Q factor, resonant frequency and me banging my head against a wall...really hurts

Discussion in 'Homework Help' started by ninjaman, Dec 14, 2014.

  1. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello

    I have a series RLC circuit. I have a resistor of 4620 ohms (measured), capacitor of 1uF and an inductor of 151mH.

    I have found the reactances at 1kHz. 951 ohms for Xl, 161 ohms for Xc. approx. values.

    I have Q of 0.0841 from 1/r ((sqrt) L/C) = 1 / 4620 * (sqrt) 0.151 / 0.000001

    I have a bandwidth of 4955Hz from below. subtract first frequency from second.

    upload_2014-12-14_16-43-46.png

    BW = Fr / Q
    I have tried this a few ways by transposing for different values and I cant get any of them to stick. I get odd values all the time
    I tried calculating Z using square root of R^+(Xl-Xc)^2, I got 4687 ohms for this. so Z / R = 4687 / 4620 = 1.014

    getting very funny values here....though im not laughing!!!

    any push in the right direction would be great

    fantastic resource this!!! I really like it!!!


    simon
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    It's not clear what your problem is. What do you mean "can't get any of them to stick" and "funny values"?
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,138
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    I was wondering that myself, but you beat me to it.
     
  4. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    I tried the Q formula, 1/r * (sqrt)L/C and got 0.084
    I tried Q = Z /R, calculated Z from (sqrt) R^2 + (xl-xc)^2, for Z I got 4687 ohms. this over R, 4687 / 4620 = 1.01
    so I have two values for Q, 0.084 and 1.01
    then I have
    BW = Fr / Q
    BW from the multisim analysis was 4955Hz, Fr is 409 Hz, I tried rearranging this to find Q and got a different answer. I tried this with both values of Q from above and didn't get the 4955Hz bandwidth.
    so everything is different.
    that's what I meant by funny values

    the closest I get is Fr/Q using 409Hz / o.084 = 4869Hz. but that isn't 4955.
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,387
    497
    Your ability to present information in clear and organized manner sucks!

    Put all the values in a table. Calculate percentage errors for values that you were tasked to find. If percentage errors are less than 5%, then you fine.
     
  6. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    where does the percentage error of 5% come from. is this to do with the accuracy of measuring equipment?
    shteii thanks for the help but I feel that comment was a little rude.
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    That is what my instructors asked of me. If the percentage error was more than 5%, then there was a problem. Even with the percentage errors of 5 or less we had to had an explanation in our lab reports for why we had an error.
     
  8. MrAl

    Well-Known Member

    Jun 17, 2014
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    490
    Hi,

    So you need to calculate the Q to within 5 percent then?

    What are you trying to calculate?

    If you take F/BW you get around 0.1 so that's low.
     
  9. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    5% or less is what I said, not OP.
     
  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Here's a reminder about resonant circuits
     
  11. MrAl

    Well-Known Member

    Jun 17, 2014
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    490
    Hi,

    So what exactly is he trying to calculate then, and to what accuracy?
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    The op has not asked a question yet. He just wanted a push in a direction....
     
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