Test and Measurement Forum

First.... Trigger signal waveform is as follows
_______ ______ ______ _____
| | | | | | | |
_____| |____________| | ___________| |_________________| |_
40nsec 160nsec 40nsec 160nsec 40nsec 240nsec 40nsec

What scope Do I need to display this waveform?
40nsec pulse is 25MHZ.
Will a 25MHZ scope display a 25MHZ Square wave?

Second after the above trigger.
_________ _________
| | | |
_______| |_____________| |______________
40usec 40usec 40usec

A 25MHz scope will not display a 25MHz square wave, the waveform will look like a sine wave.

This is because the perfect sqare wave is made out of infinite frequency components more specifically out of sinewaves of odd harmonics:

http://en.wikipedia.org/wiki/Square_wave

These harmonics shape the rising and falling edges of the waveform, the more harmonics you have the steeper the rising/falling edge.

So you really need an oscilloscope to have the fifth harmonics (or higher if you want to see steeper edges!), so with the fifth harmonic the bandwidth of the scope has to be 5 times higher so 125MHz. The way I worked it out is as follows:

The fundemental frequency is 25MHz, since we are only looking at odd harmonics, we will look at 3x, 5x, fundemental.

Fundemental 25MHz
3rd harmonic 75MHz
5th harmonic 125MHz


Hope this helps.