68HC11 Function Generator:

Function Generator #2:

Function Generator:

Low Cost Function Generator: This low-cost function generator, based on the Maxim MAX038 high-frequency waveform generator, produces sine, triangle, and square waves from under 1 Hz to over 20 MHz  (See figure Below). The frequency, amplitude, and offset of all output waveforms are adjustable, as is the duty cycle of the square wave. 

Timers generate Variable Sweep Frequencies: 8/1/96  EDN-Design Ideas