# How do you interpret LTSpice graphs?

I've created a circuit in LTSpice to model a fan assembly I'm looking to build. In the below circuit the current sources represent the fan and the voltage sources represent the voltage source I'll be connecting to the fan. Filtering occurs between the two. I'm attempting to prevent the noise from the fans getting on the voltage lines.

Below is the graph for the voltage between the two inductors, L1 and L3. There are multiple lines because of the step function on C3.

According to the graph, between 1Hz and 100Hz the voltage is -40dB. If dB is referenced to 1V this would mean at low frequencies the voltage between L1 and L3 is 10^-2V. Shouldn't it be 12V? Is there a problem in how I am interpreting the graph or am I not understanding how my circuit works?

If the AC analysis does not take into account the +12V DC, what exactly is the AC analysis doing? Is it replacing the the +12V source with a sinusoidal waveform? I also have a sinusoidal currant draw from AC1. Is this varied at all with the AC analysis?

• 12v don't exist in the AC analysis..They are only used to get the operation point..Hover any schematic wire after doing the AC analysis and the DC operation point (i.e. dc voltage, current, power) will be displayed at the bottom. Commented Jan 7, 2022 at 19:18
• @tobalt after running the analysis hovering over schematic nets only says "Click to plot V(N###)" Commented Jan 7, 2022 at 19:21
• Yeah I noticed it too. Hmm strange. I am sure, that it is usually displayed there. But I don't know right now, how to get it back there. Commented Jan 7, 2022 at 19:28
• The .AC analysis first calculates the operating point for the whole linearized circuit. Any source that is not DC, such as SIN() or PULSE() will be replaced with their DC value at time=0 (the first value): I1 will be 1 A, I2 will be 0 A, I3 will be 0.2 A. That means any DC voltages may influence the behaviour of the circuit if there are active elements. Since you only have passive elements, any DC will be useless (plus C1 is completely useless across a voltage source, unless there is some series resistance or you use the current through it). Commented Jan 7, 2022 at 20:41
• @aconcernedcitizen ok then after everything is linearized what happens? Which of the 6 sources would be used as the source of the AC sweep? Commented Jan 7, 2022 at 21:42