Stupid question: SIN wave on DSO when touching probe - Page 1
Forum section: electronics / testgear

author: Bicurico
author_rank: Super Contributor
timestamp: November 21, 2016, 11:51:16 pm
content: This might be a stupid question, but heck, I ask anyway...I am trying out my latest toy, the Rigol DS1054Z.Why do I see a perfect sinus wave when I touch the probe with a finger?Regards,Vitor

author: Skimask
author_rank: Super Contributor
timestamp: November 21, 2016, 11:58:15 pm
content: And what frequency is this 'sinus' wave?

author: N2IXK
author_rank: Frequent Contributor
timestamp: November 21, 2016, 11:59:01 pm
content: Noise pickup from the mains frequency EMF that is all around you....Same thing that causes the hum if you touch the input lead to an audio amplifier.Kinda surprised that it looks like a perfect sine wave, though. Usually pretty noisy...

author: carl_lab
author_rank: Frequent Contributor
timestamp: November 22, 2016, 07:56:38 am
content: Normally a more or less distorted sine wave appears on screen when touching the probe tip because of a combination of capacitive coupling and/or not perfect insulation of the scope's input to power line supply and human body working as an antenna that receives some "electro smog" from environment.

author: cat87
author_rank: Regular Contributor
timestamp: November 22, 2016, 09:11:10 am
content: If the frequency of the sine wave is 50 or 60 Hz (depending if you've got 50 Hz or 60Hz coming out of you wall plug) then that's mains noise pick-up, that is being coupled to the scope , through your body. If it's any other frequency, it maybe another noise source other that mains powr, that is being coupled to the scope... such as an AM radio tower near by, or maybe just the mains Earth is not correctly connected/insatlled

author: timb
author_rank: Super Contributor
timestamp: November 22, 2016, 01:19:23 pm
content: My scope used to show sin waves all the time, so I cleaned all the BNC connectors with some of Peter Popoff's Anointed Holy Water. Now it just shows Jesus fish waves...

author: Bicurico
author_rank: Super Contributor
timestamp: November 22, 2016, 08:03:27 pm
content: Of course you guys are right: 50Hz it is.Strange - I am not touching anything else, am wearing rubber soles or no shoes at all. Where does this hum come from? I noticed that the amplitude increases or decreases according to what device on my desk I touch (PC, different SAT receivers, stereo, etc.).Also strange: I never noticed this hum...@timb: LOLRegards,Vitor

author: Fungus
author_rank: Super Contributor
timestamp: November 22, 2016, 08:17:39 pm
content: Quote from: Bicurico on November 22, 2016, 08:03:27 pmThis is how radio receivers work, only this timeyou'rethe antenna.

author: tautech
author_rank: Super Contributor
timestamp: November 22, 2016, 08:22:45 pm
content: Recent thread:https://www.eevblog.com/forum/beginners/50hz-signal-on-oscilloscope-probe/

author: JPortici
author_rank: Super Contributor
timestamp: November 22, 2016, 08:29:06 pm
content: depending on your local radio stations you may also see a ~90-110 MHz oscillation superimposed on the signal if you don't probe correctly. don't panic, it may be the probe coax resonating

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2016, 09:01:22 pm
content: Quote from: Bicurico on November 22, 2016, 08:03:27 pmProbably you never noticed it because you have never been around sensitive audio amplifiers. If you had ever used your fingers to touch the input of a moving coil pickup pre-amplifier, or a microphone amplifier input, you would be familiar with the deafening 50 Hz hum that comes out of the speakersWhere does it come from? Your house is full of unshielded mains wiring which is emitting 50 Hz fields all through your house. Your body acts as a very effective antenna to pick up these signals and inject them into anything you touch.

author: metrologist
author_rank: Super Contributor
timestamp: November 22, 2016, 10:51:34 pm
content: Quote from: Bicurico on November 22, 2016, 08:03:27 pmsaid the marshmallow in a microwave...

author: tggzzz
author_rank: Super Contributor
timestamp: November 22, 2016, 10:57:54 pm
content: Quote from: Bicurico on November 22, 2016, 08:03:27 pmAs Einstein put it, "... the only difference is that there is no cat".

author: Bicurico
author_rank: Super Contributor
timestamp: November 22, 2016, 11:08:13 pm
content: yeah, yeah, keep making fun of me...The whole story was this: I spend the first weekend with my DS1054Z and on the rainy Sunday, after all family related things have been acomplished, I wanted to try and build a simple sin wave generator with my Arduino.It did not work as expected - welcome to pulsed analogue output... Nice to see, though.So I had a DAC PCB for the Arduino, which I did not solder to the pins (as opposed of Adafruits CLEAR instructions) and it did not work.So I soldered it and it still did not work: no sin output.And then there was the sin curve I was looking for!But something was wrong: the probe wasn't touching the output pin of the DAC but my finger!WTF!So at like midnight I was in stupid mode and wondered what connection in the universe was created between my finger and the DAC.Next day I had to ask, when I was in the office thinking about it.So now you have the whole story and how one can get into trouble for not having a clear head.The DAC, btw. I assume got fried or was a DOA.Interestingly I tried to decode the i2c communication of pin A4 and A5. The device ID is decoded correctly, but not the data itself. The signal looks bad. That is another thing I have to look into. I managed to decode RS232 fine, so I gues the Arduino UNO is not outputting a good signal on A4/A5? On both my Arduinos? hmmm...So right, suits me well, you have a laugh!Cheers,Vitor

author: metrologist
author_rank: Super Contributor
timestamp: November 22, 2016, 11:35:30 pm
content: no, not meant for a laugh. Just awareness of what's happening in the environment and how.It might the code giving you issue. Which dac chip and how's it wired?

author: Fungus
author_rank: Super Contributor
timestamp: November 22, 2016, 11:46:45 pm
content: Quote from: Bicurico on November 22, 2016, 11:08:13 pmThe thing about I2C is that it only 'outputs' half the signal.The Arduino can only make the lines go low, the pullup resistors you place on the lines are what makes them go high.You do have pullup resistors, right?Your DS1054Z will help you choose good resistor values to get a nice clean signal.

author: metrologist
author_rank: Super Contributor
timestamp: November 22, 2016, 11:48:36 pm
content: I thought Arduino has built in pull-up resistors? You need to configure the pin as input pullup, I think...?

author: Fungus
author_rank: Super Contributor
timestamp: November 23, 2016, 02:05:04 am
content: Quote from: metrologist on November 22, 2016, 11:48:36 pmThe built in pullups are 30-40k Ohms - that's far too high a value to pull a wire high iquickly.I2C needs anywhere from 500-5k Ohms, depending on your wires and number of devices. I think the official value is 4.7k (although that's usually too high in my experience - for very very short wires only).Experiment a bit. Watch what happens to the signal - that's what the 'scope is for.

author: AlfBaz
author_rank: Super Contributor
timestamp: November 23, 2016, 02:34:07 am
content: Quote from: IanB on November 22, 2016, 09:01:22 pmHaving played guitar for years, I can't, for the life of me, imagine a world where touching the tip of the guitar lead doesn't produce that sound. I'd love to take a battery powered guitar amp into the bush, far from any mains power and hear what noise comes out of the speaker. Alien signals perhaps

author: HighVoltage
author_rank: Super Contributor
timestamp: November 23, 2016, 10:14:43 am
content: Now, just clip a simple diode to your probe tip and touch the other end of the diode with your finger and you see half a sin wave and have a diode quick test.