Oscilloscope specs


I'm looking to buy my first oscilloscope, no experience, hence need a bit advice on typical lower-budget scopes.

A particular purpose is to look at rise time of rectangular waves with rise time from ~5ns, of frequencies typically 10-100Hz, and up. And harmonics, for whatever features it may have for that.

I figure just from the rise time that would require a 100MHz scope. But I'm not sure of how the other specs matter.

Considering some typical specs;

 A  100MHz 500MSa/s   32k
 B  100MHz 1GSa/s     40kpts
 C  100MHz 1GSa/s     2Mpts
 D  150MHz 1GSa/s     2Mpts
 E  200MHz 500MSa/s   32k
 F  200MHz 2GSa/s/CH  18kpts/CH

So, am I reading this correctly by assuming the (F) has 4 times the sample rate of the (D) when using both channels? If so that is perhaps of major importance? I don't really understand why there often is, like above, such a large differences in memory depth.. What limitations and importance does that really make?

So, any advice on these typical spec variations would be very welcome.

Best Answer

I think this question will be closed since it's a classic shopping question, but if you really want to measure 5ns edges with reasonable accuracy, you'd need more like a 300MHz oscilloscope (which won't be cheap).

The math behind this (and answers to some of your other questions) can be found in a Tektronix document called Understanding Oscilloscope Bandwidth Rise Time and Signal Fidelity.

If you get a cheaper (and more typical entry level) scope that is 50-100MHz you'll not be able to make accurate measurements of the rise time, but of course you'll still be able to see edges (the rise time you see will be dominated by the oscillscope perfomance, so you'll be able to set an upper bound on rise time, but not a lower bound). Deeper memory is more important in digital and mixed signal applications where you're looking for a relatively rare event.

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