ADI LTspice LISN Article "How to Get the Best Results Using LTspice for EMC Simulation—Part 1"

Hello,

The ADI article link I am wanting help with can be found here: https://www.analog.com/en/resources/technical-articles/how-to-get-the-best-results-using-ltspice-part-1.html

I've also attached the buck converter LTspice schematic below.

1121.Buck_Converter_LISN.asc

What I am wanting to learn more about is:

1. How to implement LISN within LTspice 
2. Steps for designing an input EMI filter for my simple buck converter

I like the idea of using a LISN within LTspice and implementing limit lines to see conducted EMI assuming you model your system correctly.

I follow the article well up until figure 7, where the figure shows the input voltage source now pulsing with some format into the LISN, where the LISN also connects to the input of the LT8618.

Regarding my point number 1, I don't understand why the input source to the LISN changed? Can you help me set up the LISN within my attached circuit?

To my point number 2, my understand of designing an input filter is I first need to know the amplitude of the harmonic or FFT plot at the switching frequency, then design the filter based on the needed attenuation. I'm not sure where to start though for getting that amplitude within LTspice, can you help me along with this process?