Well, you could go with a Wein Bridge oscillator. It consists of a
non-inverting amplifier with a gain of three (generally made with an op
amp and a couple resistors), a series RC from the output to the input,
and a parallel RC from the input to ground. At 1/(2piRC), the "gain" of
the series/parallel network is 1/3 at an angle of 0 degrees. Make up the
gain with the amplifier and it oscillates. You can put a small light bulb
in place of the resistor between the inverting op amp input and ground go
get automatic gain control. As the signal gets larger, the lamp current
goes up, increasing resistance and decreasing gain). The lamp probably
won't work at real low frequencies (it will follow the waveform).
        A nice chip for generating sine waves is the XR2206. It has square,
sine, and triangle wave outputs (you switch between triangle and sine).
It has two timing resistors and another pin to select between the two, so
it's nice for generating frequency shift keyed signals. You can also
inject current into the timing resistor pins to do linear FM. It has an
analog multiplier (balanced modulator) in the output, so you can generate
AM, BSBSC AM, and BPSK. The sine/triange output has a source resistance
of 600 ohms, ideal for driving phone lines and stuff.
        Nice chip! I first used it in the mid 1970's.
        Finally, if you want to move to a PIC, for a low frequency, you can use
the PWM output to drive an LPF as a D/A. Then use a sine lookup table to
output values for a sine wave. You could even do "variable phase
addition" in the PIC to do direct digital synthesis... Of course, there
are DDS chips available (for much higher frequencies).

Harold




FCC Rules Online at http://hallikainen.com/FccRules
Lighting control for theatre and television at http://www.dovesystems.com

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