With a conventional tone burst generator, a considerable number of harmonics are generated. These extend to sub-audible frequencies, and excite resonances that have nothing to do with the actual test frequency. By gradually increasing the amplitude of each cycle of the test waveform, then decreasing it again using a cosine waveform envelope, these unwanted harmonics are minimised.
http://sound.westhost.com/project58.htm
http://ourworld.compuserve.com/homepages/Bill_Bowden/page8.htm#phase.gif
with Crystal Accuracy
http://www.web-ee.com/Schematics/XTAL%20Sine/XTAL_Sine.htm
http://www.interq.or.jp/japan/se-inoue/e_ckt18.htm
http://octopus.freeyellow.com/sine.html
A circuit generates three-phase sine waves with excellent amplitude and phase symmetry. A variable-frequency, three-phase, sine-wave generator circuit has been designed for use as a source of polyphase excitation in studies of the propagation of traveling
http://www.nasatech.com/Briefs/Aug99/LEW16696.html
which outputs the Sine wave and the Cosine wave at the same time using the operational amplifier.
http://www.hobby-elec.org/e_ckt20.htm
The Wien bridge sine wave oscillator to introduce is the oscillator which works in the oscillation by returning(positive feedback) the oscillation output to the input.
http://www.hobby-elec.org/e_ckt18.htm