Tree branches swaying in the wind, waves lapping at a shore, a rubber ball bouncing down a sidewalk -- these are all examples of oscillations. Our lives -- indeed, our selves -- are filled with oscillations. From the sound that fills the air to the chemicals that fill our bodies, systems that oscillate are everywhere. Sometimes, the systems just respond to an impulse, then oscillate freely, but sometimes, something pushes the oscillating system. That's a forced oscillation.

ring-oscillator

http://www.ehow.com/facts_7517506_ring-oscillator.html

voltage-controlled oscillator

http://www.ehow.com/info_8737267_forced-oscillations.html

http://www.ehow.com/how_7197210_voltage-controlled-oscillator-tutorial.html