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Low and high impedance of low pass stepped impedance filter

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igortomiatti

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Hello,

I'm designing a low pass chebyshev filter using stepped impedance.

I'm using the book 'Microstrip Filters for RF/Microwave Applications' as a reference.

I have difficulty understanding how low and high impedances are defined. The book uses Z0L = 93 ohms and Z0C = 24 ohms. I could not find an explanation for these values, I searched several articles but all uses an impedance between 20 and 120 ohms but does not explain how to calculate.

How can I get to this definition?

Thanks.
 

BradtheRad

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Telephone companies found it practical to make 600 ohms a standard in various types of circuitry, equipment and measurements, going back decades. Thus 1k ohm or less is regarded as low impedance.

Dynamic microphones can be included since they typically are several hundred ohms.

It's a relative scale for considering low versus high impedance. Each has its characteristics which a designer should be aware of.
 

FvM

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The low and high microstrip impedances can be chosen arbitrarily within certain useful limits. In the first place, substrate properties and feasible minimal and maximal conductor width are determining the choice.

As far as I'm aware of, the criteria are discussed in the text books like the said one.

There are filter design tools that suggest impedances based on performance criteria, e.g. Nuhertz Filter Solutions.
 

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