I am trying out the following parallel RLC circuit configuration: enter image description here

On trying out other similar parallel circuits (like with an RL coil in parallel with C or with RC and RL branches in parallel) I got the frequency curves I had expected and seen in textbooks. However with this one, I get the following response:

enter image description here

which appears to be pretty erratic. There is a "dip" at the resonant frequency (of around 1000 rad/s or 160 Hz) but I dont understand why the curve leading upto it and after it is so jagged?

enter image description here

When I step the resistances, this gets even more erratic. Is this normal behavior for this circuit. There was nothing in my text book to suggest this kind of response. I expected something like this:

enter image description here

  • \$\begingroup\$ It happens; I missed it too at first glance. Andy's on the ball though. \$\endgroup\$
    – rdtsc
    Jan 24 at 18:53
  • 2
    \$\begingroup\$ "-300dB" on the vertical scale is a pretty strong hint you are looking at rounding noise from a calculation that really should produce 0. ANd rounding noise tells you nothing useful about analog circuit behaviour. \$\endgroup\$ Jan 24 at 22:42

1 Answer 1


This is a likely candidate for show-stopper of the month: -

enter image description here

Then, when you fix that you'll realize that a voltage source (V1) is ideal and won't care a damn about C1 and L1 being present i.e. you'll just plot the spectrum of I(R1) as a straight horizontal line.

  • \$\begingroup\$ That was easy. Thanks, I'll certainly be more careful in the future about that. Actually, I was more interested in the current through the source (IR + IC + IL) and the plot for that seems to be inline with what I expected. Lastly should I take this question down or keep it up ? \$\endgroup\$
    – First User
    Jan 24 at 18:28
  • 1
    \$\begingroup\$ @FirstUser given that I've taken the time (albeit only 5 minutes) to answer and, that this site runs on upvotes and answer-acceptance (and a lot of goodwill), I'd be livid if you took it down!! Oh, I see you've done the right thing now. \$\endgroup\$
    – Andy aka
    Jan 24 at 18:29

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