What to look for in an oscilloscope?

I'm getting back into basic electronics with an Arduino after many years away from the hobby, and I've forgotten a lot of my knowledge.

One thing I'm finding is that I really need to get an oscilloscope to assist in designing filtering circuits to give me smooth output from PWM.

I'm not after anything fancy and my budget it small, so what sort of things are on a an oscilloscope must have list?

Answer

If you want a good scope for not a lot of money, pick up just about any analog scope from ebay. I've had a Beckman Industrial 9022 and never found it lacking for hobby use. It's a 20 MHz scope, and there are few things that require anything faster. The fastest thing I have on my desk is a micocontroller clock, but even if for some odd reason I wanted to run my AVR at maximum speed and I wanted to scope the clock, at 16 MHz it's still visible. Maybe not super sharp, but fully resolved at least. Unless you plan to build any RF circuits soon, I doubt speed will be a concern.

Despite the tech gadget factor of a digital scope, there are some things that show up better on an analog scope. The cheap digital scopes I've had the displeasure of using have the problem that they can't update the display very fast, while an analog scope updates the display instantly each time it triggers. This is great when you are looking at noise superimposed on another signal; with an analog scope you can see the trace brightest at the most frequent values, and dimmer regions of the trace for the rarer excursions. On a digital scope all you get is a sharp trace that wobbles, to whatever the noise happens to be each time the display updates. I'm sure there are digital scopes that address these shortcomings, but they will be out of your price range.

You might miss the storage capabilities that every digital scope has, but I don't. For things I need to store, a logic analyzer is usually the right tool, and for the other times, dimming the lights, turning up the brightness, and using the single trigger mode works well enough usually. Analog storage scopes do exist; maybe you can find one for a good price.

In all, I think what you lose in high-tech gadgetry you will gain in quality construction and robust engineering. New, inexpensive digital scopes are made in China and will break in a couple years. A used analog scope for the same price was made in the same country that designed it with parts made to last 200 years.