is it true, oscilloscope must reach at least 4x observed freq?

[tggzzz]

some people say, if I wanna observe 100MHz, I must use an oscilloscope that reaches at least 400MHz.

Is it true?

No, for 100 MHz signals, the oscilloscope bandwidth should be about 1 GHz (10 times).

This is because you're needs to see at least 10 harmonics of square wave in order to see waveform distortions. Otherwise you will see just a sine and cannot see signal distortions and glitches.

You can still use 400-600 MHz bandwidth oscilloscope, but it's ability to see 100 MHz waveform details will be very limited. For example, you can see if signal exists, and can see if it's clean sine. But cannot see if it has glitches or distortions.

Note, that oscilloscope bandwidth is applied for a clean sine wave. If you're dealing with other kind of waveform it consists of high order harmonics and oscilloscope should be able to see it, otherwise your scope input will works like low pass filter and all what you can see is just a 100 MHz sine despite the fact that signal waveform is actually square wave, triangle or other type.

Oscilloscopes are time domain instruments; spectrum analysers are their frequency domain equivalent. The principal use of an oscilloscope is to show waveform shape, especially a waveform's risetime, overshoot etc.

If you concentrate on the fidelity of displaying the risetime and ignore frequency domain characteristics, all of the frequency domain "specifications" are seen to be misleading and ambiguous rules-of-thumb.

[BU508A]

Screenshot from a datasheet of the DSO5054A:



Datasheet source:
http://www.testequipmenthq.com/datasheets/Agilent-DSO5054A-Datasheet.pdf

[balnazzar]

Just to say that I found this thread to be quite useful and informative. Not all the details examined are clear to me, but it's a very good starting point for further investigation.
Especially the stuff about sampling rate and accurate signal reconstruction (which is not what the OP originally asked, I think).

Thanks!