Thanks for all the info so far, everyone!
Sciguy - As I said, I can't talk too much about specifics, but this unit will power an electrolysis chamber and resonate the electrolyte for increased output. That's where the frequency portion comes in.
The dual sine-waves with phase shift come in, because when shifted a minimal amount, the upper limit of the voltage is effectively the same as it was prior to generating the waves. The lower limit, however, is shifted slightly and therefore keeps the minimum voltage above 0 (say 6V, but that's just a guess). By not dropping to 0, production does not stop but rather becomes endothermic while the "active" production on the electrodes pauses and the product is allowed to clear.
The file at
http://www.spline-designs.com/FrequencyGenerator.xls shows what I'm trying to achieve, though now I think if I could set a voltage range, I would only need one generator (ie., one sine wave from +6V to +12V at a variable frequency), which would make this much easier.
Dave - Thanks for the link, every little bit that gets me and others thinking helps
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Alan - Good idea with the amp output, my only concern is the efficiency lost in the additional device, but I'm probably not going to be extremely efficient to begin with. The output is going to an electrolysis chamber, as described above.
Ken - I don't need DC in the strictest sense, otherwise I'd go straight in from my power supply (a DC car battery). I effectively need "pulsed" DC with a minimum "low" voltage, a max "high" voltage and some form of curve (not square wave) in between. Triangle wave would work, though I'd much prefer sine waves if possible, because sine waves have more of the type of derivative I'm looking for (sharp increase, quick level, sharp decrease).
I did think about tearing apart both an audio amplifier and a sine-wave inverter for this project, but the inverter isn't variable frequency and has a transformer, while I wouldn't know how to tune the amp.
Pete - Thanks for the link, I'll take a look at the data sheet and see what I can figure out about it.
Thank you all for your time in reading this, and I really appreciate the help.