# Matlab plotting

Discussion in 'Homework Help' started by Niel Hong, Aug 13, 2013.

1. ### Niel Hong Thread Starter New Member

Jul 24, 2013
3
0
Hi guys, I have a problem here. Need some experts to guide me.
I have a parallel resonant circuit with variable capacitance, and need to get it magnitude response in Matlab.
At fr=198kHz, C max=2064.5pF ; fr=304kHz, C min=2062.2pF
R=11 ohms, Q=35, L=0.25uF
I've tried to get the magnitude by varying the frequency from 160kHz to 400kHz, but all I got was just dots, nothing was being shown.
I know the Current is extremely small, is there any possible way to have the magnitude of I against frequency by using Matlab?

Last edited: Aug 13, 2013
2. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
You might want to show your work on how you calculated the fr

3. ### Niel Hong Thread Starter New Member

Jul 24, 2013
3
0
Hi Joe,

Question: A variable Capacitor needed to adjust the designated resonant frequency over the band by giving fr1=198kHz and fr2=304kHz with Q=35 at one end of the band, and Q≤35 throughout the band. Let R=11Ω, and specify the values for L, Cmin, Cmax. Using matlab to draw the magnitude response of the resonant circuit for both values of C.

My calculation for L, Cmin, Cmax:
Q=R/XL
XL=R/Q
L=R/2*pi*fr*Q
L=0.25uH

From formula, fr=(1/2pi)*[√(1/LC)-(R^2/L^2)]
I got,
C=1/L [ (2*pi*fr)^2 + (R^2/L^2) ]
then,
substitute fr1,
Cmax=2064.5pF

substitute fr2,
Cmin=2062.2pF

These are the calculations that I've done. I've tried using MultiSim, AC analysis to get the current curve. But with the capacitance that I assigned, the resonant current isn't at between Cmax and Cmin. For Matlab part, after keying and run the .m file, it shows nothing in the figure.

Did I make some mistakes?

Last edited: Aug 14, 2013
4. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
Did you cross check your answers by using the correct formula for fr?

fr = 1 / (2*pi*(LC)^.5)

It would have been nice to have all the information you were provided to start this problem. It sounds like a homework problem to me.

Last edited: Aug 14, 2013
5. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
You need to be "realistic".

When you figure out components you will need, then go to mouser.com or digitkey.com and select the actual one's you would use if you built the circuit.

Then begin the process of analyzing that circuit to be within your objective.

6. ### Niel Hong Thread Starter New Member

Jul 24, 2013
3
0
I think I get it.
Thanks Joe. I'll try it out.