# Plotting Stability Circles on a Smith Chart

Discussion in 'General Electronics Chat' started by sjgallagher2, Jun 23, 2014.

1. ### sjgallagher2 Thread Starter Member

Feb 6, 2013
106
7
I need to plot a few stability circles on a smith chart to make sure this basic RF amp example design will be stable. The problem is, the center of one of the circles is WAY off the smith chart, by 10 units actually. If I wanted to graph it how I know, I would need a meter stick. I'm thinking of printing a smaller version a the smith chart to accommodate but I think I would lose a lot of accuracy. The radius is 10.3 (or something near there) so it puts part of the circle back on the chart. Is there a better way to plot the circle? My current method, let me explain, is to take the reflection coefficient length I get from the bottom of the smith chart and drawing a line that long in whatever direction. Then I use a compass to draw the circle itself. This would work for the maximum length of the compass, which as for most compasses isn't very wide, and certainly not almost 80cm, the radius of this circle on my smith chart.

I know software would do the trick for me, but I haven't needed it yet. There's something natural and rewarding about not using software, like comparing myself to engineers of the past who didn't have it .

The question: how do I plot circles on a smith chart if the circle's center is very far away from the center of the smith chart without resorting to software?

2. ### dougc314 Member

Dec 20, 2013
38
11
I think with the circles that far away the amplifier is going to be stable. What are the radius's like? The gain that you are trying to achieve has to have a match that lies on the Smith chart, and with the stability circles that far away any realizable gain should be stable. Has the calculation for stability factor indicated that the amplifier is potentially unstable?

As for plotting it, you have to use an extended smith chart, but I don't think I ever saw one with that large a radius before, R=2 was more like it.

3. ### sjgallagher2 Thread Starter Member

Feb 6, 2013
106
7
The radius plots the circle within the Smith chart, because it's slightly more than the distance from the center of the circle to the center of the Smith chart. Yes, the stability factor showed it to be potentially unstable I have a picture of what it Should look like, which is nearly a straight line.