Someone mentioned that it's possible to generate RF with a Raspberry Pi PICO with no external oscillator. There is a library by Roman Piksaykin, called pico hf oscillator and it's used in a simple WSPR transmitter available here.
To build this project you need the native C/C++ toolchain. There are good instructions around for getting this going on a Raspberry Pi, but I did run in to a few issues getting it going on my Apple Silicon Mac. In the end it compiles.
The code dedicates one of the two CPU cores to generating the RF signal and it can go up to 33Mhz.
The waveform out of GPIO pin 6 looks like it has a bit of jitter on it.
It sounds good on the receiver and was easily decoded locally.
The BNC to the CRO was unplugged and connected to a 40m dipole and stations heard me.
It looks like it drifts slightly during a transmission but not too much. Thanks to Roman Piksaykin for his good work on this.
The Pico is a remarkable platform and I've started looking at this amazing work to produce other signals including sideband from 101 Things.