Bandwidth and sampling rate on oscilloscope spec

I see on some oscilloscope datasheet specification.
"sampling rate 1Gs/s"
"Bandwidth 50-100 MHz"
I know, what does the sampling rate mean but I don't understand about the bandwidth, is it the maximum of frequency that oscilloscope can test. If so, why this oscilloscope has two bandwidth value.

Hi

I do not know why there are two bandwidths, but if we say the analog bandwith is 100MHz. It is the highest frequency you can reliably measure. Usually it is the 3-dB frequency input filter of the scope. So the 100 MHz is going to the limit.

BUT because it is a digital scope it needs to sample the input. A rule of thumb is to sample a least 5-points on a whole sine periode. If this is the case then you see that this scope can display a 200MHz sine. But your analog bandwith is only 100 MHz so this will be heavily distorted anyway.

If you measure a square wave then you need to measure 1, 3 and 5 harmonic to get at least a resonable readout on the scope. Again on a 100MHz scope this means only a 20MHz squarewave.

As you see it is important that you have the right requirements of a scope. If you are going to measure freqeuncies as high as 100MHz then you need a larger analog bandwidth. Squarewaves then you need to look at the spec again.

Hope this helped.

thanks a lot tyassin

Nyquist once said that to sample a 100 MHz signal, you need at least a 200 MSPS clock rate. And, assuming that you could probably pass a 250 MHz signal (albeit with severe attenuation) thru your 100 MHz analog bandwidth input, you would need at least a 500 MSPS ADC to see that signal unmolested.

So, 1 GSPS is not that much overkill.

If you did not have a high sample rate, you would get digital processing artifacts in the displayed waveform, and they would be very confusing to you to figure out what glitches were real, and which ones were phantoms.