This is a wien bridge sine wave oscillator which works by returning (positive feedback) the oscillation output to the input.
http://www.hobby-elec.org/e_ckt18.htm
This circuit is based on a classic GIC (generalized impedance converter). The sine-wave-oscillator circuit has inherent amplitude stabilization and normally operates from dual power supplies. However, if you connect a few additional resistors, you can ope
http://www.edn.com/article/CA238420.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
generates a 50-Hz to 1-kHz sinusoidal waveform that exhibits lower than 60dB THD
http://www.edn.com/archives/1995/081795/17di4.htm
To vary the frequency of any sine-wave oscillator, you should use a pair of ganged variable resistors, and you should thoroughly match their characteristics over the entire variation range to satisfy the oscillator's balancing conditions. This restriction
http://www.edn.com/article/CA300030.html
http://www.web-ee.com/Schematics/Digital_Sine/DigitalSine.htm
http://www.musicfromouterspace.com/analogsynth/NewAugustSIN_TRI_SQR_LFO.html
with Crystal Accuracy
http://www.web-ee.com/Schematics/XTAL%20Sine/XTAL_Sine.htm