Intro: Measuring Impedance Using LTspice
Hey everyone this is going to be a simple introduction to generating an AC sweep of a circuit and finding the impedance at any given point, this came up several times in my courses and it was very difficult for me to find any way to do it online so hopefully this will help everyone especially those people (like me) trying to find answers at 3am.
Step 1: How to Set Up the Simulation
To do this its very simple the first step is to make your desired circuit (I will do another instructable on how to actually do this) but leave the voltage source blank.
The next step is to right click on the voltage source and select advanced you will see several options and on the right there will be small signal AC analysis, you can set that to anything however I do 1v at 0 degrees.
Then you will want to set up the sweep since this is an ac analysis you will do AC Analysis as the sweep type then select decade and use 101 points per decade you can change this to match your needs but I have yet to experience a problem with this method, and then set the desired frequency range.
Finally you will want to label the input node as you can see it says V1 in the schematic above the voltage source, of course this can be at whatever point you are measuring the impedance at.
Step 2: Results of the First Simulation
After you simulate and plot the results you will notice they dont look very conducive to take them and find the impedance, the plots images here are the voltage and current at the battery of course you can select them anywhere in the circuit and you will find a variety of results.
Step 3: Converting From Current and Voltage to Impedance
Just a defining relationship impedance Z=V/I (phasors) so to get that plotted its rather simple you will want to right click on the label of the voltage plot it should be V(v1) or whatever node was used and in the window that pops up you will simply change it from having V(v1) to V(v1)/I(V1) then hit ok. When you change this area you can make a more complex expression like (V(v1)-V(v2))/(I(v1)-I(v3)) ... the goal is to just vet V/I.
This will change the graph but the units are still going to be in decibels so you will want to right click on the Y axis and change it to linear then hit ok and the units will now be in Ohms .
Step 4: Reading the Results
After you have converted to impedance it still may be hard to read the graph and a simple fix is to right click on the label of the graph and select a cursor under Attached Cursor I used 1 and 2 so I could measure at multiple points as you can see in the image with a results window showing up.
Thanks for reading feel free to ask any questions and if this goes well I will try to create more like it. :D