Smith Chart

[bostonman]

Although it's been many years since I learned about Smith Charts, I've never understood how to look at one and understand if it represents a good circuit or a bad one.

We just dotted the associated points and connected them, and, I've seen them in datasheets, but again, never grasp the concept of whether one is bad or good.

Is there an easy way to tell?

[pdenisowski]

I don't think there's a simple answer to that question - "good" or "bad" depends on what you're trying to do.   For example, in impedance matching, you're often trying get as close to the center of the Smith Chart as possible, while in load pull, you can use constant performance contours plotted on the Smith Chart to determine the optimum impedance for a given measured parameter (PAE, Pout, NF, etc.).  There are lots of other examples.

https://youtu.be/JtMeIdlEe40?t=961

[xrunner]

I don't think there's a simple answer to that question - "good" or "bad" depends on what you're trying to do.  

Exactly.

Although it's been many years since I learned about Smith Charts, I've never understood how to look at one and understand if it represents a good circuit or a bad one.

Same answer as measuring a voltage with a DMM or inspecting a waveform with a scope. Is what they are representing on their screens "good" or "bad"? It depends on a lot of things that you have to take into account with the information you are looking at. So in a simple example if you are looking at what you believe is a "pure" resistance and you also see inductive reactance on the smith chart, then you must determine why it's showing that. Is that "good" (or in other words explainable) because the resistance leads are presenting inductive reactance, so it's what you should expect - it's acting as it should be in this circumstance. Or perhaps it's a SMD resistor and the resistance element is poor, so in that case you know the result is "bad"?

[bostonman]

Good points, I hadn't thought about that aspect before posting my question.

Would I have been better off asking whether a large circle is more/less resistive or inductive? I mean, if a hundred Smith Charts were in front of you, and you wanted to pick one based on a specific criteria, could it be done easily enough?

Maybe a 50ohm load to use a practical example.

[pdenisowski]

Maybe this would help?  (It's the most-liked video on the R&S YouTube channel)