What is causing the 50kHz signal on my oscilloscope?

[rachmaninoff]

I've been wondering for a while what the ~50kHz signal is often appears on the oscilloscope. There is an antenna-like electrode involved, so I know how I'm receiving it, the question is where does it come from? I suspected the CRT monitors nearby so they're already powered off. This is a long-standing mystery to me, I've encountered it in different experiments over the years - often it involves a long unshielded oscilloscope probe acting as an antenna.

Your thoughts?

 

[Intuitive]

I've been wondering for a while what the ~50kHz signal is often appears on the oscilloscope. There is an antenna-like electrode involved, so I know how I'm receiving it, the question is where does it come from? I suspected the CRT monitors nearby so they're already powered off. This is a long-standing mystery to me, I've encountered it in different experiments over the years - often it involves a long unshielded oscilloscope probe acting as an antenna.

Your thoughts?

Try turning off the breakers in the house individually until the signal goes away, if it doesn't it's probably an internal signal or it's coming from around your area outside your local.

Maybe somebody is vacuuming their rugs in another house, I know that vacuum cleaners can send signal spikes sometimes.

Side from this, let us know if you find out what it was.

 

[rachmaninoff]

I don't have a 200MHz oscilloscope in my house. It's in a lab of course. I've already tried succesively turning off the electronics in the room - it's coming from outside the room, or possibly the walls. It's not part of the setup itself, because it occurs in entirely different experiments with no equipment in common, and because it goes away when the antenna (electrode, long oscilloscope probe) is removed. The surrounding rooms are empty classrooms (including above/below). There's a computer lab 15m and two walls away. There's a low-power FM transmitting tower about 300m away, but that's order of 90MHz I believe.

 

[Bystander]

50 kHz ain't too far off switching power supply frequencies, suggests the computer lab, or, the scope supply.

 

[Intuitive]

I don't have a 200MHz oscilloscope in my house. It's in a lab of course. I've already tried succesively turning off the electronics in the room - it's coming from outside the room, or possibly the walls. It's not part of the setup itself, because it occurs in entirely different experiments with no equipment in common, and because it goes away when the antenna (electrode, long oscilloscope probe) is removed. The surrounding rooms are empty classrooms (including above/below). There's a computer lab 15m and two walls away. There's a low-power FM transmitting tower about 300m away, but that's order of 90MHz I believe.

I think a lot of Computer Monitors are between 50KHz to 75KHz.

 

[russ_watters]

No, computer monitors are ~50-75Hz.

 

[Bystander]

What shape?

 

[Ivan Seeking]

Fluorescent lights

 

[rachmaninoff]

Fluorescent lights!

I've now figured out that there are actually two audio-frequency signals being picked up. There is a stronger 53.7(3) kHz signal and a weaker 26.(2) kHz signal; the fluorescents were responsible for the weaker 26kHz signal (which I hadn't noticed before). Now there remains a very clean 53.7kHz signal, not caused by the lights! (Not forgetting the 60Hz mains signal!)

Mysteries...

 

[rachmaninoff]

Yup, the 53.7kHz is from the oscilloscope itself! The signal strength correlates nicely with the proximity of the electrodes to the scope.

26kHz: fluorescent lights ballast
53.7kHz: oscilloscope power supply

Strange physics students. Running around in a totally dark lab waving electrodes in the air.

 

[Bystander]

Cheat. Tie a folding chair, or some other metal object to the case ground with a long wire, and see if you get the same correlation with distance between probe and chair. If you do, you've still got some hunting to do.

 

[Averagesupernova]

No, computer monitors are ~50-75Hz.

The verticle frequency may be that (and higher) but the horizontal frequency is usually well above 15 Khz.

 

[Ivan Seeking]

Here is a rather obscure fact that caused me some problems once: The wiring in buildings tends to resonate at around 100 kHz.

 

[dlgoff]

Power companies send rf signals on transmission lines from substation to substation (used for breakers and reclosers operations). Most of the signal probably gets filtered out by the time you get to 115volts. And then there's signals used for residentual billing. Out here in the country its better than sending someone around to read the meter. I don't know what frequecies are used in these cases however.

Regards

 

[nbo10]

Check out any LCD monitors that are around. Physical unplug them or put them on the same ground as the scope.