This beats em all for electronic design. Its
TOTTALLY FREE and incorporates a SPICE simulator. So, it helps out a lot with the math.
Here's a direct link to the download:
http://ltspice.linear.com/software/swcadiii.exe
Here's a link to LinearTechnology's website:
http://www.linear.com
You'll be amazed how good this is for a free circuit simulator. PM me with any ?'s/comments you might have.
-Q