Will a matching impedance network also act as a filter? I'm using a pi network and am wondering if I should also add a filter. I designed the pi network by using the program SimSmith and it consists of a shunt capacitor, series inductor, and another shunt capacitor.
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3 Answers
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Will a matching impedance network also act as a filter?
A pi network, as described, is a filter and it can sometimes be used to match impedances. However, a straight impedance matcher is normally only resistive (such as for the termination of cables to prevent reflections at RF).
I'm using a pi network and am wondering if I should also add a filter
The pi network will act like a filter and if you are not happy about its performance as a filter then add a filter but, the filter will need to match the impedances you may be seeking to maintain.
answered Mar 19, 2015 at 22:22
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\$\begingroup\$ If an impedance matching network includes resistors then part (or even all, as in your example) of the available power is not delivered to the load -which is the purpose of most RF matching networks. \$\endgroup\$– PetrusCommented Apr 4, 2017 at 14:51
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They can act as a filter in the sense that the matching network will have a inherent frequency response out of band. This performance is definitely secondary to the impedance matching functionality.
If you do add a filter to the circuit, keep in mind that most impedance matching approaches assume that you're matching to a fixed resistance, even well out of band. This is not what a filter will present as an input / output impedance. You should separate the impedance match with some sort of impedance buffering. The details of this is left to your system design.
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The purpose of impedance matching networks is that all the available power is delivered to the load. There are many matching networks that perform this task at a given frequency of operation. Once you depart from it, the frequency dependence of the matching elements (and the frequency dependence of the load) give an overall non-constant frequency response which may be called a "filter": a network whose response depends on the input frequency.
Some matching structures are designed to offer matching and performing an useful filter function, such as a bandpass response. For this, you need a suitable structure and sufficient degrees of freedom in the element values: a two-element matching circuit can only match two requirements (real and imaginary part of impedance). With a three element structure you may be able to achieve a given bandwidth, etc.
