How to Choose an Oscilloscope

Oleg10011001

Rule of thumb: get a scope that has a bandwidth of at least the 9th harmonic.  So if you're going for square waves, it's better to get a scope with a bandwidth of at least 10x the frequency of your square wave.

Is this enough? Firstly, I heard that for normal reconstruction of the shape of a rectangular signal, one must have at least 10th harmonics of this signal, and better - 13th. Secondly, these harmonics will also be measured by the oscilloscope and, accordingly, the frequency of the last harmonic we need (9th or 10th or 13th) must not exceed 1/3 of the bandwidth of the oscilloscope, so that the measurement error does not exceed 5% or 1/5 of the passband if the measurement error is not more than 3%. Thus, it turns out that the maximum frequency of a rectangular signal with which the oscilloscope can work normally is much less than the bandwidth specified by you of 1/10. In addition, I think that to restore the waveform is important not so much the bandwidth as the sampling rate ... How much minimum is required samples for the period of the rectangular signal for the normal recovery of its shape, so that for example it was possible to measure its fronts? As a result, it turns out that an oscilloscope with a bandwidth of 200 Mhz at a sampling rate of 1GS / sec will be able to operate normally with a square signal having a frequency of no more than 2-4Mhz (If the measurement error is not more than 3%), and not 20Mhz (1/10 of 200Mhz) ... Am I wrong?

P.S: Sorry my bad english.

WisdomAugust

Evaluate the rise time. Rapid transitions can potentially be lost if rise time measures are not up to scratch. So, the faster the rise time, the higher the accuracy of measurement for those rapid transitions. The rule of five can apply here too, meaning the oscilloscope should be less than 1/5 times the fastest rise time of the signal being measured.

WisdomAugust

Bandwidth is defined as the frequency at which a sin wave input signal is attenuated to 70.7% of its true amplitude.
The -3dB or "half power" point, shown here for a 1GHz oscilloscope.

WisdomAugust

Understand that today's signals are no longer pure sine waves, but most of the time square waves.
These are built by "adding" the odd harmonics of the fundamental sine wave together. So a 10 MHz square wave is "built" by adding a 10MHz sine wave + a 30MHz sine wave + a 50MHz sine wave and so on.

Rule of thumb: get a scope that has a bandwidth of at least the 9th harmonic. So if you're going for square waves, it's better to get a scope with a bandwidth of at least 10x the frequency of your square wave. For 100MHz square waves, get a 1GHz scope...