Why is my oscilloscope showing a signal? - Page 1
Forum section: electronics / beginners

author: n_2
author_rank: Newbie
timestamp: November 17, 2019, 04:02:54 pm
content: My oscilloscope channel 2 is showing a signal (2.5V P/P, 88MHz, roughly sine wave) when :channel 1 ground, channel 2 ground, and channel 2 probe are all connected to the same point in my test circuitchannel 1 probe is connected to a different point in my test circuit.Channel 1 is showing the same shape with 4.5V P/P.I don't understand why my channel 2 is showing a signal when its probe and ground are connected to the same point. What could I be doing wrong?

author: StillTrying
author_rank: Super Contributor
timestamp: November 17, 2019, 04:12:23 pm
content: Make sure both probes are on the same X1 or X10. At that high frequency the GND clips and their short length of wire make a lot of difference even if both probes are are connected to the same point.

author: n_2
author_rank: Newbie
timestamp: November 17, 2019, 04:36:54 pm
content: Quote from: StillTrying on November 17, 2019, 04:12:23 pmThanks for the suggestion (and the explanation!).Both probes are on X10 though.

author: StillTrying
author_rank: Super Contributor
timestamp: November 17, 2019, 04:46:30 pm
content: 80MHz to 115MHz often comes from a nearby FM? radio station, but your up to 4.5Vpp seems very high, so maybe a nearby SMPS.You could show a screen shot on about 250ns/Div. so we could see it.

author: German_EE
author_rank: Super Contributor
timestamp: November 17, 2019, 05:00:08 pm
content: Best guess, pickup through either a long ground lead on a scope probe or some sort of PCB loop acting as an antenna. You could also check that you don't have any math modes enabled on the scope that have the second channel as the output.

author: radiolistener
author_rank: Super Contributor
timestamp: November 17, 2019, 05:24:47 pm
content: 80 MHz is too high, you cannot make short circuit on so high frequency, because even short wire has inductance which has significant impedance at 80 MHz.In short, there is no short circuit, you're measuring signal from magnetic loop antenna formed by the probe wires.

author: n_2
author_rank: Newbie
timestamp: November 17, 2019, 06:08:23 pm
content: Thanks!I moved the circuit away from any SMPS (except for the power supply and scope, which I think have linear supplies?). Channel 2 still shows 2.5V p/p.With channel 2 detached from the circuit but with channel 2 ground connected to channel 2 probe, channel 2 signal drops to 80mV p/p still at ~90MHz.The channel 1 and 2 probe cords were touching; if I separate them I still get noise but it's much higher frequency.With cords touching:With cords separated:[ Specified attachment is not available ]

author: n_2
author_rank: Newbie
timestamp: November 17, 2019, 06:09:15 pm
content: With cords separated:

author: radiolistener
author_rank: Super Contributor
timestamp: November 17, 2019, 07:35:48 pm
content: on second picture amplitude of CH2 is too weak for sync. Switch to sync from CH1 or change vertical gain to get large wave on CH2 for sync.

author: n_2
author_rank: Newbie
timestamp: November 17, 2019, 07:58:11 pm
content: Quote from: radiolistener on November 17, 2019, 07:35:48 pm

author: StillTrying
author_rank: Super Contributor
timestamp: November 17, 2019, 09:16:47 pm
content: With only 2.5 samples per division and in dots mode, and going by its large amplitude for that high a frequency, is that 84MHz real or could it just be an aliasing effect....I'm going for aliasing.

author: hermitengineer
author_rank: Regular Contributor
timestamp: November 18, 2019, 06:19:46 pm
content: Quote from: StillTrying on November 17, 2019, 09:16:47 pmIt may even be half that. Isn't the 500Ms/s divided between the two channels? If so, each trace gets 1.25 samples per division. Or does the scope sample one trace on each pass and alternate between them? I remember old analog scopes that did that.

author: tautech
author_rank: Super Contributor
timestamp: November 19, 2019, 08:01:03 am
content: Change Trigger Holdoff to Close (minimum). (press the button)Remove Ch2 probe, is the signal still there ?If yes, then one by one turn OFF ever other thing within some meters proximity.Wallwarts and anything running SMPS are notorious for EMI.Careful elimination will find the culprit.

author: StillTrying
author_rank: Super Contributor
timestamp: November 20, 2019, 06:54:07 pm
content: What's the 84.11MHz 5Vpp signal like on 500 and 250ns/div.

author: hamster_nz
author_rank: Super Contributor
timestamp: November 21, 2019, 03:58:14 am
content: Last time I had something like this it was a "Baby Monitor" on the other side of the wall, in the room next door.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: November 21, 2019, 04:25:36 am
content: I did some work on an electronic dog flap, like a cat flap but bigger.The door output a pulse and the collar had a LC on it that rang back if close enough.I found after lunch every day the circuit got swamped with about 15KHz signal.Turned out next door turned their TV on about that time and the TV line oscillator ran at that speed !Spent a good couple of hours tracking down oscillation in a power amp.Turned out to be my desktop lamp had a CFL lamp in it that radiated badly.Used to work on modems in the 1980's and one day the modem oscillated instead of just amplifying signals.Someone had put the sig gen on top of the scope and it interfered with the scope.Had trouble with 50Hz in a power amp.Turned out my amplifier testbed was poorly laid out with the transformer beneath the amplifier.The magnetic field of the transformer got into the power amp pcb.

author: rf-loop
author_rank: Super Contributor
timestamp: November 21, 2019, 08:10:19 am
content: Quote from: hermitengineer on November 18, 2019, 06:19:46 pmNo. There is not "may be" things.There is facts how it works exactly.Siglent 1GSa/s ADC is divided to Ch1 and 2. Also as can see in image it display sampling rate and it is 500MSa/s. (and IF it is 4 channel model it have two 1GSa/s ADC's, one for CH1+2 and other for Ch3+4 and these models can run 2 channels simutaneously, both using 1GSa/s. For make this possible user need select ch1 OR 2 AND ch3 OR 4 for use)

author: StillTrying
author_rank: Super Contributor
timestamp: November 26, 2019, 08:22:54 pm
content: Quote from: hamster_nz on November 21, 2019, 03:58:14 amI'd bet it wasn't 5Vpp.What did the 84.11MHz @ 5Vpp turn out to be.