Electronic – way to include a legend in LTSpice graphs for a sweep variable

circuit-designltspicesimulation

I am using LTspice parametric sweeps to generate a series of different cases and plot waveforms and measured values. I have two sweep variables and 5-6 points for each variable. Now I want to see how my measured quantities (using .meas) vary with respect to the sweep variables.

Currently, when I want to figure out which line corresponds to which sweep variable value, I am looking at the colour preferences order. This becomes a bit troublesome when I have many sweep points.

Is there a way to include a legend in LTSpice graphs for a sweep variable?
Of course, I can export the data and use some other tool to do all the further analysis. But here I am wondering if there is a way to make it easier with LTspice itself. (for waveforms, the solution provided by @aconcernedcitizen is very useful to group the results and display separately. But for measured quantities it does not work.)

Best Answer

Took me a while to figure out but yes, you can do this!

Right click the plot window. Select View > Step Legend.

View > Select Steps will let you change which plots are shown.

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Electronic – LTspice MOSFET width sweep

LTspice uses the .step command to accomplish up to 3 nested sweep, to do two nested sweeps add directives like this:

.step param width 1u 10u 1u      
.step param length 1u 10u 1u

LTspice measure for rise time and fall time:

 .MEAS TRAN tr1 WHEN V(out)=V(vdd)*0.1 RISE=1
 .MEAS TRAN tr2 WHEN V(out)=V(vdd)*0.9 RISE=1
 .MEAS TRAN tr PARAM (tr2-tr1)
 .MEAS TRAN tf1 WHEN V(out)=V(vdd)*0.9 FALL=1
 .MEAS TRAN tf2 WHEN V(out)=V(vdd)*0.1 FALL=1
 .MEAS TRAN tf PARAM (tf2-tf1)

You will find the output of the measure commands in the log (Ctrl-L).

If you are drawing the transistors you may need to wrap the parameters in {}-brackets.

If you write the netlist yourself you need a .param directive, e.g.:

 .param width=1u length=1u
 Mnmos1 D G S B model w={width} l={length}

Below is a working example:

example of two parameter sweep in LTspice

After running it hit Ctrl-L or use the menu item 'spice error log' to see the measure results. For simpler measure setups you cant plot the results by right clicking in the 'spice error log' window, but it didn't work in this case so you need to process it in another program.

Electronic – Solved: How to Sweep NMOS4 Device Parameters in LTspice IV

The syntax of the step Spice directive is correct, however I misinterpreted the use of the period character.

I have changed the directive to read ".step param w list 20u 80u 320u" and got the intended results.

The problem was that with the period the values were being read as 200n, 800n, and 320n respectively. This is smaller than the model I was using allowed for which resulted in the error.

Best Answer

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Electronic – LTspice MOSFET width sweep

Electronic – Solved: How to Sweep NMOS4 Device Parameters in LTspice IV

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