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Spice Simulation of Parallel LRC Circuit

Discussion in 'CAD' started by Thomas D. Dean, Nov 11, 2003.

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  1. I am using spice3f5. I don't get what I expect when I simulate a parallel LRC
    circuit.

    * parallel RLC Circuit
    V1 1 0 0 ac 1
    V2 3 0 0
    R1 1 2 10
    L 2 3 10uh
    C 1 3 1000pf
    R2 1 3 100meg
    ..end

    run
    ac lin 10k 1meg 3meg
    plot v2#branch

    Does not display what I expect, v2#branch approaching zero at approx 1.6Mhz.
    What am I doing wrong?

    The series LRC circuit behaves as expected.

    * Series RLC Circuit
    V1 1 0 dc 0 ac 1
    V2 4 0 0
    R 1 2 10
    L 2 3 10uh
    C 3 4 1000pf
    ..end

    run
    ac lin 10k 1meg 3meg
    plot v2#branch

    v2#branch peaks at approx 1.6Mhz

    tomdean
     
  2. Sorry to answer my own post. v2#branch is a complex current. So, I should plot
    the magnitude.

    * parallel RLC Circuit
    V1 1 0 0 ac 1
    V2 3 0 0
    R1 1 2 10
    L 2 3 10uh
    C 1 3 1000pf
    R2 1 3 100meg
    ..end

    run
    ac lin 10k 1meg 3meg
    plot mag(v2#branch)
    plot phase(v2#branch)

    tomdean
     
  3. qrk

    qrk Guest

    Are you sure you want R1 = 10 Ohms? That gives you a bandwidth of 15.9
    MHz. Kind of silly for a 1.59 MHz center freq.

    Fun with filters: to calculate the bandwidth of a simple LRC filter:
    BW = 1/(2*pi*R*C)
    In real life, you need to figure in the effective parallel resistance
    across the inductor which is generally frequency dependent.

    Mark
     
  4. I've simulated your circuit in LT spice and it does appear to show a
    clear peak at 1.6Mhz. I'll post a screen shot of the result to a
    binary group if that'll be any help to you.
    Alt.binaries.schematics.electronic is the group we usually use for
    this purpose. Can you find that okay if I do?

    p.
     
  5. Hello Thomas,
    not the voltage v(2) has to be zero at the resonance frequency.
    It is the current i(V2) who is zero at this frequency if you
    wouldn't have any resistance in your circuit. Give it a try.

    The only purpose of the zero Volt source V2 is to measure current,
    because there is no other way to do it in standard SPICE.

    Best Regards
    Helmut
     
  6. Paul Burridge wrote...
    Thomas, I don't understand why you're "grounding" both
    ends of the resonant circuit. Keep in mind that voltage
    sources have zero impedance by definition, so you'll not
    be able to see energy flowing from the inductor to the
    capacitor and back, right?

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  7. Hello Mark,
    sorry your formula is wrong.

    BW = f0 / Q = f0 / ( sqrt(L/C) / R ) = f0 / 12 = 132kHz


    Best Regards
    Helmut
     
  8. qrk

    qrk Guest

    Sorry for taking so long to reply, but I was on holiday. My equation
    stands for a standard RLC bandpass. I assumed that the OP wanted a
    bandpass filter since his comment card stated "parallel RLC Circuit".
    Your equation is correct for a band-reject filter which he was working
    on previous to this circuit.

    Upon further examination of his deck, I don't know what he is after
    since the capacitor is directly across the AC source (i.e., he has the
    equivalent of an RL circuit).

    Aloha, Mark
     
  9. Hello Mark,
    have you seen my other posting?
    I think that Thomas has tried to plot the wrong value.
    If we plot I(V2), we will get a current peak for the series RLC
    circuit and we will get a notch for the parallel RLC circuit.
    So both cases show a resonance when looking for the current I(V2).

    As usual, we will never know what the correct question/answer is,
    because we have no feedback from the questioner.

    Best Regards,
    Helmut
     
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