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#11
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I have a doubt in smith chart
money wrote: K7ITM wrote: money wrote: 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? Owen's right. A picture is worth, generally, about a kiloword. I'd also recommend you download one of the free Smith chart programs. You may also even find a decent web aplet that will plot directly in your browser window. For a quick more direct answer: for a series capacitance, plot on the impedance grid. The capacitance must be converted first to a capacitive reactance: Xc = -1/(2*pi*f*C). That is the length you need to move along an arc of a circle passes through your starting point and is tangent to the unit circle at infinite impedance. You will see some of these plotted on an impedance grid; your starting point may already lie on one of the plotted ones. For example, if the chart is normalized to 50 ohms, the 50+j0 point lies at the center of the chart. Let's say that the starting point is 25 ohms, and the frequency is 9.36MHz. Then the capacitive reactance will move you counterclockwise (in the normal presentation of the chart) along the arc that passes through the 25 ohm point (or the 0.5 point on a chart normalized to 1 ohm), about 35 degrees, to 25-j25 ohms. That should not be a surprise: the reactance of the capacitor at 7MHz is 25 ohms capacitive; an impedance of -j25 ohms. Series impedances simply add. For a parallel capacitance, find the suseptance of the capacitor at the frequency of interest, and follow a arc of constant conductance on the admittance overlay for the Smith chart. Remember, admittances of parallel components add, just as impedances of series components add. For the case in point, again at 9.36 MHz, you should go clockwise about 75 degrees along the arc of a circle which is tangent to the unit circle at impedance = 0 (infinite admittance) and end up at an admittance of 0.04+j0.04 mhos, or an impedance of 12.5-j12.5 ohms. If you go back to the first example, with a series 680pF capacitance, and then add a shunt inductance, you will travel from the 25-j25 ohm point, counterclockwise along a constant conductance circle. If you make the inductance 850nH, you will find that you end up at an impedance of 50 ohms: you have matched the original 25 ohms to a 50 ohm system, at that frequency, with an "L" network. The Smith chart lets you visualize the match very quickly, once you are comfortable with it. A computerized Smith chart made the numbers above much easier than the words! Cheers, Tom The circuit is actually an impedance transformation circuit to match it with line impedance 50 ohms. the frequency of operation is 900 MHz. With a software i matched an amplfier circuit with line impedence 50 ohms. 680pF(series capacitance) is part of the impedance transformation circuit. I do get the point of traversal in a smith chart. For series capacitance i need to move along the constant resistance circle anti-clockwise. But if i use the formula capacitive reactance = 1 / C W then normalise it to 50 ohms i get negligible reactance of 0.05.... Is it correct? Yes, right. It is a very small reactance at 900MHz. In other words, I assume it is just a DC blocking capacitor, if it is part of a properly designed network. It must be other parts which are doing the actual matching job. Do you know the impedance of the port which you are matching to 50 ohms? Or do you have the network already, and you want to "reverse-engineer" the circuit to discover what the impedance is that is matched to 50 ohms? I would expect that parts used in a 900MHz matching network would be in the range from perhaps 0.5pF to 10pF--maybe a little beyond, and also inductances in the low nanohenry range. And such small capacitances and inductances can be even small copper areas and short, narrow traces on printed circuit boards. Often it is better to analyze everything in terms of transmission lines, instead of discrete capacitances and inductances, when you are operating at such high frequencies. Cheers, Tom |
#12
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I have a doubt in smith chart
"Frank's" wrote in message news:YOhdh.17285$YV4.7275@edtnps89... "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? Try the free Smith Chart demo at: http://fritz.dellsperger.net/. Probably the best electronic version I have seen. 73, Frank (VE6CB) That is a good Smith Chart program. I have been looking for one. |
#13
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I have a doubt in smith chart
"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 |
#14
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I have a doubt in smith chart
Try the free Smith Chart demo at: http://fritz.dellsperger.net/.
Probably the best electronic version I have seen. 73, Frank (VE6CB) Yeah frank i just downloaded the smith chart demo..... I am looking into it. It is great & is surely helping me a lot. Thanks a ton frank... :-) Glad you found it interesting. If you need to know more of the theory behind the Smith Chart I would recommend "RF Circuit Design" by Christopher Bowick. It is available on www.amazon.com for $26.37. The treatment is fairly non- mathematical, but it does give great insight into how the Smith Chart works. Frank |
#15
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I have a doubt in smith chart
On Tue, 05 Dec 2006 17:15:36 GMT, "Frank's"
wrote: "money" wrote in message ups.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? Try the free Smith Chart demo at: http://fritz.dellsperger.net/. Probably the best electronic version I have seen. Hi Frank, Why is it that when people develop electronic implementations of the Smith Chart (like this one) that they don't implement things like complex Zo and attenuation in transmission line elements, complex reference impedance (incorrectly termed Zo in this prog)? Owen -- |
#16
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I have a doubt in smith chart
Hi Frank,
Why is it that when people develop electronic implementations of the Smith Chart (like this one) that they don't implement things like complex Zo and attenuation in transmission line elements, complex reference impedance (incorrectly termed Zo in this prog)? Owen Hi Owen, Yes that has always bugged me. Not so much a complex Zo, but transmission line losses would be nice. Frank |
#17
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I have a doubt in smith chart
Jerry Martes wrote:
In 1966 I wrote a short article on matching with a Smith Chart for Electronic Design. Aha, that's where I ran across your name. I still have a copy of that article somewhere. -- 73, Cecil http://www.w5dxp.com |
#18
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I have a doubt in smith chart
Yeah sure....
Please do send it... |
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