Line and shadow trace on oscilloscope. What is it?

[erik_]

Hi!

I have recently acquired an oscilloscope and amplified photodetector (Siglent SDS 1202X-E, Thorlabs PDA100A2) to make some measurements, and when testing today I got a strange line and a "shadow trace" on the scope. I'm new to the world of oscilloscopes so I don't know what it is and what causes it.

What is it I am looking at, and why does it happen? In this case I had a 5 kOhm load resistor attached to a BNC T-adapter and the PDA100A2 connected to the T-adapter with a 65 cm BNC cable. A Mini Maglite LED was shining on the sensor from some distance.



I also measured if the screen of my old Nexus 5 phone had any signs of PWM flicker, and I was surprised to consistently see 501 kHz on the screen of the scope. I would have expected a maximum of 50 kHz so I'm not sure if I have some other problem too. The screenshot below does not show a very clear line.



In contrast I also measured an old Galaxy Nexus with a OLED display and got more realistic results in terms of frequency, but I'm not sure it's triggered right.
100% brightness


50% brightness


25% brightness



Any ideas about what's going on and any pointers on how to get it right?

[tautech]

Autoset chooses the most common settings to display a waveform that are not always optimal.

Signals like PWM often show a ghosting either side of the horizontal trigger point and this is normal. If you slow the timebase further this ghosting will become more pronounced.

Start taking control of the scope yourself and learn how to get the best from it.
Have fun.

[erik_]

I see. That explains it then I guess. Thanks 

I have used manual settings too but I'm still getting the hang of it. Have only used it for an hour or two so far.

[tautech]

The trigger is your most powerful tool and it's the most important feature of a DSO to get to master and will take a while except for basic use.

When probing some circuits the probe tip capacitance when set at 1x will influence the signal so for most use we use a 10x probe setting....be sure to set both the probe and its channel to 10x for measurements to be accurate.

Observe and learn what all the display boxes and flags are telling you and when posting any screenshots try to have menus displayed to show us what scope setting are in use so we have better info to guide you when you're seeing something you don't understand.
Have fun.

[erik_]

Some greats pointers. Thank you!

I rarely use the probes since the main purpose for me is to test for flicker in displays and light sources, so the photo detector will be used 99+% of the time.
I'll do some more testing and try to get better at it.

[Brumby]

Yes, scopes are great instruments - but nothing beats experience in knowing how to wrangle them.

Sometimes they lie - and experience is invaluable in sorting out what are lies and what are problems with the operator.  We've all been there at some point - and some of us are still wading through it on occasion.