I was wondering if a scope that only has 20mhz bandwidth and the signal you're probing is say 120mhz, can you you still use the same scope to see if a clock is present? Or do you have buy another scope?
Oscilloscope bandwidth is defined as the lowest frequency at which the input signal is attenuated by 3 dB, that is, where a sine wave signal would be attenuated to 70.7% of its true amplitude. So, if you're using a 20 MHz scope, the amplitude of the 120 MHz signal may be almost impossible to discern. Also, keep in mind that a digital signal will contain frequency components at harmonics of the fundamental frequency. The frequency spectrum of this signal consists of a signal at the fundamental frequency and odd harmonics. The amplitude of the harmonics follow a sin(x)/x function in frequency so the third harmonic is about 13.5 dB below the fundamental and the fifth harmonic is 27 dB below it. The next harmonic, the 7th, is 54 dB and below the noise floor of most oscilloscopes. A common rule of thumb for choosing oscilloscope bandwidth is the so-called "fifth harmonic rule." Using this rule, you'd want a scope with a bandwidth of at least 600 MHz. In many cases, however, this rule leads to an overly high bandwidth choice.
You can attempt it but the signal will most likely be too attenuated/distorted to discern. A better solution is to either heterodyne the signal into the scopes bandwidth or get a scope that has higher bandwidth.
An oscilloscope cannot 'see' signals faster than it can sample. This is dependent on two things, sampling and bandwidth. Generally it's the bandwidth that makes the difference. If the signal that you want to observe is being undersampled it looks like this:
It means that you need a faster scope to really tell what is going on, because the signal is moving faster than you can see it. Get a 200MHz scope.