# Calculate the Break Frequency

Discussion in 'Homework Help' started by millotorres, May 19, 2012.

1. ### millotorres Thread Starter New Member

May 18, 2012
5
0
I need help calculating the break frequency of an RC and an RL circuit when given only the resistor of 1000 ohms value and the magnitude and phase plots.

I've been trying to solve this without luck. Please guide me here of the steps I should take.

RL - Red Plot
RC - Blue Plot

Thanks

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,773
1,340
When the gain Vout/Vin reach 1/√2 ≈ 0.707 ≈ -3.01dB
The Xc = R and also XL = R

3. ### millotorres Thread Starter New Member

May 18, 2012
5
0
Ok I still don't get it, can you show me how I get fc. I cannot tie the knot on this one. I have tao = RC, fc=1/2piRC. I have two unknowns. Show me the light here someone!

4. ### MrChips Moderator

Oct 2, 2009
18,337
5,795
When Xc = R.

i.e. when R = 1/ωC

5. ### crutschow Expert

Mar 14, 2008
21,701
6,253
The light is that you have to supply two of the values to calculate the third. For example, if you want a certain fc, then you need to also specify a value for either R or C, and solve for the missing value.

6. ### millotorres Thread Starter New Member

May 18, 2012
5
0
Ok, This is the puzzle, I must get fc by looking at the graphs and the frequency i.e. 4.6 is told not to be = fc. I only was given what I posted..
A magnitude and a phase graph of the circuit and the resistor value of 1000 ohms. So can you solve this one?? fc = ...

7. ### MrChips Moderator

Oct 2, 2009
18,337
5,795
The formula for Xc is:

$X_c = \frac{1}{2\pi fC}$

We solve for f when R = Xc

$f = \frac{1}{2\pi RC}$

Last edited: May 20, 2012
8. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,773
1,340
Use the plot to find frequency when gain drop to -3.01dB or 45 degree of a phase shift.

9. ### millotorres Thread Starter New Member

May 18, 2012
5
0
Please no more theory, fc = ? someone???

10. ### MrChips Moderator

Oct 2, 2009
18,337
5,795
Geez... did you not see my posts #4 and #7? Also post #2.

We keep giving you the answer.

11. ### millotorres Thread Starter New Member

May 18, 2012
5
0
I'm getting ~50000 rad/sec but still it is marked as incorrect

12. ### MrChips Moderator

Oct 2, 2009
18,337
5,795
What is the answer you are expecting?