Are Smith Charts used to help match the impedance of filter circuits?

Thread Starter

talikarng

Joined May 27, 2017
21
I'd just like to check my understanding: I assume that Smith Charts are not used to design impedance transforming circuits for LC filters because LC filters are designed with a nominal impedance in mind (nominally 50 ohms). Is this correct?

(The reason I ask: I have built a LC low pass filter (7th order Chebyshev) and the Smith chart (from a NanoVNA) shows that the impedance of the filter varies with frequency - which I think makes sense because the output of the filter will be the result of the complex impedances of each component.)
 

Papabravo

Joined Feb 24, 2006
16,132
I'd just like to check my understanding: I assume that Smith Charts are not used to design impedance transforming circuits for LC filters because LC filters are designed with a nominal impedance in mind (nominally 50 ohms). Is this correct?

(The reason I ask: I have built a LC low pass filter (7th order Chebyshev) and the Smith chart (from a NanoVNA) shows that the impedance of the filter varies with frequency - which I think makes sense because the output of the filter will be the result of the complex impedances of each component.)
In RF work the impedance does vary with frequency, but not that much over a narrow bandwidth. Also any point on a circle in the complex plane centered on the origin will have the same impedance. Points close to the circle will also have an impedance that is very close to the impedance on the circle.
 
The Smith chart is not necessary in the design phase but is useful when analysing your design. When looking at how it interfaces with surrounding circuits for instance you may want to know what the stop band impedance is, which will be close to the edge of the Smith chart. A Chebyshev is not a perfect match in the passband either, how far from perfect is a function of the ripple selected for the design. The Smith chart can be used for visualising this.
 
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