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LC parallel circuit at resonant frequency

baristaskin

Dear All,

I am trying to simulate a simple LC parallel circuit when it is drived by a voltage source.

I'm expecting the current of the voltage source V1 to get smaller as its frequency gets closer to the resonant frequency.

When I simulate this circuit at the resonant frequency, with spectre default values, I get:

I tried to play around with the following parameters:

maxstep, reltol, vabstol, iabstol

without success.

My question is: how do I setup Spectre in order to get consistent and accurate results?

What I expect to see is a sine-shaped current signal with no DC value.

I'm including the netlist of the simulation shown above:

// Generated for: spectre
// Generated on: Mar 2 16:11:27 2015
// Design library name: paper3
// Design cell name: LC_osc
// Design view name: schematic
simulator lang=spectre
global 0
parameters _EXPR_8=1.986858915135295e-08 C=1p L=100n vdd=1 \
freqC=503.30696M cycles=10 L_IC=-sqrt(C/L)*vdd/2

// Library name: paper3
// Cell name: LC_osc
// View name: schematic
L1 (Vin 0) inductor l=L r=1a ic=0
V1 (Vin 0) vsource type=sine freq=freqC ampl=vdd/2 sinephase=90 sinedc=0
C1 (Vin 0) capacitor c=C ic=vdd/2
simulatorOptions options reltol=1e-3 vabstol=1e-6 iabstol=1e-12 temp=27 \
tnom=27 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 maxnotes=5 maxwarns=5 \
digits=5 cols=80 pivrel=1e-3 sensfile="../psf/sens.output" \
checklimitdest=psf
tran tran stop=_EXPR_8 errpreset=conservative write="spectre.ic" \
writefinal="spectre.fc" annotate=status maxiters=5
finalTimeOP info what=oppoint where=rawfile
modelParameter info what=models where=rawfile
element info what=inst where=rawfile
outputParameter info what=output where=rawfile
designParamVals info what=parameters where=rawfile
primitives info what=primitives where=rawfile
subckts info what=subckts where=rawfile
save Vin V1:p L1:1
saveOptions options save=allpub

 

Thank you in advance.

Parents

  • Dear baristaskin,

    Thank you for the added information. You've mentioned a few things that I may not fully understand, but I will try to comment. My apologies if I am not interpreting the issue correctly!

    1. You noted , "The Idea is to make this non linear system oscillate with the inductor, since RL can be fairly small.". There is no guarantee that placing an inductor in parallel with an arbitrary non-linear network with a capacitive impedance is going to produce an oscillation. If you also add a voltage source (as shown in your handwritten image), the voltage source acts as an infinite capacitance and probably defeats any study of the non-linear system.

    If you are trying to study the impedance of your non-linear system, I would recommend using a linear and non-linear (transient) analysis with only an ideal current source in parallel with your non-linear network - i.e., remove the inductor L and voltage source. With the current source, from a small signal point of view and transient point of view, set its amplitude to some value and sweep its frequency. Record the voltage across the current source in an AC or transient simulation and divide the voltage by the input current. The result will be the real and imaginary parts of the input impedance of your non-linear network. From this information, you can deduce whether it will oscillate with an inductor. You can also determine the magnitude of any possible oscillation by setting the current to a value that produces an impedance equal and opposite to that of the series resistance and inductor you place in parallel with it (in lieu of the current source).

    2. You also note "With such a system, and a required oscillation of 1V, ...". I'm not sure I understand your concern. Specifically, are you trying to force an oscillation amplitude of 1 V? If so, using the above mentioned technique, you can determine the oscillation amplitude, but its amplitude is set by the non-linearity of your network to produce an impedance equal and opposite to that of the impedance you place in parallel with it - if it oscillates under that condition.

    Shawn

Reply

  • Dear baristaskin,

    Thank you for the added information. You've mentioned a few things that I may not fully understand, but I will try to comment. My apologies if I am not interpreting the issue correctly!

    1. You noted , "The Idea is to make this non linear system oscillate with the inductor, since RL can be fairly small.". There is no guarantee that placing an inductor in parallel with an arbitrary non-linear network with a capacitive impedance is going to produce an oscillation. If you also add a voltage source (as shown in your handwritten image), the voltage source acts as an infinite capacitance and probably defeats any study of the non-linear system.

    If you are trying to study the impedance of your non-linear system, I would recommend using a linear and non-linear (transient) analysis with only an ideal current source in parallel with your non-linear network - i.e., remove the inductor L and voltage source. With the current source, from a small signal point of view and transient point of view, set its amplitude to some value and sweep its frequency. Record the voltage across the current source in an AC or transient simulation and divide the voltage by the input current. The result will be the real and imaginary parts of the input impedance of your non-linear network. From this information, you can deduce whether it will oscillate with an inductor. You can also determine the magnitude of any possible oscillation by setting the current to a value that produces an impedance equal and opposite to that of the series resistance and inductor you place in parallel with it (in lieu of the current source).

    2. You also note "With such a system, and a required oscillation of 1V, ...". I'm not sure I understand your concern. Specifically, are you trying to force an oscillation amplitude of 1 V? If so, using the above mentioned technique, you can determine the oscillation amplitude, but its amplitude is set by the non-linearity of your network to produce an impedance equal and opposite to that of the impedance you place in parallel with it - if it oscillates under that condition.

    Shawn

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