Jess Chen

I agree with mg777 regarding SPW. Unless you are in a digital signal processing class and perhaps already know how to use SPW, I think you'll find yourself spending most of your time learning SPW and very little of it learning about PLLs.
In my opinion, Matlab (or Mathematica) would be a good tool for studying the linear behavior of PLLs (loop gain, phase noise, settling time). You would use a linear phase domain model of your PLL. This model is extremely useful for analyzing or deriving PLL specifications but it masks some of the details. If you are taking a class in control theory, I'd use Matlab or Mathematica.
However, if want to learn about how a PLL works in more detail, I'd use a circuit simulator like SPICE or Spectre, preferably one that supports a behavioral modeling language, like VerilogA, VerilogAMS, or VHDLAMS. If you are just starting out, you could construct behavioral models of the phasefrequency detector, charge pump, VCO, and divider and really see how everything works together. If you eventually want to swap your behavioral models with real circuits, you don't have much choice but to use a circuit simulator.
If you are taking a digital design course, you could even use Verilog or VHDL to model the PLL in a fair amount of detail. In fact, we often use a digital simulator because today's PLLs are mostly digital.
I would use a mixed signal behavioral language like VerilogAMS or VHDLAMS because they gives you the best of both simulators (analog and digital).
Of course, a practical decision may have more to do with what tools you have at hand or time to learn.
Jess
