ECE3300 Lecture 4-3 Transmission LInes


Uploaded by cfurse on 26.08.2009

Transcript:
Now let's talk about the different kinds of
effects that we will see on a transmission line. If our
transmission line is long enough that we need to
consider it, one of the primary effects is going to be
delay. Imagine what would happen if we had two
different memory chips and we wanted to trigger them
at exactly the same time. Suppose that we used a piece
of wire that was longer in one case than the other.
Here's a delta L that's longer than our first
transmission line. If that delta L is long enough to be
considered a transmission line, the clock pulse that we
are sending to the first chip is going to be delayed
when it reaches the second chip.
Another effect that we're going to see is
reflection. In this case, the pulse that we send down
the line gets right here so it's moving down the line
and it gets to where the chip is and then a portion of it
reflects back. That reflection might be positive or the
reflection might be negative. The reflection might be
separated from the initial pulse as shown here or it
might overlap on the initial pulse. It could be possible
to have a negative reflection that would combine with
the positive and pulse make it go away, make it appear
as if there was no pulse at all. So reflections can also
be fairly serious problems.
Another problem that we see is power loss.
Power is lost in two places. One is in the insulator
between the two pieces of wire that make up the
transmission line, and the other one is in the conductor
of the transmission line itself. With a perfect
insulator and a perfect conductor there would be no
power loss and our line would be considered lossless
but you virtually can never have a perfect insulator and
a perfect conductor so you do get some loss. Typically,
as you increase your frequency, you also increase your
loss.
Now one other effect that we will see in
transmission lines is dispersion. In dispersion, the
velocity of propagation is a function of frequency. That
means that different frequencies propagate at
different speeds.
So suppose that I had this pulse. This pulse is
made up of an infinite number of frequencies, and if
they do not always travel at the same rate then this
pulse is going to be dispersed. What that usually ends
up looking like is something like this where you get a
tail on the front end of your pulse; you get a tail on the
back end of your pulse. Dispersion means that you
don't get as nice a rise time or as nice and well defined
a fall time on your pulses, and sometimes the bounces
that you are seeing because of dispersion may even
interfere with your system as well.
Now, let's talk about the different
propagation modes on transmission lines. The most
common kind of transmission line and the kind that we
are going to limit ourselves to in this class is called
TEM. What that means is transverse, electric, and
magnetic. It means that the electric field is
perpendicular to the magnetic field and that both of
those are also perpendicular to the direction of
propagation. TEM transmission lines are NA2
conductor line. NA2 conductor line will behave as a
TEM line so our coax, for instance, a micro strip, which
has a piece of metal up here, an insulator and then a
ground plane below, all two conductor lines act as TEM
lines.
Now, let's figure out which direction the
electric and magnetic fields are seen. Let's use our
right hand again, put your thumb up here. That's going
to be the direction of propagation, and then use your
first finger, that's your pointer finger. Your pointer
finger is going to point in the direction of the electric
field and then your third finger, right there, is going
to be the magnetic field so your middle finger is H. So
just move those so that they are all perpendicular to
each other. It's kind of like holding out a gun and then
moving your third finger perpendicular to that, and you
can see the direction of propagation and the
perpendicular E and H fields.