interesting thing about oscilloscope at different SEC/DIV

[Chengjun Li]

Hi,

I use oscilloscope to measure a 1000Hz 6Vpp AC signal. I observed different traces shown on the oscilloscope screen which is interesting.

When sec/div is 1ms,

When sec/div is 5ms,

When sec/div is 100ms,

When sec/div is 250ms.

When the sec/div is 50s,


Observation:
1.As you can see, as sec/div changed from 1ms to 5ms, the screen showed more cycles, which is the common sense.
2.When the sec/div is increased to 100ms, more cycles were shown on the screen, but the oscilloscope changes to scan mode.
3.But when the sec/div is increased to 250ms, the oscilloscope couldn't show the true input signal.
4.As the sec/div increased to 50s, what the oscilloscope shown is the same amplitude with the input signal but different frequency.

I have two questions,
1. when the sec/div is increased to certain value, why the oscilloscope change to scan mode?
2. what's the reason of above observation 3 and 4?

Thanks.

 

[Gryd3]

Hi,

I use oscilloscope to measure a 1000Hz 6Vpp AC signal. I observed different traces shown on the oscilloscope screen which is interesting.

When sec/div is 1ms,
View attachment 19565
When sec/div is 5ms,
View attachment 19566
When sec/div is 100ms,
View attachment 19567
When sec/div is 250ms.
View attachment 19568
When the sec/div is 50s,
View attachment 19569

Observation:
1.As you can see, as sec/div changed from 1ms to 5ms, the screen showed more cycles, which is the common sense.
2.When the sec/div is increased to 100ms, more cycles were shown on the screen, but the oscilloscope changes to scan mode.
3.But when the sec/div is increased to 250ms, the oscilloscope couldn't show the true input signal.
4.As the sec/div increased to 50s, what the oscilloscope shown is the same amplitude with the input signal but different frequency.

I have two questions,
1. when the sec/div is increased to certain value, why the oscilloscope change to scan mode?
2. what's the reason of above observation 3 and 4?

Thanks.

What waveform are you vieweing, and can you change the frequency to something slightly different.
I suspect, that it's merely a fluke in that the samples being taken just so happen to line up to appear to be a different waveform. Changing the frequency you are observing will show a drastic change in observations 3 and 4.

 

[JWHassler]

The phenomenon involved is undersampling, also called aliasing... if you're lucky, you will only hear it explained a few dozen times in your life, so start now

 

[Chengjun Li]

What waveform are you vieweing, and can you change the frequency to something slightly different.
I suspect, that it's merely a fluke in that the samples being taken just so happen to line up to appear to be a different waveform. Changing the frequency you are observing will show a drastic change in observations 3 and 4.

I did the thing as you said and did observe different waveform for 500ms.
For 800Hz,

For 15kHz,

I also test other frequency, but I didn't show it here.
However, for sec/div is 50s, I could always get the right amplitude but wrong frequency.
For 800Hz,

For 15kHz,


I think you are right, it is about sampling.