Electronic – Why does the gain of a RC filter vary with input amplitude in LTspice

With the photos, I'd venture that you have stray capacitances all over your circuit because it is on a solderless breadboard. You shouldn't be doing high frequency work like this on one of those. Of course, this doesn't rule out parasitic effects addressed in the comments to your question, but there is a gorilla in the room, and it's the breadboard.

In a circuit like yours the frequency of any voltage or current will be that of the source signal, hence there is no need to retrieve it from the simulation result. If your situation is much more complex, e.g. you want to measure the frequency of some resonant ringing or the like, you will have to do this graphically from the output plot of a transient simulation.

Now to your question: If you want to vary several parameters and execute a simulation with every possible combination of these you need to make a parametric simulation. To do so, add the relevant parameters with arbitrary values to your netlist. You can do this by adding the following SPICE directive:

.PARAM + pAmplitude = 20 + pFrequency = 60

Also add the following directive to make you transient simulation a parametric one:

.STEP PARAM pAmplitude 20 200 10 Change pAmplitude from 20 to 200 in steps of 10

.STEP PARAM pFrequency 60 300 10 Change pFrequency from 60 to 300 in steps of 10

To include the parameters into your schematic simply replace the amplitude value of V1 by {pAmplitude} and the frequency by {pFrequency}.