Oscilloscopes - Bandwidth
Intended audience: Those with little electronics knowledge/experience.
Let us pretend that you are in the market for an oscilloscope, with the primary target being some vintage computers that you have. The fastest vintage computer that you have is an IBM 5160 (IBM XT). You look at the diagram at here and figure that the 14.31818 MHz clock is the highest frequency signal in that computer, and accordingly, because of your tight budget, choose an oscilloscope that has a frequency bandwidth of only 20 MHz.
You bring out your fully-functional IBM 5160, then hook up your chosen oscilloscope to the clock pin of the motherboards's CPU, expecting to see a 'squarish' waveform of 4.77 MHz. But instead, you see what is shown in the top-right photo at here.
What is going on !!!
Well, a sine wave contains only one frequency. But signals other than sine waves, contain multiple frequencies.
For example, per the following diagram, a square wave of 25 Hz contains 25 Hz, plus, all the odd harmonics of 25 Hz in a decending amplitude.
If your oscilloscope's bandwidth does not capture a significant frequency component of a signal, you will not see what you expect.
So, getting back to the CPU clock pin on your IBM 5160 motherboard, your 20 MHz oscilloscope only captures the fundamental frequency (4.77 MHz) and the first odd harmonic frequency (14.31 MHz), but none of the other odd harmonic frequencies.
Had you instead chosen a 10 MHz oscilloscope, the situation would have been worse - see the top-right photo at here.
For more information on this subject, do an online search using: choosing oscilloscope bandwidth