For the *Best* scope get one of the following 3 makes:
1) Tektronix.
2) Tektronix.
3) Tektronix.
Now there ARE other good scopes out there but, generally they target specialties.
The type (technology) of scope is important too.
First a point about the number of channels.. One channel is okay but two channels let you observe a control signal and the resulting response of the controlled device/system. This is what a vast majority of what people do with scopes. In my opinion you want two channels.
Now for the technology.
The standard scope (least expensive) will be your standard sweep oscilloscope. BUT! They are useful only for studying PERIODIC events. This means a repeating signal that can continually refresh the screen so you can actually see the signal. You can, if you are good with a scope, setup see non repetitive events with this type of scope. It must have sublime triggering control so you can force a single trigger when you want it then turn up the scope brightness to damaging levels and have one image flash past and stick to your eyeballs.
A more useful scope is the digital scope. Instead of just funneling a signal into a sweeping screen painter the signal is digitized and stuck into digital memory. It is fished out of this memory and displayed onto a screen (an LCD).
The LARGE benefit of this method is that very slow or non repetitive signals can be caught and stuck on the screen for your perusal. Often stored and printed out too.
Down side they cost 2-3 times more. Personally, to me, use-wise they are 5-6 times better. They are easier to get what you need out of, faster.
The last thing to mention that a lot of people miss is the scopes front end. A standard scope of any type needs the probe and the a return signal path. Normally this signal return is the scopes chassis ground->back to the wall outlet. If you are digging around in a computer, the whole thing is likely referenced to a ground. Just hook the scope ground to the computers DC supply ground and you are good to go. However if you want to look at un referenced signals you have to be very careful.... If you want to look at any power system on the line voltage side you need to consider EXACTLY where you put the scope ground lead because it is the GROUND. If you hook it to the wrong place, your scope's ground system will likely vaporize including the ground clip that you are foolishly using in your fingers as you clip it onto the wrong spot of your circuit.
There are some ways around this. Good scopes like those mentioned in the list above recognize that this situation can come up. The classic (but dangerous) method used by the canny investigator is to isolate the ground prong of the scope. This means the entire scope becomes live as it floats on the line voltage point you hooked the scope's ground to. This is why the scope maker will specify how much isolation is provided between the inside electronics and the scope's metal face plate. There are several alternatives both are much safer. First, you can buy (relatively) expensive isolated probes. The other is to buy a scope with isolated inputs. I have owned a dozen scopes. I own three at present. I always turn to my isolated scope! It is wonderful to be able to ask, "am I below the isolation limit voltage?". If the answer is yes, then I can clip on anywhere. The second channel can then be clipped on anywhere else because both channels are totally isolated from each other and the scope chassis.
A classic example would be looking at a Solid State Relay. One side is run by a 5V computer signal the other side is controlling a 480V heater. You can look at both sides of this SSR with an isolated scope at the same time, watching cause and effect.
Down side? They cost, yet again, more.
Given this information you need to consider the type circuits you are thinking about studying. If you will be messing about with line voltage circuits then you should consider an isolated scope from the get-go.
If you have other specialized interests they should, of course, be addressed too. For instance if you are going to be doin three phase power studies there are scopes that will do ALL the power measurements with push button ease. These scopes can get really pricey too.
One scope I highly recommend is Tektronix THS700 Series.
It is digital. It is Isolated. It is battery powered if desired. It is NOT cheap. But oh so effective.
If you are just working on the above product it sounds like you could probably pick a place on the output of your stepper generator, and ground the system. (No isolation needed?) If you are going to be looking at signals above 40Hz repetition rates then a digital scope isn't absolutely required. If you are only interested in the output (the input being rotation), then a single channel will probably suffice.
Just get one of the types listed above and you will do okay.
