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Why trigger behavior changes with vertical scale?

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bitbanger:
Hi folks - this has long been something that's really bothered me. Why is it when I change the vertical scaling even one division, I often run into a situation where I have to adjust my trigger to reliably trigger again?

Happened today, using a DSOX4034A for an inrush measurement. I was in trigger normal mode (so I didn't have to repeatedly arm), powering on and off the DUT to capture the current probe signal. As I'm honing in on the capture settings, I'm increasing vertical scale (higher V/div ie 1V/div to 5V/div), powering on, adjusting, powering on, adjusting timebase, delay, holdoff, etc, rinse and repeat. Well after I reach a certain point when I increase the vertical scale, I stop triggering. Drop back and I can trigger again. Say I'm measuring a peak inrush of 40A, my trigger was set to rising at 2A level. Now, just for example, I have to raise my trigger level to 10V to trigger again. I didn't honestly remember if I was increasing V/div or decreasing but you probably get the idea.

My suspicion is what I'm seeing in screen is essentially used to "software" trigger? And so with more on-screen vertical dynamic range with increasing V/division, I need to pull my trigger level out of the weeds so to speak?

Is there absolutely no analog trigger circuitry in modern storage oscilloscopes? Why is trigger ability dependant on vertical scaling?

I guess maybe I'm hung up on how an analog scope trigger worked, you set a level and that was that.

This has been something I've struggled to explain eloquently so would love to hear your thoughts.

nctnico:
The reason is simple: the trigger input is taken from the vertical signal. So if the vertical signal gets larger / smaller, the signal to the trigger changes amplitude as well. And thus the triggering is affected.

bitbanger:
Thanks for the reply. I suppose I was confused because I thought the signal used to compare for the trigger would not change - it is absolute. As does the trigger level setting. Perhaps I need to find a good block diagram of a modern front end (or check my assumptions about what is no different than a traditional setup)

eTobey:
You have probably set the trigger quite high with falling edge or vise versa? There is something called hysteresis.

rf-loop:

--- Quote from: bitbanger on May 21, 2024, 09:43:52 am ---Thanks for the reply. I suppose I was confused because I thought the signal used to compare for the trigger would not change - it is absolute. As does the trigger level setting. Perhaps I need to find a good block diagram of a modern front end (or check my assumptions about what is no different than a traditional setup)

--- End quote ---

In digital oscilloscopes what use analog trig, typically signal is splitted  to trigger pathway just  before ADC
So level and offset changes in frontend before this split affect.

Lot of different if compare with old analog oscilloscopes where trigger system see whole signal. (when digital trigger see only this vertical band what is ADC range) 

If look 10V signal and just top of it using 100mV/div then ADC range is around 1V or bit more... and all what trigger system can know is inside this band)

If trigger system is fully digital. Full digital trigger engine is reading digital data from ADC only. What is not inside this data, trigger engine can not know.
Very extremely different if compare with old analog scopes or these older digital oscilloscopes what ahe analog sidepahway trigger.


Old simple image from my some "teaching" material when Siglent just started produce full digital trigger. With full igital trigger all happen after ADC using data from ADC.



Old a rough overview of what it consists of.

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