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Finding the bandwidth of a parallel RLC circuit (+MATLAB)

  1. Oct 1, 2018 #1
    The problem statement, all variables and given/known data
    The attempt at a solution
    Constructing the total impedance of the circuit as follows,
    where $Z_R=R$, $Z_C=-j\frac{1}{\omega C}$ and $Z_L=j\omega L$.
    $$\frac{1}{Z_T}=\frac{1}{R}+j\omega C+\frac{1}{j\omega L}$$
    solving for $Z_T$ gives us,
    $$Z_T=\frac{Rj\omega L}{j\omega L-RCL\omega^2+R}$$
    where $\omega=2\pi f$
    $$Z_T=\frac{2\pi fRLj }{2\pi fLj-RCL(2\pi f)^2+R}$$
    Plotting this function over the frequencies, whilst $L=10\ \mu H$, $C=5\ pF$ and $R=10^6\ \Omega

    From here, I created the following MATLAB code to find:
    - The magnitude of the impedance (Z_Mag) for a range of frequencies.
    - Finding the maximum of Z_mag and it's corresponding frequency.
    - finding the -3db points and the frequencies that correspond to these points.
    - taking the difference of said frequencies in order to calculate the bandwidth.
    Code (Text):

    clear all
    close all

    N=100000; %number of frequency samples
    L=10*10^(-6); %inductance
    C=5*10^(-12); %capacitance
    R=10^6; %resistance
    f=linspace(1,10^8,N); %frequency of 1 Hz to 100 MHz

    Z_T=(2*pi*f*R*L*1i)./(2*pi*f*L*1i-R*C*L*(2*pi*f).^2+R); %impedance
    Z_mag=abs(Z_T); %magnitude of the complex impedance

    plot(f,Z_mag); %plotting the frequency against the total impedance
    title('Total impedance per frequency')
    ylabel('total impedance')

    [max_Z, max_index]=max(Z_mag); %maximum value of impedance
    threedb=max_Z*sqrt(2)/2; %the 3db point

    [Z_db,index_db] = min(abs(Z_mag-threedb)); %closest value and index to the 3db point
    f1=index_db*df; %the first frequency at the 3db point
    %the second frequency at the 3db point (first frequency mirrored around the
    %frequency at max magnitude

    BW=abs(f2-f1); %the bandwidth

    Now when I vary the N (number of frequency samples), the bandwidth I find seems to change quite drastically. Ideally, I would expect the bandwidth to asymptotically approach some value with a greater accuracy for a greater N.

    I have been trying to found out what I did wrong for some time now, does anyone know where I went wrong? Thanks!

    Attached Files:

    Last edited: Oct 1, 2018
  2. jcsd
  3. Oct 1, 2018 #2
    I found an error in my program, which solved the problem.
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