Electronic – dB scale in LTspice

decibelltspicescaling

I have a question about the dB scale in Ltspice. As far as I know it uses 1 of the unit (so 1V in case of Voltage) as a reference and then plots the dB value, which is defined by 20*log(V/Vref). (So the lower image is the voltage of my circuit with the y axis property set to dB.)

But when I plot this exact formula in LTspice the plot is actually mirrored at the x-Axis. (Upper plot) Why is that? I think it should be the same. What is my mistake?

Hope you can help!

enter image description here

Edit: I plottet abs(V(n009)) since the phase is not important for my application and just gives a messy plot. I also have the same problem with any schematic, so I guess the problem has nothing to do with the circuit.

For clarification: easy schematic, same problem (this time it's the current).

enter image description here

Best Answer

It's not an LTspice problem, it's the way the waveforms are displayed: magnitude, instead of amplitude. If you plot sin(w), you won't see a sin(x) waveform, but an abs(sin(x)). The difference will be in the phase: in your plots, the phase appears centered around 180 degrees for the formula, while the "normal" plot shows it's centered around 0. That's why exporting the waveform will make them "look" fine -- it's a matter of appearance.

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