EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Doogs on February 07, 2019, 10:18:46 pm
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I fired up my DS1054 (hacked) today to test an Arduino RF tx/rx circuit and at boot-up my scope was showing a sine wave of 100mv p-p ac amplitude and a duration of 20ms duration (60Hz) which indicates mains power interference which it's never done before. I tested all four channels with all the probes separately, and it was the same signal on all of them, the signal does disappear when I disconnect the probes. The probe calibration signal is showing 1khz at 3 volts dc with a 500us @ 50% duty cycle square wave so that looks normal. I also disconnected power to all my test equipment and my laptop with no change. I tried a self calibration and no change. I'm stumped so if anyone has any ideas it would be appreciated.
Update:
I plugged my scope into the kitchen GFI with no change so tried my isolation transformer also no change, then tried the ungrounded receptacle on the Isolation transformer and the amplitude doubled to 200mv at the same frequency?! :wtf: Interference on the neutral/ground possibly? I wish I had another scope to compare.
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So what's the problem? Floating probes act like an antenna. This is the expected behavior.
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I've never had them float at 100-200mv before! I would expect 20mv or lower which is usually where my scope starts getting noisy. Maybe I have a better spec'd scope... or used to.
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It depends on a lot of factors, including tip orientation in the space, proximity to wiring in the wall, your body location, etc. It is unpredictable.
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Yep, it's entirely normal to pick up AC noise from the environment like that. If you attach the probe to ground it should go away.
I've once measured in excess of 1Vpp at 50Hz by using my body as an antenna coupling to the probe by touching it!
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Thanks for the replies lads:
Whatever it was it's back down to the 20mv range. I realised we had some freezing rain last night and I have some high tension lines outside the window which were pretty covered posssibly radiating more than normal (1.5 meters away) It's thawed up a bit so maybe that was it. I also had the grabber attached which adds a fair bit.
Well I'm glad that is cleared up as I just lost my Spectrum Analyser (it won't lock) so I'd be bummed if my scope had of been on the way out. The SA is pretty damn old anyway (Anritsu MS2601A) and I can justify replacing it now ;).
Cheers