Is my oscilloscope faulty?

[davenorton]

Hi,

I just purchased a SDS1072CML+ 70MHz dual-channel DSO, but I'm puzzled about how it misrepresents some signals and fails to trigger.

Problem 1) If I change the voltage scale, there's a point where a relay clicks inside the DSO, and the waveform grows a big hump.
Problem 2) With the same signal, which is repeating, adjusting the trigger level within the waveform doesn't cause the DSO to trigger.

The issues can be seen in a youtube video here:

(or if URLs are stripped for new users, here: youtu(dot)be(slash)xdEF_QTQlIs )

Am I missing something obvious, or is this bad behaviour?

Cheers.

 

[dr pepper]

Looks Ok to me,
You'd probably have to spend a lot more for significantly better performance.

 

[dougy83]

...

 

[dr pepper]

I watched the vid again, I didnt see any change in the signal except for around 20 seconds, the tail did look different.
What probe are you using, have you tried a x10 probe and calibrating it first?
Looks like there is some sort of dc effect on the trigger, was the 'scope ac or dc coupled.

 

[davenorton]

The tail is the bit that changes from a flat line to a hump when the range relay clicks. The channel is AC coupled. Using 10x probe doesn't seem to have the issue.

The trigger issue occurs for AC or DC coupled input. It's more than a third of a division that's ignored when the signal is near the centre, and becomes worse as the signal is moved up the screen; e.g. when moved to the top div, the trigger level had to be 0.9 divisions above the desired trigger level. There's a similar but opposite issue when the signal is moved to the bottom of the screen. It's as if the signal going to the ADC and that going to the trigger have markedly different gains or offsets

 

[dr pepper]

Yep sounds like your right.
I wonder of the manufacturers would have anyone who'd take a look at the vid.
Probably not.

 

[davenorton]

It doesn't hurt to ask the distributer, I guess. They're in my country.