Electronic – Difference between .MACRO and .SUBCKT in SPICE

I know of two methods for impedance measurement. The first deals with DC impedance, and the second deals with AC impedance. They work regardless of whether you are looking at the input or output.

1. Measuring input impedance: Apply a DC voltage source to the input of your circuit. If any output loading, power supplies, or additional biasing is required, include this in the schematic as well. Sweep the input DC voltage source over the voltage range you expect to use the circuit. Use the equation below to give you the input resistance. Note that it is very possible to not have a flat curve.

   R = 1/(Iin dI/dV)
   

   Explained, this is the inverse of the derivative of the input source's current w.r.t. the input voltage.

2. Output impedance (or some other port): Apply the required bias voltages to the inputs to cause the device to be at its quiescent point. Apply a voltage source to the node you want to measure, and sweep the DC voltage over the range of expected node voltages. Calculate resistance in the same method as the first technique.

3. AC Impedance: Superimpose a small (1 uV) sine wave on the port you want to measure. Measure the amplitude of the current, and the phase difference between the voltage and current waveform. This will give you a current phasor, which can then be used to solve for a value of Z. You may be able to do this with an AC simulation, but a transient simulation with different frequencies should give the result you are looking for.

I would say that depends heavily on what you need it for. Often the expensive spices are part of some e.g PCB design tool. I'm just trying out MultiSIm from NI (expensive orcad type tool), and it has lots of pretty virtual instruments (e.g scope, distortion analyser, etc) and monte carlo analysis (which LTspice does not have a "convenient" version of - it does have some functions you can use though as Vlad points out, here is a link on using them) but to be honest I find that 99% of things I could do on LTspice.
I find the LTspice setup is by far the quickest out of any spice I have tried, once you get used to the key commands. R for resistor, D for diode, is much easier than clicking the picture (or even selecting from a pop up box in MultiSims case, arghh) and dragging to the right place every time.
You can have a circuit done in seconds this way.

The manual is not as pretty either, but all the info you need is there regarding how to use, eg. the .param, .step and .measure commands for doing things like running an analysis many times and varying parameters. I just tried to run a transfer function analysis in MultiSim in this manner (i.e. run may times and vary a parameter then plot results) but despite reading/wrestling for hours with it, I couldn't manage it, but a quick addition of .step V2 -15 15 1 to the sheet made it possible in LTspice.
I'm sure some of the above is simply as I'm new to MultiSim, and no doubt I am missing something (as the above example simply must be possible in a tool like that) and no I don't work for LT :-) but it has been the only spice that I have used regularly for the last few years. The main point is that it will do all the normal stuff as good (and probably faster) than the expensive tools, but if you need the extras (e.g. monte carle, PCB level anaysis based on actual routing/IBIS models - Altium does this excellently) and all wrapped up in one design tool then you may need more than if can offer.
In my view it can't hurt much to have around even if you do need a more powerful tool anyway.