pspice analysis

Discussion in 'General Electronics Chat' started by sandhyapt, May 19, 2013.

  1. sandhyapt

    Thread Starter New Member

    May 19, 2013
    I am using LTspice to view the graph of a circuit. i see transient analyis and frequencey analysis options using .TRAN and .AC commands.

    In my circuit i am changing the capacitance using .STEP command. i want to see the voltage variations of a node based on the varying capacitance. so, i need to plot capacitance on X-axis and voltage on Y-axis.

    However, i do not see any option in PSPICE code where i can plot anything other than time or frequency.

    If anyone knows how to do this in PSPICE or any tool that has this capability please let me know.

  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    What program exactly do you use? LTspice?
    Anyway transient analysis is like looking at an oscilloscope - the x axis is time and y is the value measured. Stepping will produce mutliple waveforms overlapped, each one for different value.
  3. sandhyapt

    Thread Starter New Member

    May 19, 2013
    Hello Kubeek,
    I'm using LTSpice. I could see the graph for transient analysis and change the capacitance using .STEP. But instead of seeing the time on x axis, I want to see the capacitance. I saw a few charts like that in some papers but not sure what tool did the author use to get such charts. Please let me know if anyone has any idea.
  4. tindel

    Active Member

    Sep 16, 2012
    It doesn't make sense to me why you'd want to have a transient analysis plot capacitance on the x axis. Your capacitance will be constant for each sweep - I'm not sure you can do that in LT-spice... I'd suggest a AC sweep of C then plot the parameters you're interested in versus C. [Edit: it won't let me do that]

    When you do a .step command you should have several traces based on the capacitance of your .step command, so the analysis should run several times. When you do this you can find out what capacitance value is best in the time domain by pressing [Shift+left mouse button] on the trace you'd like to see - this will bring up the cursor, then place the cursor where you'd like to compare data. Then press the [up] and [down] arrows to select a trace that you like. Then [left click] on the cursor to determine which value of your capacitor gave you that curve.

    Hope this helps.
    Last edited: May 20, 2013
  5. WBahn


    Mar 31, 2012
    You can (in general using pSPICE, which I'm pretty sure LTSPICE is pretty compatible) generate plots with other variables on the horizontal axis, but what you need to do (IIRC, it's been many years since I've done it) is use something to capture a single value from each simulation, such as the average voltage over the simulation or the voltage at a particular time, and plot that versus the stepped parameter. It's a bit tricky to get set up, but once you figure it out it is not too bad and very useful.
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Sandhyapt, post the .asc file of the circuit you are simulating. Maybe we can help you if we can see what you are trying to do.