This happened to me in xschem. The ngspice is different to the wave plot from xschem. The neagtive part of the graph is absolute. This might be a bug

It's not a bug, but a different way of displaying data. XSchem uses polar representation (magnitude and phase) for AC simulations (while ngspice stores vectors as complex real/imaginary data). For any variable , like

pssrmag

there are two vectors: • `pssrmag`: the magnitude (always >=0) •

ph(pssrmag)

the phase in deg. if you plot the phase of pssrmag

ph(pssrmag)

it will probably go to 180 deg. If you want the same plot in xschem just use this expression in the xschem graph (including double quotes):

"pssrmag; vref db20() -1 *"

this means: • Use

pssrmag

label • take

vref

magnitude • calculate

dB

value • multiply by

-1

Get in touch again if above considerations do not apply in your case.