Yet another oscilloscope input noise question

[Joel_l]

So all these responses got me curious. I went back and ran more test with a probe set to 10:1 ( was my worse case ).

Probe not connected to anything - about 50mV of noise

Probe and probe ground connected to scope ground - about 50mV of noise ( very faint and fuzzy compared to above)

Probe tip connected to probe ground ( no scope ground ) - no visible noise

Probe tip connected to scope ground, probe ground unterminated - about 200 mV. I think my earlier measurement must not have had a good ground.

I also ran an FFT with CF at 102MHz and could see some peaks from the local stations. None are very strong, my bench is down in my basement cornered by two concrete walls and dirt. Above me is a bunch of sheet metal duct work.

In the end, I'm not worried about anything I see. A good refresher on probing though.

Joel

[macboy]

For those who wondered, here's the FFT of the signal. You've guessed it, it starts at 88 and ends at 108 MHz: it's the entire FM band! ("duh" moment, as JPortici said ).

So, the question remains. Is there any way of reducing/shielding from this kind of noise? ("no" would be a sad but valid answer). Should I cover the entire room with tin foil? 

Not tin foil, but chicken wire would work at these frequencies. Save the tin foil for microwave frequencies like radar and Wifi.  I'm only half joking.

You are obviously close to radio transmitters. Every piece of wire is an antenna. Try snap-on ferrite beads on the power cable, near the scope. You might need more than one. For ~100 MHz I would avoid looping the cable through the ferrite multiple times, this is effective for lower frequencies but hurts high frequency performance. You can also try a ferrite on the probe cable. The signal should be "safe" inside the shield, but it will reduce the RFI. You will want to test for negative effect on the signal at high frequencies.

[guillep2k]

You are obviously close to radio transmitters. Every piece of wire is an antenna. Try snap-on ferrite beads on the power cable, near the scope. You might need more than one. For ~100 MHz I would avoid looping the cable through the ferrite multiple times, this is effective for lower frequencies but hurts high frequency performance. You can also try a ferrite on the probe cable. The signal should be "safe" inside the shield, but it will reduce the RFI. You will want to test for negative effect on the signal at high frequencies.

Thanks, I'll do that! (It might take me a while to get those ferrite beads).