It's an SBC-scale OXCO. I half wonder if adding a larger heatsink, or even putting thermal mass around the existing oscillator could also help, or if the heating is more localized in the PCB itself.

Always fun new things to learn when doing something "simple" like setting up an NTP server!

Flirc makes a metal Pi case where the CPU is pressed against the metal body of the case, resulting in a huge thermal mass for passive cooling. I have a bunch of them and it works very well. No fan necessary.

> or even putting thermal mass around the existing oscillator

I was thinking along these lines as well. Put a metal block on the CPU and oscillator for thermal mass (not sure if separate blocks would be better). Ideally, with a large enough thermal capacity, the block should reach an average temperature and remain there.

Inertia is also good even if the temperature is not constant: clock drift can be measured and compensated. If the temperature rises slowly, the clock speed will increase slowly: the rate can be measured and compensated for. Jitter is the issue here, and thermal inertia should dampen it.

It may also be worth preventing convection from happening on the board. Putting the Pi in a wool sock may not be the best idea depending on its temperature, but an electrically insulating thermal conductor (or an electrical insulation layer + steel wool may do it).

Heatsinks may also be counter-productive (if they have a small thermal capacity), as their temperature depends on room temperature, which changes during the day.

I was thinking it might be nice to add some insulation around some the pi's enclosure, to reduce its cooling significantly. A little bit to tamp down any potential rapid fluctuations in the room's temperature (if someone opens a window, steps out of the bath, whatever). But more so that it could save a watt or two of power, by having the time-burner cores working much less.

You're right that this is a over-controller oscillator. The goal generally with ovens is to keep heat! (To an extent of course.)

> I half wonder if adding a larger heatsink, or even putting thermal mass around the existing oscillator could also help, or if the heating is more localized in the PCB itself.

That would likely make it worse. The trick here is that the other cores are running at essentially their maximum temperature and and will dynamically reduce their clockspeed if required to keep from going above that limit. In essence, the environment becomes actively temperature controlled. If the ambient heat goes higher, the cores clock lower, if it gets colder the cores clock higher (up to a point).

If you add too much heat dissipation, the total power used by those cores might not be enough to keep well above ambient.