Electronic – LTspice: Vary a resistor’s value over time

Well, yes and no. From the ltspice help:

There is also a general nonlinear capacitor available. Instead of specifying the capacitance, one writes an expression for the charge.

LTspice will compile this expression and symbolically differentiate it with respect to all the variables, finding the partial derivative's that correspond to capacitances.

Syntax: Cnnn n1 n2 Q=<expression> [ic=<value>] [m=<value>]

There is a special variable, x, that means the voltage across the device. Therefore, a 100pF constant capacitance can be written as

Cnnn n1 n2 Q=100p*x

A capacitance with an abrupt change from 100p to 300p at zero volts can be written as

Cnnn n1 n2 Q=x*if(x<0,100p,300p)

This requires a bit of different thinking, and although you can make use of the special time variable, I have never verified that things work as expected. Especially intresting would be the point where the capacity will suddenly be lower than what the cap is currently holding in energy, what will ltspice do here? Or if when for the next timestep the capacity is now higher, will the stored energy be the same or scaled too?

You should test if it behaves the way you want it in a seperate test circuit first.

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}.