[SI-LIST] Re: ISI
- From: Khalid Ansari <KANSARI@xxxxxxxxxx>
- To: "'ikhan@xxxxxxxxxxxxxxxxxxx'" <ikhan@xxxxxxxxxxxxxxxxxxx>,si-list@xxxxxxxxxxxxx
- Date: Sun, 11 Aug 2002 11:41:05 -0700
Ibrahim,
1. Inter-Symbol-Interference
A transmission line plays a low pass filter
effect on a signal going through. This is due
to skin effect, as we go higher in frequency
resistance develops within (starting from the center)
a transmission line pushing the signal, which
basically travels in the form of energy, towards
the outside of the conductor. Each frequency
component will see different amount of resistance.
The depth of the energy travelling within the
conductor from the walls of the conductor is
known as the skin depth which is
skin depth = sqrt (1/pie*f*uo*ur*conductivity), from
the formula it is apparent that as the frequency
increases the skin depth decreases, the signal is
pushed out even further.
Basically when you have a data going through a
transmission line the higher frequency components
will not go as high in amplitude as the lower
frequency component, due to which the edges will
be scattered. If you look at the eye diagram for
your data, depending on the length of transmission
line you are going through, the eye will have more
closure.
2. How to predict (based on the data sheet etc) that we will encounter
ISI
ISI is not mentioned in data sheets. It depends on
how you are designing your system, how long are your
traces. The longer the trace the more the ISI. You
can somewhat predict the amount of ISI using some a
simulation tool. Use a 2D field solver to extract
the RLGC for your transmission line and then send your
data through and see what happens to it after a certain
length of transmission line. You can also go into a
Field theory book and get formula's from there but do you
really wanna do that?? A good book to refer to is 'Fields
and Waves in Communication Electronics'.
To predict
3. How to avoid it
Use shorter transmission lines. Add pre-emphasis to your
driver and/or equalization on your receiver. To understand
pre-emphasis and equalization, there are a lot of good
documents on the web. All these factors will help you reduce ISI.
Khalid
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