I came across this Short Video on Youtube by MicroType Engineering, link here.

His simulation on LTSpice was the below:

enter image description here

I tried the simulation on LTSpice myself. However I am curious as to what the voltage means. I entered the following values:

enter image description here

Does this mean the voltage source is a DC voltage of 12V with a 1V AC ripple on it?

enter image description here


1 Answer 1



Your top plot is in the frequency domain and is called a frequency Bode plot. Notice the axes. X is frequency in decades, and Y is output voltage ( somewhere on top of C1) relative to 1V in decibels. That's calculated as:-

$$ \text{dB} = 20 \log_{10} \big( \frac{V_{out}}{V_{in}} \big) $$

The 1V you entered into the small signal analysis box is the amplitude of a test signal applied across source V1. That's fed into the circuit, and the plot is the output frequency response. The 12V isn't used in a .ac analysis as the frequency of a DC voltage is 0 Hz.

By entering specifically 1V as the test signal, the decibel calculation makes for real world sense as Spice assumes that the dB loss is relative to 1V. It's kinda a convention for such analysis and typically works.

So as the frequency of the test signal increases, the amplitude of the output signal drops due to L1, R1 and C1 forming a low pass filter.

  • \$\begingroup\$ That makes sense, so that explains if I probe V1+, I get a band pass filter (V1+ sees a RLC) and if I probe C1 I get a low pass filter. \$\endgroup\$
    – JoeyB
    Apr 17, 2022 at 11:56
  • \$\begingroup\$ So the frequency of the AC signal goes from 150kHz to 1GHz, since this is the ac sweep window? \$\endgroup\$
    – JoeyB
    Apr 17, 2022 at 12:03
  • 1
    \$\begingroup\$ @JoeyB 150-1: Yes, that's it. However, notice the 1k. That means a 1000 data points per decade. I normally do 10. Pragmatically, can you spot all 1000 points between 10 MHz and 100 MHz? More than 10ish is semantically redundant. \$\endgroup\$
    – Paul Uszak
    Apr 17, 2022 at 12:11
  • 3
    \$\begingroup\$ One minor correction: "The 12V isn't used in a .ac analysis" -- for this case, it isn't, since the circuit is linear, overall. But if there were active devices, the DC voltage would have been used for calculating the operating point, so it's not a universal case of "no, it isn't", but rather an "it is, depending on the circuit". \$\endgroup\$ Apr 17, 2022 at 17:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.