All About Circuits Forum problems with RLC bandpass filter simulation in LTspice
 Register Blogs FAQ Members List Today's Posts Search Today's Posts Mark Forums Read

 Homework Help Stuck on a textbook question or coursework? Cramming for a test and need help understanding something? Post your questions and attempts here and let others help.

#1
10-03-2010, 05:18 PM
 finley New Member Join Date: Oct 2010 Posts: 6
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 Hz--1.0kHz 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
10-03-2010, 05:30 PM
 finley New Member Join Date: Oct 2010 Posts: 6

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
10-03-2010, 06:22 PM
 Jony130 Senior Member Join Date: Feb 2009 Location: Poland/Wroclaw Posts: 2,459

It seems that you forgot about Q factor.
Becaues in LCR filter Q factor determine bandwidth of a filter.
#4
10-03-2010, 06:40 PM
 finley New Member Join Date: Oct 2010 Posts: 6

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 Q-factor to solve this problem?
#5
10-03-2010, 07:10 PM
 Jony130 Senior Member Join Date: Feb 2009 Location: Poland/Wroclaw Posts: 2,459

This equation show everything
Q = XL/R = Fo/ (F2 - F1)
Fo = √(F1*F2)

Last edited by Jony130; 10-03-2010 at 07:18 PM.
#6
10-03-2010, 08:06 PM
 finley New Member Join Date: Oct 2010 Posts: 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 Q-factor applies?
#7
10-03-2010, 08:27 PM
 finley New Member Join Date: Oct 2010 Posts: 6

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
10-03-2010, 08:55 PM
 Jony130 Senior Member Join Date: Feb 2009 Location: Poland/Wroclaw Posts: 2,459

Quote:
 Originally Posted by finley Is this correct? and that it should be more around 100Hz.
Yes, fo =100Hz

Quote:
 Originally Posted by finley 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?.
Fo for LC circuit is equal 1/sqrt(LC) becaues only for this frequency
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
10-04-2010, 06:55 AM
 finley New Member Join Date: Oct 2010 Posts: 6

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

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

 Related Site Pages Section Title Worksheet Active filters Worksheet AC network analysis Textbook Credits : Contributor List Worksheet Passive filter circuits Worksheet Mixed-frequency signals Worksheet Resonance Textbook Harmonics in polyphase power systems : Polyphase Ac Circuits Textbook Resonant filters : Filters Textbook Low-pass filters : Filters Textbook Resonance in series-parallel circuits : Resonance

 Similar Threads Thread Thread Starter Forum Replies Last Post simply_me The Projects Forum 22 06-09-2010 05:11 PM 1233 Homework Help 2 12-28-2009 12:07 AM alialiali Homework Help 31 03-16-2009 07:10 PM windell747 General Electronics Chat 2 06-22-2008 08:00 AM diwa2000@gmail.com General Electronics Chat 3 06-13-2008 02:35 PM

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Electronics Forums     General Electronics Chat     The Projects Forum     Homework Help     Electronics Resources Software, Microcomputing, and Communications Forums     Programmer's Corner     Embedded Systems and Microcontrollers     Computing and Networks     Radio and Communications Circuits and Projects     The Completed Projects Collection Abstract Forums     Math     Physics     General Science All About Circuits Commmunity Forums     Off-Topic     The Flea Market     Feedback and Suggestions

All times are GMT. The time now is 03:09 PM.