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Digital Storage Oscilloscope - newbie questions

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tip35

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Hello, I have a UTD2025CL digital storage oscilloscope. In this thread I want to post a few questions regarding this oscilloscope.
1. Is the wave form from the attached screenshots normal ? - I measured this on the output of a simple TL431 shunt regulator, which has 5V output.
2. When measuring ripple voltage across power supply output, the trigger notifications show fast switching between "Trig'd" and "Armed" - is this normal ?
 

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1) what do you mean "normal"? Yes, there will be ripple on a regulator output. It depends on load, input, filtering...
 

1. I mean that on the oscilloscope screen appears more than 1 sine wave. There are a few sine wave overlapped.

2. I saw that when measuring the output of a 24Vac 50Hz or 12Vac 50Hz power transformer, the triggered symbol on the oscilloscope's screen is switching quickly between "Trig'd" and "Armed". Is that normal ? I saw that when visualizing a square wave, then on the screen appears only "Trig'd".
 

tip35 said:
Hello, I have a UTD2025CL digital storage oscilloscope. In this thread I want to post a few questions regarding this oscilloscope.
1. Is the wave form from the attached screenshots normal ? - I measured this on the output of a simple TL431 shunt regulator, which has 5V output.
2. When measuring ripple voltage across power supply output, the trigger notifications show fast switching between "Trig'd" and "Armed" - is this normal ?
Click to expand...

The screenshot on the left shows about 7 mV peak to peak ripple (scope says 9.20 mV peak)
without checking the specification of the TL431, that's probably about right

armed means the scope is ready and waiting for the trigger conditions to be met
when the trigger conditions are met, the scope is triggered and shows a trace on the screen
that is probably normal for your scope and the settings in force when you took the screenshots

your scope is set to trigger on channel 1
the trigger level is shown by the small blue arrow on the right side of the screenshot, just above the center horizontal line
at the bottom center of the screenshot, triggering (reverse video blue T) on channel 1 (CH 1) on rising edge at 3.20 mV

the screenshot on left has a time scale of 20 ns, while on the right it is 100 ns
 

The display is "normal" if you have a persistence mode enabled, which will continue to show previous traces that were generated.
 

I checked the settings and the persistence mode was disabled (off), but if I remove the probe, that waveform disappears.
 

And if I use a higher time base, then on the screen appear only one trace.
 

I have also checked if the wave forms persist on all time base settings and I found that the persistence of the wave form is only from 200nS to 10nS.
 

I think what you are seeing is not one trace but several similar traces appearing to overlap. There is only one trace at a time but it is repeating rapidly and giving the impression of a more complicated waveform. If you change to single sweep mode you will see only one trace before having to manually reset it, it should make it easier to understand.

The traces look absolutely normal to me and it is to be expected that some residual signals will be seen when the sensitivity is set so high. What looks big on the trace is actually very small in reality.

The trigger/armed is also quite normal, it is armed until triggered and the trigger will occur when the height of the trace exceeds the trigger level threshold. If you switch to single sweep mode it will disable re-triggering so you see how the triggering process works.

Brian.
 

betwixt said:
I think what you are seeing is not one trace but several similar traces appearing to overlap. There is only one trace at a time but it is repeating rapidly and giving the impression of a more complicated waveform. If you change to single sweep mode you will see only one trace before having to manually reset it, it should make it easier to understand.

The traces look absolutely normal to me and it is to be expected that some residual signals will be seen when the sensitivity is set so high. What looks big on the trace is actually very small in reality.

The trigger/armed is also quite normal, it is armed until triggered and the trigger will occur when the height of the trace exceeds the trigger level threshold. If you switch to single sweep mode it will disable re-triggering so you see how the triggering process works.

Brian.
Click to expand...
The screen shot utility of the scope is suspect as they are not in persistence mode and the scope appears to be in continuous mode so, I would think the scope would only show a single trace not multiple ones. At least that is what all the Tektronics DSO (and an HP) I've used do. The only way on those brands to get multiple traces is to run it in persistence mode and then do a screen capture after it's run for a while (it's how I've gotten eye diagrams for high speed transceivers signals).

Single shot as Brian says might be the only way to get around this issue on that low end scope.
 

Can you please explain why in the attached screenshot the trace is in a little bit lower position than the CH1 mark on the screen ?
 

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I have checked again, with no probe or other thing connected to the input.

It is normal that the trace to appear lower than the CH1 marking on the screen ?
 

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Spikes in noise suggest that there's still something connected to the channel input. If not, it's input DC offset, can be often removed with oscilloscope calibration procedure. Check the manual.

- - - Updated - - -

Question was already answered. Read manual about Utility/Self Adjust function.
 

I have read the manual, and I already made the self adjustment procedure, as given in the manual.
 

And I found that if I connect the probe to ground right after the moment when I turn the oscilloscope on, then the offset is present on the screen, but if I wait a few minutes after turning it on and then I connect again the probe to ground then the offset is gone.
 

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