I've been advised to say that, although I believe the package to be bug-free, you use the results at your own peril, and you're advised to check them for correctness before using them. Don't blame me if your aircraft falls out of the sky!
The Java code generated by this package is the way it is for pedagogic reasons, and could easily be ``tightened up''.
The graphs are produced with the help of the excellent gd GIF manipulation library, originally from Quest Protein Database Center.
Here are some references to textbooks which will tell you about analogue and digital PLLs, and here is a short glossary of technical terms.
A companion page generates digital filters.
An email message, saying what you like and don't like about this package, will be appreciated.