--- lcdpublishing <lcdpublishing@...> wrote:
Roy, I will do when I get the cable in that I plan
on using for
this. By chance, can you give me a hint as to what
sort of good and
bad things I should be looking for?
Transmission lines all have four basic properties
inherient to them. They are: 1) resistance 2)
inductance 3) capacitance 4) conductance.
The resistance and inductance are in series with the
load at the other end and the capacitance and
conductance are in parallel with the load. What this
means is that your transmission line will behave as a
filter when a signal is passed along it. This has
several side affects associated with it. I will
discuss these in a moment. First let me digress into
an aside about signals.
According to Fourier, any real signal can be expressed
as a sumation of an infinate combination of sines and
cosines; each with a different frequency and
amplitude. The frequencies of each "component" are
integer multiples of the fundimental frequency. This
is what people are refering to when they talk about
n-th order harmonic frequencies (i.e. the third
harmonic for example). In digital circuits, our
transmission signal is a square wave. Transforming
this into its Fourier series yeilds a combination of
sine waves with odd-numbered multiples of the
fundimental frequency. You don't really need to grasp
this fully right now but it is revelant for your case.
Getting back to the transmission line properties, let
me remind you that a transmission line will behave as
a passive filter when the line lengths get long
enough. This results in two distinct effects on a
signal that is being transmitted on it. 1) Phase
shift 2) Voltage reflection.
PHASE SHIFT -- All passive filters will create some
sort of a phase shift of any AC signal that is passed
through it. Whether or not the shift is positive or
negative will depend upon the frequency of the signal
and the type of filter it passes through. Recall that
our digital square-wave is actually composed of an
infinate combination of sine waves. When this type of
wave is sent along a transmission line the wave can
become "distorted" when it comes out the other end.
This is because the "line filter" has shifted the
phase of some of the square wave's harmonics. So what
you should be looking for on the output end of your
line will be ringing on your transitions and rounding
of your signal edges. If your line is really long, or
poorly constructed, you might even see something that
looks more like an audio signal rather than a
square-wave.
VOLTAGE REFLECTION -- The second characteristic that
must be accounted for is voltage reflection. What
this describes is the fact that that not all power
transmitted on the line will be transfered to the
load. Some of it will be reflected back to the
source. The effect is similar to the one observed
when you take a string and tie one end to a wall and
then take the other end and shake it up and down to
create a wave on the string. When the wave reaches
the wall, the wave "bounces back" towards you. This
is onset by a mismatch in impedances between the
source, transmission line, and load. When the voltage
is reflected back to the source, if the source and
line impedences don't match the wave will then be
reflected back down to the load again. This results
in what is called a "standing wave" and in "ideal"
conditions can continue forever. One effect that can
be caused by this standing wave is if the standing
wave happens to be in phase with the signal at the
load then the voltages add together. This can cause
the voltage across the load to increase greatly. The
same is true at the source. When the standing wave is
in phase with the signal at the source, the voltage at
the source can increase greatly.
The opposite is also true. If the standing wave is
180 degrees out of phase with the signal and the
relative amplitudes of each are similar, then the
signal will in effect be canceled out at the load (or
source depending on which end of the line you're on.)
This is why impedence matching is so important between
the source, line, and load. If the impedences match
then a standing wave will not be seen. What you
should look for to determine if this is happening is
check your voltages. If you have a dramatic increase
or decrease on either side of the line then you
probably need to adjust the length of your line.
Also, check each of the components on either side of
the line. If they are getting hot then that probably
indicates a problem as well.
I forget the exact length, but I believe these effects
start to become noticeable when the length of your
line is 1/100th that of the signal wavelength but I'll
have to look this up to be sure. At your voltages and
frequencies, you will probably have to worry more
about phase shift than voltage reflection but it never
hurts to check for both. Also, the degree in which
these effects are manifested depend upon the type of
transmission line used.
If you want more information on either the Fourier
transform or transmission line properties do a google
search. There is quite a bit of information out there
on both topics.
Hope this helps you find what you are looking for.
Shawn
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