Keith Dysart wrote:
On Jan 1, 6:00 pm, Roger wrote:
Roy Lewallen wrote:
Roger wrote:
Could you better describe how you determine that the source has a Z0
equal to the line Z0? I can guess that you use a Thévenin equivalent
circuit and set the series resistor to Z0.
Probably the simplest way is to put the entire source circuitry into a
black box. Measure the terminal voltage with the box terminals open
circuited, and the current with the terminals short circuited. The ratio
of these is the source impedance. If you replace the box with a Thevenin
or Norton equivalent, this will be the value of the equivalent circuit's
impedance component (a resistor for most of our examples).
If the driving circuitry consists of a perfect voltage source in series
with a resistance, the source Z will be the resistance; if it consists
of a perfect current source in parallel with a resistance, the source Z
will be the resistance. You can readily see that the open circuit V
divided by the short circuit I of these two simple circuits equals the
value of the resistance.
The power output of the Thévenin equivalent circuit follows the load.
Sorry, I don't understand this. Can you express it as an equation?
There seems to be some confusion as to the terms "Thévenin equivalent
circuit", "ideal voltage source", and how impedance follows these
sources.

Two sources we all have access to are these links:

Voltage source:
http://en.wikipedia.org/wiki/Voltage_source
Thévenin equivalent circuit:http://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem

I don't disagree with anything I read there.

But you may not quite have the concept of impedance
correct.

The impedance of the Thevenin/Norton equivalent source
is not V/I but rather the slope of the line representing
the relationship of the voltage to the current.
When there is a source present, this
line does not pass through the origin. Only for
passive components does this line pass through
the origin in which case it becomes V/I.

Because the voltage to current plot for an ideal
voltage source is horizontal, the slope is 0 and hence
so is the impedance. For an ideal current source
the slope is vertical and the impedance is infinite.

Hoping this helps clarify....

...Keith

Yes, I can understand that there is no change in voltage no matter what
the current load is, so there can be no resistance or reactive component
in the source. The ideal voltage link also said that the ideal source
could maintain voltage no matter what current was applied. Presumably a
negative current through the source would result in the same voltage as
a positive current, which is logical if the source has zero impedance.

During the transition from negative current passing through the source
to positive current passing through the source, the current at some time
must be zero. How is the impedance of the perfect source defined at
this zero current point? How is the impedance of the attached system
defined?

The resistor in the Thevenin/Norton equivalent source is selected with
some criteria in mind. What I would like to do is to design a Thevenin
source to provide 1v across a 50 ohm transmission line INFINATELY
long, ignoring ohmic resistance. I would like the source resistor to
absorb as much power as the line, so that if power ever returns under
reflected wave conditions, it can all be absorbed by the resistor. I
think the size of such a resistor will be 50 ohms.

Thanks for the clarification.

73, Roger, W7WKB