Q-Factor and Resistance

  • Thread starter Mzzed
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  • #1
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Main Question or Discussion Point

I am trying to calculate the correct component values to use in a simple series LC filter, however I am not sure which value of resistance is to be included in these equations. When talking about a series LC filter, is the resistance required for calculating q factor the resistance of the inductor and capacitor? Or is it what ever the load resistance is further down the line in the circuit?

Thanks!
 

Answers and Replies

  • #2
Tom.G
Science Advisor
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the resistance of the inductor and capacitor? Or is it what ever the load resistance is further down the line in the circuit?
Both.
Create an equivalent circuit using the DC resistance of each component, load, and source resistance.
Calculate the effective DC resistance of all series and parallel branches combined (Thevenin Equivalent). Use that resistance to calculate the Q.
Note that in this sense, Q is defined only around the resonant frequency.

Cheers,
Tom
 
  • #3
LvW
793
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Normally, the capacitor has a loss resistance Rp in parallel to the capacitance.
Therefore, in order to combine this resistance Rp with the series loss resistance of the inductor Ri, the value of Rp (relatively large) must be transferred into an equivalent (small) series loss resistance Rs.
 
  • #4
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Thankyou both, exact answers I needed, really appreciate it!
 
  • #5
tech99
Gold Member
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Both.
Create an equivalent circuit using the DC resistance of each component, load, and source resistance.
Calculate the effective DC resistance of all series and parallel branches combined (Thevenin Equivalent). Use that resistance to calculate the Q.
Note that in this sense, Q is defined only around the resonant frequency.

Cheers,
Tom
Apart from simple text book questions, I think you need to use the resistance at the operating frequency, because DC and AC resistance differ due to skin effect and dielectric losses.
 

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