How do I use .MEAS to get the risetime of a pulse? I seem to always fight LTspice for everything but the simplest stuff.

Here is my circuit:

 

I just measure it from the expanded plot (left-click and draw a rectangle around the rise portion of the signal).

 

I just measure it from the expanded plot (left-click and draw a rectangle around the rise portion of the signal).

Yes, this works but is a real pain if I am making changes to the circuit to optimize the risetime.

And... I would like to know how to do the .meas operation for future use.

 

Yes, this works but is a real pain if I am making changes to the circuit to optimize the risetime.

And... I would like to know how to do the .meas operation for future use.

Hi

That could get difficult because the waveform is “ringing”.

But as an example..
Suppose the output voltage is s pulse of 0-10 volts and you want 10% and 90% values.
You could do something like this:

Rise time:
.meas tr1 time TRIG V(out)=1 TD=0.2u RISE=1 TARG V(out)=9 TD=0.2u RISE=1

Fall time:
.meas tf1 time TRIG V(out)=9 TD=0.2u FALL=1 TARG V(out)=1 TD=0.2u FALL=1

TD specifies the measurement delay time.
In the example, It means “wait 0.2u seconds before measuring”

Hope that helps...

eT

 

Hope that helps...

eT

Thanks, it helps a lot. That gives me a good place to start. Some more thoughts and questions though.

The ringing (and also overshoot) is a problem. I can't decide whether I should measure to the "flat" part of the waveform or to the peak of the waveform. That's an issue I am willing to ignore for now. (I'm opting for the peak for now).

My amplitude varies when I make changes. This makes the fixed voltages at the 10% and 90% values a problem. Can you think of a way to do the measurement without using the fixed voltages. Maybe measure the minimum and maximum voltages and calculate the voltages at the 10% and 90% points?


I will try your examples and see what other questions I have.

 

Maybe measure the minimum and maximum voltages and calculate the voltages at the 10% and 90% points?

That’s a better way for this type of waveform.

There are a many .measure switches and options to play with...try the help info and experiment.

eT

 

See

 

See
View attachment 158066

Wow! That is a lot to understand. Thanks a lot.

I will take me some time to digest all of it.

 

Wow! That is a lot to understand. Thanks a lot.

I will take me some time to digest all of it.

PSpice/Probe search commands are much easier...

 

See
View attachment 158066

this is amazing. thanks for sharing!

 

PSpice/Probe search commands are much easier...

hi can you show some examples about that..? thanks!