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The boundary between a simple point-to-point interconnection and a transmission line, which has distributed elements, sometimes
is difficult to define.
Because the effects of these distributed elements in transmission lines can crop up
in unexpected places, designers often overcompensate by preparing for the wave reflections that cannot possibly occur.
How to identify a transmission line?
In the real world all wires have finite signal propagation delay. Practical numbers are about 1-2nS per foot ,
depending on characteristics of the wire.
When wire delays are on the same order as the transition times of the signals that they carry, the wires must be treated not as zero-delay
perfect conductors, but as the "transmission lines" that they really are.
When wire delays are insignificant in comparison with "slow" transition times of the signal - then we deal with simple
interconnections that permit the use of Kirchhoff's circuit laws.
For transmission lines the mathematics of the system becomes more complex. You can analyze any interconnection from either
a frequency or time domain perspective.
When using frequency domain analysis, if the length of the interconnection is greater than 1/15 of the wavelength of the signal
it carries, consider the interconnection to be a transmission line.
L = v / f ,
where L - Wavelength;
v - velocity of propagation;
f - highest frequency, present in the signal.
Cable manufacturers instead of velocity of propagation sometimes specify the inverse value t- propagation delay.
Propagation delay is proportional to the square root of the dielectric constant. For example the propagation delay of signals
traveling in air is 85 ps/inch, for coax cable it is 113 ps/inch, for pc-board traces it is about 180 ps/inch.
When using time domain analysis, if the time required for the signal to travel the length of the interconnection is greater
than 1/8 of the signal rise time, consider the interconnection to be a transmission line.
Time T required for the signal to travel the length of the interconnection can be calculated from the above Wavelength equation,
L = v * T.
Rise time information is usually specified between 10% and 90% amplitude points. For example for ECL family rise time is about 1 nS.
References:
1.John F. Wakerly Digital design,principles and practices,Second Edition,Prentice Hall,1994
2.Transmission lines & interconnections,David Royle,EDN,June23,1988
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