Q-Factor and Resistance

  • Thread starter Mzzed
  • Start date
  • #1
Mzzed
67
5
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
Gold Member
4,459
3,226
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
906
244
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
Mzzed
67
5
Thankyou both, exact answers I needed, really appreciate it!
 
  • #5
tech99
Science Advisor
Gold Member
2,507
1,065
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.
 

Suggested for: Q-Factor and Resistance

  • Last Post
Replies
4
Views
2K
  • Last Post
Replies
10
Views
3K
Replies
1
Views
5K
  • Last Post
Replies
3
Views
24K
  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
11
Views
9K
  • Last Post
Replies
3
Views
11K
  • Last Post
Replies
7
Views
3K
Replies
3
Views
2K
Replies
4
Views
3K
Top