signal generator to test circuits, frequecy range is 20 to 20 kHz, outpus sinewave, triangle wave and square wave.
http://home.maine.rr.com/randylinscott/may99.htm
http://margo.student.utwente.nl/el/analog/audosc.gif
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
http://www.web-ee.com/Schematics/Function%20Generator/function%20generator.htm
http://www.evenfall.com/diy/pwm.html
Ramp or sawtooth waveforms are useful for a broad range of applications, including automatic-test equipment, benchtest equipment, and actuator control.
http://www.edn.com/article/CA307866.html
http://www.web-ee.com/Schematics/Function%20Generator/function%20generator.htm
pseudo-random signal generating circuit ideas
http://www.geofex.com/Article_Folders/LFOs/psuedorandom.htm
Schematics and code for a 68HC11 based frequency counter, square wave synthesizer, sine wave synthesizer, programmable filter, capacitance and inductance meter, with a digitalker speech synthesizer output.
http://members.tripod.com/michaelgellis/68hc11.html