# ltspice, plot Vpp

Maybe this is extremely trivial.

I have a transient analysis over a buck regulator and can't figure out how to plot Vpp. I just want to sweep some capacitors and see the ripple.

In PSpice you can add expression directly to plot (like V*sqrt(2)), I can't figure out how to do this in LtSpice. If I click the name of the plot, I can enter some expression but always gave me error such as "undefined symbol" Vpp(noo4)

I also added a .measure directive in schematic but nothing happens to the plot

.meas TRAN x PP v(n004)


What I miss? Thank you.

My buck's output

• Look in the log! That's where the result appears. – winny Feb 15 '17 at 10:33

## 2 Answers

vpp measurement in ltspice is a single value over the whole plot (or a section thereof, check .meas parameters). Look in the error log (weird place, I know)

For sweeping multiple values of a single component, use the .step param facility, and then you can right click in the error log and plot the .meas values vs. component values.

• Not sure if this is ok! the total Vpp will include ramping from zero, isn't it? please see attached picture, this is actual buck output and I need to plot Vpp in certain region (skip startup) to see what is it. A real scope on my bench will compute this on fly over zoomed region only... – orfruit Feb 15 '17 at 11:29
• @user1734108: the .meas command includes x axis ranges for the measurement, just let it start far enough after the ramp up phase – PlasmaHH Feb 15 '17 at 11:44

Your .meas command will put results in the error log, found in View-->SPICE Error Log. Alternatively you can use the keyboard shortcut 'Ctrl+L'.