I've read there are 3-4 basic ways to generate a function. XR2206, Max038, xr8038 and DDS.

Read more.

First, the 2206, MAX038, and XR8038 are functionally equal, so your list has only two basic ways. Here are some others:

Wein Bridge Oscillator
Phase Shift Oscillator
Bubba Oscillator
Hartley Oscillator
Colpitts Oscillator
Twin-T Oscillator
GIC Resonator (Gyrator or negative impedance)
Counter - D/A

etc.

https://sound-au.com/articles/sinewave.htm

https://www.ti.com/lit/an/snoa665c/snoa665c.pdf

https://www.ti.com/lit/an/sloa060/sloa060.pdf

ak

 

When creating an oscillator for the purpose of generating a low distortion sine wave, it is important to avoid using excessive feedback that forces the active element into non-linear operation.
I mention this because some published designers seem to be fixated on very rapid startup of an oscillator, with the result that the distorted wave contains a lot of harmonics.
So if excess distortion is a problem with an oscillator, reducing the feedback may be the solution.

 

ok just downloaded it.
I’m 51. My hearing tops out around 15.5 kHz. Is that normal?

My teenagers top out at 17k.

I've seen charts of what is considered normal, but just off hand that sounds right. If you get an "official" hearing test, the results are adjusted for age. That makes the results seem less awful. I'm 66 and top out around 7 kHz. Sucks, but I think that's also pretty normal.