I want to calculate harmonic currents of the following circuit in LTSpice. Circuit operates with 100MHz square wave with 1ns rising and 1ns falling edge and 5V:


In FFT view, LTSPice shows me that I1eff=3.254mA, I3eff=304µA:


I calculate first the impedance of the circuit: Z1=sqrt(R2^2+(2*pi*f1*L)^2); (R2=100, f1=100e6, L=10e-6), then I1=U/Z, Ieff1=I1/sqrt(2).

Similarly the impedance for 300MHz: Z3=sqrt(R2^2+(2*pi*f3*L)^2); (R2=100, f3=300e6, L=10e-6), then I3=U/Z, Ieff3=I3/sqrt(2).

But I get Ieff1=5.55mA, Ieff3=1.87mA.

How can I calculate these values manually in a correct way?

  • 2
    \$\begingroup\$ Why don't you start with a source that is bipolar (not unipolar) so that there is no DC content. Then do an FFT on the source's voltage waveform and try and figure out what the source harmonic levels are at 100 MHz and 300 MHz. In other words, debug your thoughts by going simpler and checking things properly step by step. I can see issues with what you have done but, it's easier for you to prove this to yourself first. For instance, if you had a square wave of +1/-1 amplitude, what is the fundamental frequency amplitude you are currently assuming? \$\endgroup\$
    – Andy aka
    Jan 19 at 10:47
  • 1
    \$\begingroup\$ Don't forget that "harmonic" means... "steady state" waveforms ... \$\endgroup\$
    – Antonio51
    Jan 19 at 13:02


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