Stupid question: SIN wave on DSO when touching probe

[Bicurico]

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

[Skimask]

And what frequency is this 'sinus' wave?

[N2IXK]

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...

[carl_lab]

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.

[cat87]

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

[timb]

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...

[Bicurico]

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: LOL

Regards,
Vitor

[Fungus]

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...

This is how radio receivers work, only this time you're the antenna.

[tautech]

Recent thread:
https://www.eevblog.com/forum/beginners/50hz-signal-on-oscilloscope-probe/

[JPortici]

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

[IanB]

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...

Probably 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 speakers 

Where 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.

[metrologist]

Strange - I am not touching anything else...

said the marshmallow in a microwave...

[tggzzz]

Strange - I am not touching anything else

As Einstein put it, "... the only difference is that there is no cat".

[Bicurico]

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

[metrologist]

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?

[Fungus]

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...

The 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.

[metrologist]

I thought Arduino has built in pull-up resistors? You need to configure the pin as input pullup, I think...?

[Fungus]

I thought Arduino has built in pull-up resistors? You need to configure the pin as input pullup, I think...?

The 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.

[AlfBaz]

...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 speakers 

Where 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.

Having 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

[HighVoltage]

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.