A question about oscilloscopes' bandwidth

[mehdi]

Hi
I've an R&S (Hameg) HMO1002 base model (50MHZ bandwidth)
You can upgrade the license to a 70 or 100MHZ bandwidth.
I connected the output of my signal generator to this oscilloscope and was able to see frequencies above 50MHZ (even 125MHZ)
How's that possible?
What am I doing wrong?

Thanks

[VK5RC]

This is normal, the flat response bandwidth is what is specified, there is a slow roll off of amplitude as you go above this point till very little at double the frequency, this is important in Digital scopes to avoid aliasing of very high frequency/out of range signals giving false lower frequency signals appearing on the screen.

[Srbel]

You are not doing anything wrong. 50 MHz bandwidth means that the signal that oscilloscope is showing, at 50 MHz, will have -3 dB (or 0,707) of the actual signal amplitude. Oscilloscope is still capable of showing signs of higher frequencies, but their amplitude will be lower the higher you go.
Your scope has a sampling rate of 1Gsps. So you could even maybe see a signal of 500 MHz, but it will have very low amplitude (if any) and will only show it as triangles.

[w2aew]

As others have stated, the BW simply indicates the guaranteed 3dB bandwidth of the vertical response.  Here's a video I did on the subject a while back with an analog scope:


and one using a older digital oscilloscope: