I have some concerns about a UNI-T UTD2025CL oscilloscope:

  1. Why there is more than 1 sine wave in the screenshots? Is that normal? The screenshots show a TL431 regulator output. The coupling is set to AC, the trigger is set to CH1.

    oscilloscope waveform 1

    oscilloscope waveform 2

  2. While measuring output of a 50Hz 24Vac transformer, the trigger notification on the DSO screen is switching quickly between "Trig'd" and "Armed"? Is that normal?


1) There are several waves plotted on top of each other. If you want that: it is normal. If you don't want that: try adjusting the scope settings such that you get only one trace. For example use "single shot" mode.

2) It might be normal, depending on how you set the triggering to work the scope might not trigger reliably making it wait in between. If you don't like this: adjust the trigger settings such that you get reliable triggering.

General remark: an oscilloscope is a complex (to use) instrument, it requires practice. Don't be afraid to try changing some settings to see what they do. Read the manual of the oscilloscope!

Don't ask "is this normal" because if you set the scope in the wrong way you will get a weird/wrong/no plot on the screen. That is normal. You have to use the scope properly to get a usable picture. That also is normal.

  • \$\begingroup\$ I love how you phrased it. This is indeed true for any complex instrument, the concept of "normality" doesn't really apply. The instrument does what you tell it to do :) \$\endgroup\$ – anrieff Sep 3 '19 at 15:45
  • \$\begingroup\$ @Bimpelrekkie I checked the settings of the oscilloscope and the "persistence" mode is Off but the several waves plotted on the screen are present from 200nS to 10nS, then they dissapear when the time base is higher than 200nS. \$\endgroup\$ – mike_mike Sep 3 '19 at 18:16
  • \$\begingroup\$ You're looking at signals with a quite small amplitude and it seems to vary a lot over time. Then you get a different trace depending when the scope triggers. If you "zoom out" (increase timebase to a couple of us or even ms) you might see the amplitude variations. It is normal to zoom in and out to get an idea of what the signal looks like. Not all signals look like a stable sinewave! \$\endgroup\$ – Bimpelrekkie Sep 3 '19 at 20:13
  • \$\begingroup\$ @Bimpelrekkie I found that the trace appears a little bit lower than the CH1 marking. I made a self adjustment procedure, but the same offset appeared on the screen. Is that normal ? Please have a look at the following screenshot. ibb.co/jfCN7fK \$\endgroup\$ – mike_mike Sep 4 '19 at 11:33
  • \$\begingroup\$ Why are you still asking "is that normal"? Ask yourself why the plot looks like it is. Short the input of CH1, do you still see the offset? No? Then it is caused by what you measure, the circuit has DC offset. Yes? Then the offset is inside the oscilloscope. Nothing to worry about, it's just some mV DC which is quite common. \$\endgroup\$ – Bimpelrekkie Sep 4 '19 at 11:38

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