I have analyzed the filter in question both in practice and in simulation (with LTspice)

rlc filter

The calculations I performed to determine the high and low cutoff frequency and the resonance frequency are as follows:

  • fres = 1/2pi * sqrt(LC)
  • fch = R/2pi * L
  • fcl = 1/2pi * RC

From these, I obtained:

  • fres = ~10kHz
  • fch = ~238kHz
  • fcl = ~106Hz

The results I obtained coincide with those recorded during the practical test, but not in the simulated one.

Below the trend measured at the node between the inductor and the resistor:

enter image description here

Now I'm wondering why it behaves this way. Also, since this is a passive filter, shouldn't the maximum gain be 0 dB?

  • \$\begingroup\$ The answer below is the correct one, but I have to wonder: why did you choose AC 5 in the first place? I'm interested in the reason. \$\endgroup\$ Dec 23, 2021 at 10:16
  • \$\begingroup\$ the value has been chosen for no particular reason. \$\endgroup\$
    – lasb3tas
    Dec 23, 2021 at 17:57
  • \$\begingroup\$ In your honest opinion, do you think that is a healthy attitude when searching for results? Don't forget about the GIGO. As it happens, this wasn't the case, but it did cause you to misinterpret the results. \$\endgroup\$ Dec 23, 2021 at 20:14
  • 1
    \$\begingroup\$ I asked for this very reason, unfortunately I did not know that ltspice measured in dbV and I am new to the subject. However I understand the concept and I thank you for the advice, maybe if the situation had been slightly different I would have undoubtedly made a mistake by misinterpreting the results. \$\endgroup\$
    – lasb3tas
    Dec 23, 2021 at 21:17

1 Answer 1


The reason the "gain" is above zero is that it's not gain, it's voltage expressed in dBV and you're putting 5 VAC into the circuit. Change your source AC amplitude to 1 and try it.

  • \$\begingroup\$ Thank you so much for your reply. Very helpful, but what is the relationship between db and dbV? How does the conversion take place? Thank you very much. \$\endgroup\$
    – lasb3tas
    Dec 21, 2021 at 21:16
  • 1
    \$\begingroup\$ Decibels are a way of expressing signals using logarithms so that you can add and subtract them easily. For dBV the formula is 20 * log10(volts) and 1 volt = 0dBV. In LTSpice you can right-click on the dB scale on the left and change it to linear to see it in volts. Usually if you're doing an AC analysis you want to use 1 VAC for your source so everything is referenced to 0dB. You should also notice that your simulated results should now match your calculated results. \$\endgroup\$
    – GodJihyo
    Dec 21, 2021 at 21:28

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