I apologise if this is a stupid question, hence why I am posting it in the beginners forum. I have a Rigol MSO5104 scope, and am looking at a signal from an AM antenna. What I want to know, is whether these sinusoidal waveforms riding on the main wave are noise or an actual signal? Asking since it is sinusoidal - and am wondering if this has something to do with the bandwidth limit of my scope (100MHz) and interpolation using the sinc function. Below the waveform with bandwidth limit set to 20MHz:
![Stupid oscilloscope question - noise vs bandwidth](https://www.eevblog.com/forum/index.php?action=dlattach;topic=263376.0;attach=1130246;image)
Then zoomed in a bit - the superimposed sinusoidal waveforms are clear:
Zoom in more and use cursors to measure the frequency, it seems to be 15MHz, close to the bandwidth limit of 20MHz.
![Stupid oscilloscope question - noise vs bandwidth](https://www.eevblog.com/forum/index.php?action=dlattach;topic=263376.0;attach=1130254;image)
Lastly, removing the 20MHz bandwidth limit and using the "full" 100MHz of the scope I get:
![Stupid oscilloscope question - noise vs bandwidth](https://www.eevblog.com/forum/index.php?action=dlattach;topic=263376.0;attach=1130258;image)
This measures 100MHz. So I presume this is just noise being interpolated incorrectly? Or am I missing something?