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http://cushychicken.github.io/ckt-notes-function-generator/
I [Nash Reilly] haven't done a Circuit Notes post in a long time. Years, in fact! And I'm considering picking up a function generator as a very late Christmas gift to myself. So, naturally, I started poking around online for function generators.
There are some truly cheap options available - you can get kit versions on Amazon for less than $15, but these are pretty limited in bandwidth, scope, and specs. They're basically just bistable multivibrators with some integrator stages for sawtooth and sine wave generation. One of these days, I'll take a little time to look into those - there are lots of foibles that you can run into with these designs due to the analog imperfections in the capacitors and opamps.
Today, however, I'm gonna take a look at the technology that rules the roost in function generation these days: direct digital synthesis. DDS relies on a packaged integration of a phase accumulator (fancy words for "an adding counter"), a lookup table of phase-to-amplitude conversions, and a digital-to-analog converter. DDS is another proof that digital technology makes certain subsets of electronics substantially easier: instead of a bunch of square wave oscillators and integration stages with the associated analog imperfections, you're instead limited by the linearity of your DAC, the resolution of your lookup table, and the linearity of the output stage. Certainly not as high performance as a big, highly tuned analog function generator - but it does fit on a single chip!
(Score: 4, Informative) by driverless on Sunday January 12 2020, @11:16PM
It's actually a really good analysis of how not to design a piece of electronics. I didn't follow the "where to buy it" link but went straight to the good stuff and kept muttering about "typical cheap Aliexpress junk" as I read it, until I saw it was off Tindie. However, it's fully representative of vast amounts of badly-designed Aliexpress electronic crap, most of which are the bare minimum of components you can slap together to get it to work under artificially perfect conditions, out-of-spec components, oddball Chinese-clone substitutes for standard active components, things that only work because protection circuitry in some device is holding things together, etc. So if you ever buy something off Aliexpress or similar, this is the virtual teardown for it.