http://courses.ece.ubc.ca/359/lectures/chapter2.pdf

http://web.eecs.umich.edu/~fessler/course/451/l/pdf/c3.pdf

http://www.site.uottawa.ca/~jpyao/courses/ELG3120_files/ch2.pdf

https://engineering.purdue.edu/~ipollak/ee438/FALL04/notes/Section1.1.pdf

http://www.dip.ee.uct.ac.za/~nicolls/lectures/eee401f/06_tanal.pdf