David Gesswein:
It all depends on that you want to do with it. This is written assuming you are troubleshooting old computers.
You need sufficient bandwidth to view your signal of interest and sufficient sample rate to make it look reasonable. Sampling twice the frequency of a sine wave captures the information but it doesn't look like [the original signal].
I think the sample rate will be the biggest limitation. You will need to see the character of signals you will want to look at.
For troubleshooting old equipment the degradation of the signal edges is normally not as big an issue. The faults are normally not that subtle but I have run across glitches on signals causing trouble that the low sample rate can hide.
I agree with my old friend and colleague, mostly. But I'd put stronger emphasis on detecting "glitches". I prefer analog scopes, because they can show in some fashion glitches even above their bandwidth. The following lecture will be technical. This matters to me, as this is work I've done for decades. "Bandwidth as twice sampling rate" is fine for detection of repeated events. It's not fine for reconstructing the signal, or finding one-off events. Digital sampling limits aren't the same as the bandwidth rolloff of an analog oscilloscope amplifier (or probe). Many 20th C. brand-name analog oscilloscopes gave (still give!) good performance well past their nameplate bandwidth. also: scope trigger circuits have to respond to events far above bandwidth! Triggering is very important. A "glitch" is a signal where a logic state changes, not because of a change in logic condition, but because of a timing event such as "propagation" or a "triggering delay". These are very short pulses, on the order of several nanoseconds in TTL circuits. Or, they may manifest as nanosecond delays in signals. Power supply lines carry glitches, caused by literal "spikes" in DC power to logic-switching circuits; large currents plus fast changes produce L di/dt or C dv/dt signals. These matter, because they can trigger logic events. If you can't see the trigger, you don't know "why" you have such events. Old TTL designs, sometimes generated glitches due to poor design. Aging components may cause glitches, by increasing capacitance, or sagging DC voltages. Dead or absent bypass caps, fail to filter out power-supply-line glitches. Look at schematics of the IMSAI front-panel. They actually CREATE "glitches" with a series capacitor to turn a logic-level change into a triggering pulse! Ugly ugly ugly design. All that said, any scope beats none. Looking at logic-level signal activity (any activity) is a go / no-go test for logic chips. Many faults can be diagnosed with a 20Mhz analog oscilloscope and probe. (Probe bandwidth matters. Not today's lecture.) But some faults are design problems that finally came due, or non-logic component failures, or "tired" chips. TTL logic responds to signals and events of several nanosecond duration - that's hundreds of megahertz "bandwidth" technically. and even a 20Mhz scope, will trigger on much faster events - that at least "detects" them. and you can guess if an analog "bump" is actually a pulse above your bandwidth. End of technical lecture. In my 20th century view, a $100ish old toolbox-sized HP or Tek 100Mhz analog oscilloscope, will show more information about pre 1990's computer chips, than most users can appreciate. But a $300 "100Mhz sampling" digital oscilloscope you can carry in your pocket, likely performs little better than a 30-40Mhz no-brand-I-remember analog oscilloscope that you leave behind at a hamfest. Will a 1Ghz modern DSO do much better? (shrug) I lack that experience, ask an owner, faster is better but.... And a DSO is conveniently tablet-sized, and likely does other things too. So the answer for you is: what's in your wallet? Buy BOTH if you can afford it, choose otherwise. Herb "race condition" Johnson retrotechnology.com -- Herbert R. Johnson, New Jersey in the USA http://www.retrotechnology.com OR .net preserve, recover, restore 1970's computing email: hjohnson AT retrotechnology DOT com or try later herbjohnson AT retrotechnology DOT info