What exactly does a scope do during self-cal?

[Phaedrus]

As the title. What exactly are the parameters calibrated, and what do most scopes use for their references? Also, how frequently should you self-cal?

[Psi]

I imagine it depends on the scope in question, but they likely connect the reference signal (maybe the one on the front panel terminal) to the channel inputs with relays. Then measure the frequency and voltage on each input range/timebase and compare to intended value then adjust cal values if needed to match them up.
They probably also have a 2nd higher frequency reference too, since the one on the front is only 1khz

More expensive scopes have more complex self-cal tests.

[sync]

I think the most common adjustment is the DC offset. On the DS1000Z you can see this in real time. The traces are displayed during self-cal. No square waves are used, only varying DC.

[eV1Te]

I think the most common adjustment is the DC offset. On the DS1000Z you can see this in real time. The traces are displayed during self-cal. No square waves are used, only varying DC.

Yep, I noticed this to, maybe you can connect a second scope to the input and "read" the voltages on the BNC during self cal.

I would imagine that the only thing that needs calibration is DC offset and DC gain (on all of the vertical scale positions separately).

[Phaedrus]

Cool. I use a TDS 3014C at work, and self caled it earlier this week. Probably doesn't make a huge difference for me, since 95% of my work is in the <20V, <20MHz zone. But it's good to know.

The Tek seems to do a lot of DC test modes, but also runs what look like sin waves and a couple other patterns, and also runs with nothing on the screen for a few minutes. Whole thing takes about 15 minutes I'd reckon.