How to calculate input impedance in LTSpice

 This post is a follow on to my previous post on output impedance. This time it is for calculating input impedance. Suppose we have a black box circuit that we will be sending a signal to. The black box could be Rx input pin of an RF transceiver IC, for example. 

We want to calculate Zin, the input impedance of the black box. In the case of input impedance, we'll add a 1V AC signal at the frequency of interest (915 MHz in this example), and measure the complex current that flows.

Step 1 is to add the AC source and set up the simulation parameters:

To make the example concrete, we'll pretend the black box consists of an inductor and resistor.

Step 2 is run the simlation. It should output something like:

Now we convert I(V1) from mag/phase into a complex number using this formula:

Plugging in magnitude of 0.0189064 and phase of 160.969 we get 0.01787 + 0.006165j.  

[ hand-waving ] I think because we're measuring current through the voltage source, we have to take the complex conjugate, so 0.01787 - 0.006165j.  Though I'm not totally sure that's right. [ /hand-waving ]

Now we know the voltage and current, so using the complex form of V=IR, we can solve for the impedance:

As we can see, the 50 matches the resistance, and we can convert the 17.25j, since the only reactive component is the inductor, using this equation:

Which matches as well. Voila!