ln(
120
0
d
s
Z
r
) / ( 85 in ps t r d
