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LCR Measurements

Discussion in 'Electronic Design' started by [email protected], Aug 20, 2007.

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  1. Guest


    I have a question that I hope somebody can help me with. I have a LCR
    meter and I am sweeping some components. I am recording |Z| and the
    phase angle. How can I calculate the R, L, and C at each of the
    different frequencies I have recorded.

  2. Joel Kolstad

    Joel Kolstad Guest

    Mmm... are you sure this isn't a homework question?

    Take the measurement you have (magnitude and angle) and express it in
    rectangular coordinates, Z=R+jX. R is the resistance, and then knowing that
    X=2*pi*L or -1/(2*pi*C), you can solve for C. (If X is positive, you have an
    inductor, otherwise it's a capacitor, of course.)
  3. Guest

    No.... it's not a homework question. jX will indicate if the circuit
    is capacitive or inductive. However, I have a series RLC circuit and
    would like to calculate the R,L, and C from the lcr data. The part I'm
    having a problem with is extracting the L and C values individually.
  4. Joel Kolstad

    Joel Kolstad Guest

    Ah, OK. This is impossible to do if you measure the impedance at only one
    frequency (since you have three unknowns -- R, L, and C -- but only two
    equations, Re(Z) and Im(Z) ). What you do instead is:

    1) Measure the impedance at two (or more) frequencies, set up a system of
    equations and solve (use, e.g., least-squares fitting if you use more than two
    equations). Most LCR meters will let you choose at least two different test
    2) (The way people usually do this...) Use an adjustable frequency generator,
    connect your circuit to its output, and sweep the frequency until you maximize
    the current through the circuit (this corresponds to the minimum |Z|). The
    idea here is that a current maximum is reached at resonance, at which point
    f*L=1/(f*C), and now that you have enough information to solve for all the
    3) (Seemed to be a popular lab exercise in school...) Similar to #2, you find
    the 3dB points of the impedances response as well as the resonant frequency,
    then you compute Q, and since you can reasure the resistance directly from Q
    and R you can compute L or C from Q~=2pi*fL/R or Q~=1/(2pi*fC*R). I believe
    the idea is that this approach tends to be a little more accurate than (2)
    since by measuring both 3dB points you're doing a bit of averaging and are
    somewhat out from resonances where, if you have a high-Q circuit, measurement
    accuracy is often compromised.

    If you're lucky enough to have access to a network analyzer, you just tell it
    to measure S_{11} over some frequency range and it'll then find the minimum
    |Z| for you and read out the R, L, and C directly at that point. :) The
    network analyzer approach is also useful to give you some idea of how accurate
    a simple series RLC model is for your particular circuit.

  5. Guest

    Thanks for your help!
  6. Guest

    I am not expert
    but if measures the answer to an impulse
    you do not succeed to go back to the transfer function
    and to calculate poles and zero then
    and knowing the structure of the net
    to go back to the values?
    aspect answers from who is more expert than me
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