[kazinator]

I think the author's analysis of the problem is off. He writes about ripple from the regulator:

> This switching causes ripples in the voltage line,

But what is on the scope is not that ripple:

> Take another look at the pictures of the ripples above and notice the “M: 20ns” in the top left corner. This signifies that each vertical dotted line is 20ns apart, so the ripple you see has a frequency of something like 50MHz.

The switching regulator does not operate anywhere near 50 Mhz. Those voltage fluctuations are caused by the magnetometer itself: its own internal switching causing rapid current demand fluctuations. Or, possibly, it could be some other nearby device, in which case that device needs the decoupler (also).

This is why the decoupling capacitor addresses the problem. The purpose of the decoupling capacitor isn't to filter power supply ripple, but to provide a local, low-impedance current source that can swallow changes in current demand. That's why it's placed close to the device. It not only ensures that the device has smooth power, but also reduces the noise that it generates, protecting other devices.

[Neywiny]

I don't think you're entirely correct here. The regulator will create noise that has a fundamental frequency of 500 kHz. That we agree on. However, this 50 MHz noise can most certainly be caused by the regulator. It's critical to note that it's not a constant 50 MHz noise. It's clearly attenuating quickly and occurring sharply. If the author had zoomed out to the 500 kHz time scale, I'd bet we'd see this noise every time the switches change.