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I don't have a lot of experience working with oscilloscope, it is strange to me that if I connect the scope with a wire and leave the other end of wire floating in the air, I would see a regular sine signal of frequency 600KHz. I suspect this is because of the impedance of the oscilloscope, but adding a 50 ohm terminator doesn't fix the problem.enter image description here
Is that normal ? If it is not normal, how can I fix the problem?

Thank you very much

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    \$\begingroup\$ Look around and switch things off until it disappears, or move around and see where it gets stronger, then you have the thing it picks up. Likely some kind of display or so, maybe even the scopes own LCD. \$\endgroup\$ – PlasmaHH Jul 19 '17 at 13:17
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    \$\begingroup\$ Is there a 600 kHz AM radio transmitter near by? \$\endgroup\$ – alex.forencich Jul 19 '17 at 13:34
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    \$\begingroup\$ Your test would alarm me too. A short open-end wire antenna into 50 ohm terminator? Should give you only 'scope noise. Check cable resistance (especially shield continuity) and verify 50 ohms on that terminator. I've also seen oscilloscope BNC jacks become internally disconnected from ground plane due to over-use. \$\endgroup\$ – glen_geek Jul 19 '17 at 14:21
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There's nothing to "fix". You are connecting your scope probe to an antenna, and the scope is displaying a resulting signal. If you didn't see it, that would mean your scope is brokent. I suppose you might wonder if you are really viewing a 600kHz sine, or an aliasing of a higher frequency, but that's a different issue.

If you want to not see such a signal, hook your oscilloscope up to something you actually want to measure.

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  • \$\begingroup\$ We actually connect the scope to a SIPM ( photo multiplier) board via those wire (not cable ). That also gives us the sine noise which is even larger than the signal we want to detect. So I wonder if there is anyway to diminish this noise ? \$\endgroup\$ – Yanyu Jia Jul 19 '17 at 13:23
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    \$\begingroup\$ @YanyuJia the shielding of the cable should handle this if the cable is appropriately handled in the design of the photomultiplier. Assess performance in the scenario for which it will be used, not some other situation. If noise is still a problem, work in a Faraday cage. \$\endgroup\$ – Scott Seidman Jul 19 '17 at 13:25
  • \$\begingroup\$ @YanyuJia - if you are connecting to PMTs with unshielded wires you're doing it wrong..."any way to diminish this noise" would include proper shielding, of course. If that's not "of course" to you, you need to learn some basics until it is. \$\endgroup\$ – Ecnerwal Jul 19 '17 at 14:04
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    \$\begingroup\$ Hi all. Thank you very much. The problem is fixed by creating a Faraday Cage of Al foil. I think the wire was actually picking up 600kHz radio around. \$\endgroup\$ – Yanyu Jia Jul 20 '17 at 15:29
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Turn everything off, including the lights. For example, fluorescent lightbulbs create lots of interference for measurements. Noise still there ?

You can build a directional antenna like so:

enter image description here enter image description here

If the 600kHz is modulated, you can also use an AM receiver. And then... good hunting!

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It's pretty common to pick up noise at specific frequencies, especially if there's a radio or something like the other comments mention. You can try moving the wires around in a different orientation, or just a straight BNC shunt instead of doing the wire thing like in your picture and see how that works out. Also, try a different physical orientation, room, or building.

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