re: intersymbol Interference

Del Cecchi (dcecchi@VNET.IBM.COM)
Thu, 5 Jun 1997 11:30:38 -0500 (CDT)

While I guess that you could indeed have intersymbol interference from
improperly terminated nets causing remnant energy to be bouncing around,
the normal cause is high frequency attenuation.

High frequency attenuation will cause the switching time of a given bit
to vary depending on the previous bits. You can model this very simply
by running a pulse stream through an RC low pass filter. For example,
if the rise time of the system is reduced to be equal to a bit time,
then the shape of a bit and its pulse width will be reduced if it has
been proceeded long string of bits of the opposite polarity.

While cables don't exhibit a single pole response, it is a useful
approximation. A couple hours with a pulse generator, scope and some
resistors and capacitors will give a good feeling for the problem
without expensive high frequency equipment.

Most communications textbooks discuss this sort of thing in excruciating
detail.

Del Cecchi
dcecchi@vnet.ibm.com
IBM Rochester, MN