Bandwidth importance on a scope for digital electronics

Hi. My hobby is working on old arcade boards - generally fixing dead ones. At the moment I have a Hameg scope with 20 MHz bandwidth. I've noticed a number of times that when looking at data lines on higher speed boards, I get a confusing picture. For example, watching a data line on a 12MHz 68000 gives me apparently two pictures super-imposed. I will have a clear and bright wave showing the changing logic states, but also a fainter set of logic transitions super-imposed.

I have a feeling that this is related to the bandwidth of my scope - I've searched the archive here and have learnt some more information, but I'm still not knowledgeable enough to shell out cash yet. I have a feeling I need a higher bandwidth scope. To further confuse myself, I have somehow convinced myself that I need a faster timebase. I can go down to 0.5uS but was thinking that even faster would allow me a more accurate display. As a final thought, I did wonder if the "echo" I'm seeing on the screen is actually a badly terminated bus and I'm just seeing reflections.

My understanding is that an faster timebase allows me to see less on the screen, but with more accuracy - e.g. seeing the slope on a square wave. Having been brought up in a digital world, I equate bandwidth with resolution and accuracy.

Have I made enough sense to allow you to give me advice? As I learn more I'll be progressing to CPUs with faster clock speeds, so I can only assume the problem will get worse in that I could miss logic transitions.

If none of this makes sense, I'd very much appreciate somebody explaining the relationship between bandwidth/timebase/resolution (or a pointer to a website?)

thanks, Tim

What you are probably seeing there is a typical data bus signal. Because data busses are a non-repetitive signal, and things get switched ff and onto the data bus you will typically see a "jumble" of signals on an analog oscilloscope display. A true repetitive signal like the clock line will not have this problem, and you should be able to trigger off off and display a nice clean signal.

With a 20MHz bandwidth scope, you won't be able to accurately see the true characteristics of say a 20MHz digital (square) wave, because a

20MHz square wave is made up of much higher harmonic frequencies which will not make it through. So your square wave will display more like a sine wave. In fact, the base signal frequency does not matter, it's the rise and fall time of the signal that is the key, but that's probably more than what you need to know at this point.

See:

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A 20MHz scope will be fine to "see if there is something there", but

100MHz oscilloscopes are cheap and readily available on the used market, even 4 channel ones, well worth having for this kind of work. So you are not off the mark at all in thinking you need a higher bandwidth scope, you do.

What you may find very handy though is a digital storage oscilloscope, for capturing and analysing non-repetitive signals like serial data lines. But these do not come cheap, and they have a whole new set of performance criteria. But a basic 1GS/s 60MHz+ 10KB memory scope will go a long way. All three of those numbers matter a lot in a digital store oscilloscope. Get one of these *in addition* to the analog oscilloscope if you can afford it.

Dave.