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#1




problems with RLC bandpass filter simulation in LTspice
Hello there!
I have a project for school that is aimed towards engineering students (which I am not) to build an RLC bandpass filter which will pass 10 Hz1.0kHz where those same points are the 3dB points. I have calculated that the natural frequency should be 495 Hz and, from a bandwidth of 990 Hz I have gotten these values: L=.161 H C=.642uF and R=1k. I started by deciding R=1k. My problem comes when I simulate this in LTSpice. I get the correct natural frequency, but my corner frequencies are 200 and 1201 Hz respectively. Can someone tell me why this might be occurring and how I could go about correcting for it? finley 
#2




oh, and additionally, my setup is a series RLC circuit with a capacitor, inductor, and resistor in that order. I don't know how to post pictures of the schematic and frequency response graph but if someone tells me how I could definitely do that.

#3




It seems that you forgot about Q factor.
Becaues in LCR filter Q factor determine bandwidth of a filter. 
#4




sorry if this is the most obvious thing in the world, but I've been working with this problem for about three grillion hours by now and my brain is fairly mushy. can you be a little bit more specific as to how I should use the Qfactor to solve this problem?

#5




This equation show everything
Q = XL/R = Fo/ (F2  F1) Fo = √(F1*F2) http://www.allaboutcircuits.com/vol_2/chpt_6/6.html Last edited by Jony130; 10032010 at 07:18 PM. 
#6




What I meant was could you please tell me in words why this is important. it's not hard to find equations, what's difficult for me is that I don't have enough background theory in this area to understand what precisely I should be looking for. My problem is that while I have the correct natural frequency, my corner frequencies seem to be shifted up by about 200 Hz. I'm assuming that this is due to some sort of internal resistance or something in the inductor, but I don't know how to apply adjustments to my circuit in an efficient enough way to correct for this. have you got any advice along those lines or clarification on how the Qfactor applies?

#7




Okay, I was a little confused with one of the equations that your page gave me, but after looking it up elsewhere I realized that what you mean is that I calculated the wrong natural frequency for my circuit. Is this correct? and that it should be more around 100 Hz. In this case, my question is now a little different. Everywhere else I've looked it said that the natural frequency should be 1/sqrt(LC) so could you please explain to me why f=sqrt(fh*fl) is valid? As I said, I'm missing a whole lot of theory here and kind of trying to patch together stuff that I have learned a couple of years ago. I appreciate the fact that you even replied to me at all.

#8




Yes, fo =100Hz
Quote:
Xc = XL And Fo is located "halfway" between F2, F1. But for mother nature "halfway" in not in the "linear" axis (arithmetic mean) but in geometric mean. And that why Fo = √(F1*F2) it's true. So for you circuit Fo = 100Hz and for R = 1KΩ > Q = 0.101 L = 0.160762H C = 15.7563μF 
#9




Great. Thanks for being so clear and helpful! Now...to the computer lab!

Tags 
bandpass, bandpass filter, filter, ltspice, problems, rlc, rlc circuit, simulation 
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