# 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
307
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.

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
16,532
4,453
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
11,137
2,175
I was wondering that myself, but you beat me to it.

4. ### ninjaman Thread Starter Member

May 18, 2013
307
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
3,971
616
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
307
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
3,971
616
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 Distinguished Member

Jun 17, 2014
3,741
791
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
3,971
616
5% or less is what I said, not OP.

10. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,674
1,536
Here's a reminder about resonant circuits

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11. ### MrAl Distinguished Member

Jun 17, 2014
3,741
791
Hi,

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

12. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,674
1,536
The op has not asked a question yet. He just wanted a push in a direction....