I'm starting the design of a power supply and was wondering whether my oscilloscope's 20mV (20 milivolt) per division minimum ? Resolution? will be enough?
Also, most likely it will be a switching power supply. I don't have x1 probes, what bandwidth will I need to look for?
EDIT: didn't see your last reply when I was writing this, take this into account
It depends on the specs and what you want to measure, which better be defined before designing and building anything.
I'd say it's good-enough to check the output waveform if it's not a low-noise mission-critical design. It will let you check power-on glitches, transient response, stability, and noise within its measurement capabilities. I've built my first power supply with a 200kHz+ "oscilloscope", and it was incredibly useful to stabilize it, even at that limited bandwidth. I had no problem seeing noise of SMPS, even though I could see only so much of the noise spectrum. I couldn't see ringing, but was able to see switching ripple.
Things you won't be able to do: accurately measure noise. I'd be only worried about peak-to-peak measurements within first few MHz, there shouldn't be much noise above that, unless layout is messed up etc (which is, unfortunately, can be a case when designing your first power supply). So, following datasheet advice on layout is the key.
How much bandwidth is your scope? Normally output is measuremed for 0-20MHz bandwidth, but it's ok to measure the noise in a narrower band. Passive probes (those that I have) start attenuate at 3MHz+ (~6MHz -3db bandwidth), and high-freq noise is often not a concern. If your scope can do at least 2MHz, I'd say it's good enough.
It's great to see the noise up to, say, 500MHz to see parasitic ringing, etc, but this is rarely done in practice. It's also quite challenging and pricey.
For a linear power supply you don't need as much bandwidth as for smps (noise still can get injected somehow, like through mains or environment, but that's a different story). If you build an smps, consider buying a ready module, instead of messing up with mains voltage. It will also be cheaper.