Oscilloscope trigger definition

Thread Starter

Vilius_Zalenas

Joined Jul 24, 2022
27
Hi,

I am a university student just starting to learn and discover various test equipment functions and the skills required to use the equipment. My question applies not only to the oscilloscopes, bet lets settle on those for this time.

I am trying to develop a profound and fundamental understanding of trigger concept in an oscilloscope. I watched many tutorials and read some articles but still I can not make the ends meet inside my head, as all the information is really ,,dry" and sounds really difficult and confusing to understand. One strongly simplified explanation of trigger I really liked was ,,trigger tells the oscilloscope of when to make a screenshot", however, that is not the scientifical idea I could base my knowledge on.

My first question:

Is the trigger only a defined voltage level (that could always stay as a DC voltage) or is it some kind of periodical voltage signal (such as sine, square, etc.)

I hooked a 2Vpp 100 kHz sine to my entry level Siglent oscilloscope, I tried to play with various trigger options, even tried to use the external trigger feature, but no matter which trigger option would I choose, I could not notice the difference or the pattern of what was changing.

Second question:

What does it actually change when setting the different trigger mode. Would I see a different picture on the oscilloscope display when triggering the signal (lets say a simple square wave) on the rising or falling edges (or even those other more advanced modes)?


I also noticed that when there is no signal applied, the scope still depicts noise, and it does so as long as the trigger level is bellow certain point.

So my last question:

What is really happening (from the physical point of view) in the scope as the trigger level (starting at 0V) crosses the point where chaotic scope display starts to show a nice waveform?

I know that these questions might take a lot of time and effort to answer with great details, but it would really give me a strong foundation in understanding the trigger concept. I really value your time and effort. I want to thank you all in advance.

Vilius
 

Papabravo

Joined Feb 24, 2006
19,576
On an analog oscilloscope the trigger was defined as the "input signal" crossing the trigger level in either a positive or a negative direction.

On a modern DSO, which I am not that familiar with, I can imagine that other trigger mechanisms might be possible. For example: "trigger on every 3rd rising edge". If you are looking at asynchronous serial data, you might be able to trigger on a falling edge after two bit time in the MARKING state. Thus, you would be able to see an entire character on the screen.
 

WBahn

Joined Mar 31, 2012
27,875
If you were playing around with the trigger options and not seeing any effect, then it's likely the scope was still in an "auto-trigger" mode which, for many scopes, simply means that it automatically starts a new trace as soon as the current trace is finished. This is also known as "free-running" mode.

In it's simplest form, a trigger is a voltage level such that when the input waveform rises above it, the scope will start a new trace if it currently isn't doing a trace. Once a trace starts, it always finishes, but at the point it does nothing until the next trigger event occurs. All of the scopes I've worked with let you chose whether the trigger level was positive (meaning that it would be triggered if the input was above the trigger level), or negative (meaning that it would be triggered if the input was below the trigger level). A very useful feature was the "trigger hold-off", which set a time delay after a trace finished that disabled the trigger circuit until the delay expired. This allowed you to lock onto a particular trigger point of a repetitive waveform that might be longer than your scope trace, which would permit you to zoom in on the waveform near the trigger event.

Storage oscilloscopes also generally have a "single-trigger" mode in which the screen is blanked and one trace is captured when the trigger criteria are met and then the trigger circuits are suppressed until reset while the captured trace is displayed continuously on the screen. This permits capture one-time transient events.

Modern scopes, particularly DSOs and MSOs, have a multitude of other triggering options, but the idea is still the same. Suppress the generation of a new trace until the triggering criteria are met, then generate one complete trace on the screen, then repeat this process.
 

tautech

Joined Oct 8, 2019
306
For a better understanding of the modern DSO trigger imagine that the trigger fires when a user specified condition is met.
Auto mode keeps a visible waveform on the display even with a disconnected input while Normal mode that some prefer only displays a waveform when the trigger conditions are met.
Single mode OTOH displays only the first waveform that meets the trigger conditions and ignores all following. Imagine it as a Single capture that it really is.

Vilius_Zalenas
To go into greater detail about DSO triggers it would be helpful to know which model Siglent you own.

This has the potential to develop into a pinned thread , so deep is the subject on DSO triggers as they are our most important tool and knowing which to use on each occasion is another skill to develop.
 

MrChips

Joined Oct 2, 2009
27,679
Every oscilloscope has three control sections.
1) Y amplifiers. These control the height and vertical position of the displayed waveform.
2) Time sweep. This controls the time scale on the horizontal axis.
3) Trigger control. This controls when to display something on the screen.

Of the three sections, the trigger control is the most important and difficult to understand and master. It is important that you master this section if you want to obtain full benefit of your instrument.

The way the trigger section operates on an analog oscilloscope differs from that of a digital oscilloscope. The purpose remains the same.

In the simplest form, imagine that you have a camera set out in the field. The purpose of the camera is to capture images of wildlife when something approaches the camera. The camera is triggered by a sensor circuit when an animal is detected. The camera fires and takes a photo.

That is what the trigger circuit on the oscilloscope does. It records the waveform when the trigger condition is satisfied.
 

Thread Starter

Vilius_Zalenas

Joined Jul 24, 2022
27
For a better understanding of the modern DSO trigger imagine that the trigger fires when a user specified condition is met.
Auto mode keeps a visible waveform on the display even with a disconnected input while Normal mode that some prefer only displays a waveform when the trigger conditions are met.
Single mode OTOH displays only the first waveform that meets the trigger conditions and ignores all following. Imagine it as a Single capture that it really is.

Vilius_Zalenas
To go into greater detail about DSO triggers it would be helpful to know which model Siglent you own.

This has the potential to develop into a pinned thread , so deep is the subject on DSO triggers as they are our most important tool and knowing which to use on each occasion is another skill to develop.
My scope is Siglent SDS1204X-E
 

MrChips

Joined Oct 2, 2009
27,679
Now that we know that you want to learn what triggering does on a digital sampling oscilloscope (DSO) we can focus on that.

Instead of the single frame camera in my wildlife photography example, imagine a dashcam in an automobile. The camera is running all the time saving full motion videos. Now a crash occurs. This is your trigger event. You can now access the saved video and examine seconds (or minutes) leading up to the crash and seconds (or minutes) following the crash.

This is what the triggering section does on a DSO. You get to set the conditions that define a trigger event. The DSO will playback the recorded video with the trigger time in the middle of the screen or where ever you want the trigger time to be with respect to the screen. In other words, you can view what came long before the time of the trigger event or long after the time of the trigger event depending on the memory storage capacity of your DSO.

Your Siglent SDS1204X-E can store up to 14 million data points.
 

tautech

Joined Oct 8, 2019
306
My scope is Siglent SDS1204X-E
Thanks.
For simplicity sake imagine the trigger uses a comparator of which a threshold is set by the user, normally a rising or falling edge but in this model the full range of triggering capability is much more complex and wide.

For now we should focus on edge trigger types as they are the most commonly used by far and include the types I mentioned earlier, Auto, Normal and Single.
These 3 with edge triggers are the first to master so to get rock solid triggering that results in a perfectly stable waveform on the display.

For us to understand and comment/help on what you see you must first master taking screenshots to USB so to post them here so we can also see what you do. For this you need a FAT32 formatted USB stick and grab screenshots to it using the blue Print button then attach them to a post.
Be sure to have an appropriate menu visible as it helps if we can see all settings.

BTW, ask anything you like about this model as I was a beta tester prior to public release back in 2017. Still one of the best bang for buck scopes on the market and its little brother SDS1104X-E is a strong worldwide seller.
 
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