"money" wrote in message
oups.com...

Jerry Martes wrote:

"money" wrote in message
ps.com...
I have a circuit in which there is a series capacitance of value 680
pF. The characteristic impedance of the line is 50 ohms. Can any
suggest how do i plot the capacitance in the smith chart.

If suppoe it was a parallel capacitance, how do i do it?


Hi Money

A purely reactive load, plots on the circumference (or perimeter) of
the
Smith Chart. It plots on the lower half (the minus impedance half) if it
is
capacitive, as in your case.

If you pick a frequency around 10 MHz, the capacitor will have close to
75
ohms X sub C.
Since you are working with a 50 ohm line, 75 is plotted on the lower
half
of the chart, at its perimeter and with a value of 75/50, or, 1.5 on the
chart.

I really like getting help thru Wikipedia. If you read a few of their
sites on "Smith Chart" and still have questions, I'd be happy to tell you
what little I know about Smith Charts.

Jerry


Yeah Jerry..... I have seen Wikipedia. :-) But from where did you get
75 ohms X sub C.... Wat does it mean?

Converting 680pF series capacitance to capacitive reactance we need to
use Xc = 1 / CW.... Is it not??

Hi money

I got 75 by making a mistake. The 680 pF capacitor will be about 24
ohms capacitive reactance at about 10 MHz., ?right?

When 24 ohms of capacitive reactance terminates the 50 ohm line, that
impedance is shown as R0-J24/50, which is found on the perimiter of the
chart. All impedances with R = zero are located on the outer perimiter of
the Smith Chart. Find the point on the chart where -J 0.48 is shown.
That point is in the lower left of the chart, when the chart is positioned
so the purely resistive axis is reading left to right.
So, that answers your original question "how do I plot xxx". I asummed
that you were aware that you cant actually plot capacitance on a Smith
Chart. The Smith Chart identifys only Impedance. The Smith Chart
identifies *all* impedances that have a real R value.

In 1966 I wrote a short article on matching with a Smith Chart for
Electronic Design. It isnt worth much by today's standards. But, I'd be
happy to send you a copy if you are interested in using a Smith Chart for
impedance matching.

Jerry