http://books.google.com.br/books?id=C-rbT_c69oUC&pg=PA121&dq=carrier+lock+in+amplifier#v=onepage&q=carrier%20lock%20in%20amplifier&f=false

http://books.google.com.br/books?id=qPi-SSchB6MC&pg=PA94&dq=carrier+lock+in+amplifier#v=onepage&q=carrier%20lock%20in%20amplifier&f=false

http://books.google.com.br/books?id=VXQdq0B3tnUC&pg=PT2150&dq=carrier+lock+in+amplifier#v=onepage&q=carrier%20lock%20in%20amplifier&f=false

http://books.google.com.br/books?id=DyX_dKLyZVwC&pg=PA40&dq=carrier+lock+in+amplifier#v=onepage&q=carrier%20lock%20in%20amplifier&f=false