Stupid oscilloscope question - noise vs bandwidth

[pwnell]

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:



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.



Lastly, removing the 20MHz bandwidth limit and using the "full" 100MHz of the scope I get:



This measures 100MHz.  So I presume this is just noise being interpolated incorrectly?  Or am I missing something?

[bob91343]

Nothing is perfect, and that appears to apply to your oscilloscope as well.  Perhaps viewed on a different unit to see if it's really there.

I don't trust digital oscilloscopes.  Great when they work well but confusing as to whether to believe what they display.

[pwnell]

I tried dot display and turned off vector - but the scope oversamples so it definitely is following some sinusoidal waveform - t does not appear to be interpolation.  I do not think my scope can switch off sinc interpolation though...

[bdunham7]

Your sample rate is 8GS/s, so it isn't going to be a sinx/x or Gibbs-type issue.  The fact that it seems to be directly related to the selected input bandwidth seems interesting, pehaps you should hack it for greater bandwidth and see what happens then! 

It seems pretty pronounced since you are at 50mV/div--what probe or connection are you using? 

[pwnell]

The standard passive 10x probe with the hook clip.

[radiolistener]

What I want to know, is whether these sinusoidal waveforms riding on the main wave are noise or an actual signal?

They are actual real signal. This is strongest component from some transmitter in your area. Since it is about 100 MHz, most of all this is FM radio station transmitter in your area.

Below the waveform with bandwidth limit set to 20MHz:

Note, that bandwidth limit 20 MHz does not remove frequencies above 20 MHz. It just reduces it's ampitude for a little. So, this is normal to see 100 MHz signal with enabled 20 MHz filter.

[magic]

Yes, but this 100MHz signal turns into a 15MHz signal in 20MHz mode.
This is not normal, it should just disappear or become very weak, but still 100MHz.

[radiolistener]

Yes, but this 100MHz signal turns into a 15MHz signal in 20MHz mode.
This is not normal, it should just disappear or become very weak, but still 100MHz.

it's also okay. When you enable 20 MHz filter, 100 MHz component amplitude drops down, and now you see another strongest component, which is 15 MHz.

According to the short wave database there is time signal transmitter on 15.000 MHz:
https://www.short-wave.info/index.php

It has two locations:
- Colorado Denver = 10 kW transmitter
- Hawaii Kokole Point = 2.5 kW transmitter

Also there are a lot of AM stations at 15-16 MHz