Rigol DS1054Z, what is my signal suffering of??

[Houseman]

Hi forum.
During a field measurement of a 26V PLC digital input counter (the sensor is a open collector output normally closed so D0 ticks when Input goes high) applying my CH1 Rigol to the PLC port, (ground to 0VDC) I got those strange noisy output I cannot understand. I have enabled 10x probe so the value you see must be divided by 10).
What the heck is going on??
Conducted noise from 230V mains input?
Any hints very appreciated. Best regards and thanks.
Steve

[tkamiya]

Do you have your cellular phone near by? 

It can't possibly the real reading of p-p over 400 volt.  Period of 8 second suggests your phone pinging the tower.

[Houseman]

tkamiya thank you.
No, unluckily. I was inside a natural gas cogeneration plant, nearby the turbine, where the GAS smart-meter sends its impulses. What you see in the picture is the reader of the meter that sends the cubic meters burnt pushing high the 26V PLC line.
What I was trying to understand is the ultra noise that appears on the scope
Regards

[Doctorandus_P]

Scope probes often have a switch. or 1x or 10x.
It could be that the switch is in the 1x position, and your scope thinks it measuring 400V, while it only is measuring 40V.

But 400V is also not really excessive in a noisy industrial environment.
If I touch the probe with one hand, and put my other hand near a FL lamp, or grab the (plastic outside of) 230Vac mains, I easily measure 150V, due to some capacitive coupling.

Limiting the bandwith of the scope often helps.
You can also put a resistor parallel to the scope probe, to lessen input impedance, which will attenuate noise (hopefully) much more then your signal.

Another possibility is a bad GND connection between your scope and the test circuit.

[Houseman]

HI DOCTORANDUS_P thank you for your reply.
Correct.
Probe was in 1x switch but setting was 10x
So there was actually 40V measurement.
Ground connection was ok. I have rechecked it.
I was thinking of a strong electric field inside the plant.
the sensor cable that goes to the PLC inside the switch box runs unshielded along with all other power cable also of the turbine in the tray for several meters.
Maybe it picks up the noise by some sort of capacitive or inductive coupling.
What was impressing me is the noise all superimposed to the signal.
Isn't reducing the bandwidth of the scope a way to shadow such a noise source?
Best regards

[Doctorandus_P]

Several meters of unshielded cable in an industrial environment...

Even at home I've got uC's on a breadboard which co completely bonkers If I do not put a ferrite ring on the power cable (5V from simple wall wart). Especially when switching power transformers nearby (I had a particular nasty 50W halogen desk lamp several years ago). But the ballasts of FL lighting is also notorious.

In the end it boils down to that you can not run long cables without filtering. And the amount of filtering needed "varies".
I like little smps modules nearby (on) the circuits I design. The inductor in an SMPS module keeps out lots of nasty noise that will go straigth through a lineair chip such as the LM317. A long time ago I took apart some old printers with a centronics connector. they had pi filters on each signal line from the connector.

EMC testing and compliance is also a part of this. Many thick books have been written on this subject.

So if you limit the bandwidth of your scope, you do indeed "play a bit dirty", but if it gets the job done it's a useful trick.
Better is to not measure directly on the cable, but on the PCB on the "other" side.
If the PCB is properly designed, then your signals will be filtered near to the connector.