Author Topic: oscilloscope auto trigger explained  (Read 868 times)

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Offline stern0m1

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oscilloscope auto trigger explained
« on: October 07, 2018, 12:13:39 pm »
Hi,
Im using the RIGOL DS1054Z.
I want the oscilloscope just to continuously display the voltage levels it reads.
Is auto trigger going to give me this?
Below is how the manual explains it. I dont understand it. Is there no mode where it would just continuously read the voltage levels?
Im a beginner, obviously.  :)

The manual states:

Auto:
No matter whether the trigger condition is met, there is always waveform display. A
horizontal line is displayed when no signal is input.
In this mode, the oscilloscope operates by first filling the pre-trigger buffer. It starts
searching for a trigger after the pre-trigger buffer is filled and continues to flow data
Pre-trigger Buffer Post-trigger Buffer through this buffer while it searches for the trigger. While searching for the trigger,
the oscilloscope overflows the pre-trigger buffer and the first data put into the buffer
is first pushed out (First Input First Out, FIFO). When a trigger is found, the
pre-trigger buffer would contain the data acquired just before the trigger. If no
trigger is found, the oscilloscope will trigger forcefully. If forceful trigger is invalid, the
oscilloscope still displays waveform but the waveform is not stable; if forceful trigger
is valid, the oscilloscope displays stable waveform.

 

Offline Brumby

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Re: oscilloscope auto trigger explained
« Reply #1 on: October 07, 2018, 01:20:53 pm »
The manual states:

Auto:
No matter whether the trigger condition is met, there is always waveform display. A
horizontal line is displayed when no signal is input.


This is the bit you need to understand - and it's pretty straightforward.  For a DC input, there will be a line displayed - and it will be shifted from zero exactly as you would expect.

The rest of the text you quoted is an explanation of the process that happens within the scope - which isn't really of interest for a simple voltage measurement.
 

Offline PTR_1275

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Re: oscilloscope auto trigger explained
« Reply #2 on: October 07, 2018, 01:27:30 pm »
Putting the scope in roll mode might help as well, depending on what you are wanting and hoping to see
 

Offline bitseeker

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Re: oscilloscope auto trigger explained
« Reply #3 on: October 07, 2018, 06:04:00 pm »
stern0m1, yes, auto trigger will make the scope draw a trace regardless of the trigger level that's set. As Brumby said, for a steady DC voltage you'll see a straight line. For a voltage that changes over time, you'll see that as well, but depending on how fast it changes, your time base setting, and your trigger level, it may be difficult to see clearly.

If you want to see a slowly changing voltage drawn across the screen like a graph, that's what roll mode is for. It'll make the screen like a roll of paper with the trace being drawn across it continuously. Roll mode on the DS1054Z is enabled automatically when the horizontal time base is set to 200 ms or longer.

So, give these a try to see what works best for the signal you're wanting to look at.
You don't acquire TEA. It acquires you.
 

Online vk6zgo

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Re: oscilloscope auto trigger explained
« Reply #4 on: October 07, 2018, 11:05:31 pm »
It was a common technique with analog 'scopes to put them into auto trigger,so that they would "free run", tracing a line across the screen.
In DC coupled mode, this was very useful for quick checking of DC supplies, or for adjusting DC voltages for a maximum ot minimum value.

It sounds like this is what you want to do.

Early DSOs that I used did not have an equivalent "free run" function, so we had to try to use the "roll" mode, which was pretty useless, as it wanted to display the DC variation as a slow waveform, making it very difficult to see variations in level to adjust things.

Modern DSOs do have such a mode, so can probably be used in the same way as an analog instrument
(I do not have any type of DSO, either old or new, so the comment about modern ones is based on what I have read in this forum & others).
 

Online David Hess

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Re: oscilloscope auto trigger explained
« Reply #5 on: October 08, 2018, 01:10:50 am »
Early DSOs that I used did not have an equivalent "free run" function, so we had to try to use the "roll" mode, which was pretty useless, as it wanted to display the DC variation as a slow waveform, making it very difficult to see variations in level to adjust things.

Which early DSOs were these?  All of the ones from Tektronix duplicated automatic triggering.



Just in case it is not clear exactly what automatic triggering does, after a timeout period where no valid trigger is detected, it generates a trigger.

Roll mode does not use a trigger but scan mode does.  Some rare DSOs in scan mode operate in roll mode until the trigger is received and then switch to scan mode so the current input is always visible despite the pretrigger acquisition.
 

Online vk6zgo

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Re: oscilloscope auto trigger explained
« Reply #6 on: October 08, 2018, 08:20:08 pm »
Early DSOs that I used did not have an equivalent "free run" function, so we had to try to use the "roll" mode, which was pretty useless, as it wanted to display the DC variation as a slow waveform, making it very difficult to see variations in level to adjust things.

Which early DSOs were these?  All of the ones from Tektronix duplicated automatic triggering.



Just in case it is not clear exactly what automatic triggering does, after a timeout period where no valid trigger is detected, it generates a trigger.

Roll mode does not use a trigger but scan mode does.  Some rare DSOs in scan mode operate in roll mode until the trigger is received and then switch to scan mode so the current input is always visible despite the pretrigger acquisition.

The only early DSO I used for any length of time was a Tektronix THS720A.
I tried to make it give a comparable display to the analog  auto trigger "free running" state, but even scouring the "Operator's Manual" did not reveal a way to do this.
I just now, looked at it on line, with a similar result.
http://download.tek.com/manual/070973105.pdf

A later employer had a DSO & an analog 'scope, both made by Philips.
After similar fruitless efforts to obtain a "free run" like display, I gave up on the DSO, & used the analog instrument.
After all. "Time is money", & the digital instrument did not offer any advantage at the time.p
 

Online David Hess

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Re: oscilloscope auto trigger explained
« Reply #7 on: October 08, 2018, 10:07:16 pm »
The only early DSO I used for any length of time was a Tektronix THS720A.
I tried to make it give a comparable display to the analog  auto trigger "free running" state, but even scouring the "Operator's Manual" did not reveal a way to do this.
I just now, looked at it on line, with a similar result.
http://download.tek.com/manual/070973105.pdf

The manual is not very good but I did find it on page 136:

Normal and Auto Mode. Use Normal trigger mode when you want the
oscilloscope to trigger only on a valid trigger. Use Auto trigger mode
when you want the acquisition to free-run in the absence of a valid
trigger event. Also, choose Auto when you want an untriggered,
rolling waveform at 500 ms/div or slower time base settings.

 

Online vk6zgo

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Re: oscilloscope auto trigger explained
« Reply #8 on: October 08, 2018, 11:29:12 pm »
The only early DSO I used for any length of time was a Tektronix THS720A.
I tried to make it give a comparable display to the analog  auto trigger "free running" state, but even scouring the "Operator's Manual" did not reveal a way to do this.
I just now, looked at it on line, with a similar result.
http://download.tek.com/manual/070973105.pdf

The manual is not very good but I did find it on page 136:

Normal and Auto Mode. Use Normal trigger mode when you want the
oscilloscope to trigger only on a valid trigger. Use Auto trigger mode
when you want the acquisition to free-run in the absence of a valid
trigger event. Also, choose Auto when you want an untriggered,
rolling waveform at 500 ms/div or slower time base settings.


Thanks, David, it seems like I was incorrect.
The manual is not at all up to the standard of Tektronic's classic analog instruments.

I always felt that both the UI & the documentation on these early DSOs was like we used to say about Polaroid cameras.
"As if they had never seen a camera (or in this case an Oscilloscope), & were basing their design on a description sent in a badly mutilated FAX."
 

Online Tom45

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Re: oscilloscope auto trigger explained
« Reply #9 on: October 09, 2018, 12:57:59 am »
Thanks, David, it seems like I was incorrect.
The manual is not at all up to the standard of Tektronix's classic analog instruments.

I always felt that both the UI & the documentation on these early DSOs was like we used to say about Polaroid cameras.
"As if they had never seen a camera (or in this case an Oscilloscope), & were basing their design on a description sent in a badly mutilated FAX."

The manual is actually quite clear. I downloaded it and looked at the table of contents  for Trigger Controls and was referred to page 3-54. Then went to 3-54 and 2 pages later on 3-56 is a chart and accompanying text describing auto mode. Using the manual, Auto trigger mode is easy to find.

As another person that grew up with analog scopes, I submit that the actual problem is the difference between analog scopes with knobs and buttons for everything vs. digital with many things hidden away in menus. Those of us used to analog scope front panel controls can find Auto mode right away. With new digital scopes we need to go to the manual, or poke around in menus until we find it.

The  465 and 2246 front panel examples are obvious but the THS720A menu structure isn't.



 

Online vk6zgo

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Re: oscilloscope auto trigger explained
« Reply #10 on: October 09, 2018, 03:02:41 pm »
Thanks, David, it seems like I was incorrect.
The manual is not at all up to the standard of Tektronix's classic analog instruments.

I always felt that both the UI & the documentation on these early DSOs was like we used to say about Polaroid cameras.
"As if they had never seen a camera (or in this case an Oscilloscope), & were basing their design on a description sent in a badly mutilated FAX."

The manual is actually quite clear. I downloaded it and looked at the table of contents  for Trigger Controls and was referred to page 3-54. Then went to 3-54 and 2 pages later on 3-56 is a chart and accompanying text describing auto mode. Using the manual, Auto trigger mode is easy to find.

As another person that grew up with analog scopes, I submit that the actual problem is the difference between analog scopes with knobs and buttons for everything vs. digital with many things hidden away in menus. Those of us used to analog scope front panel controls can find Auto mode right away. With new digital scopes we need to go to the manual, or poke around in menus until we find it.

The  465 and 2246 front panel examples are obvious but the THS720A menu structure isn't.

I still question the logic of putting information on an often used function 136 pages into the manual.
With the analog Tek Oscilloscopes, such things were in the first few pages.

This is symptomatic of what seemed to have been a radical change in culture between the last generation of analogs & the first generations of DSOs, in both Textronix & HP.

Representatives of both companies who visited my old work, touting the advantages of the (then) new DSOs, would "read from their script", expecting this to be greeted with "oohs! & aahs!".

Once that meagre source of information was exhausted, they seemed to have no answers to issues brought up by people who used 'scopes every day.
Things which were of great importance to us were treated as "nitpicking" by "technological dinosaurs".
 

Offline SilverSolder

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Re: oscilloscope auto trigger explained
« Reply #11 on: October 09, 2018, 04:07:00 pm »

With the analog Tek Oscilloscopes, such things were in the first few pages.



Older instruments (not just scopes) are far more "task focused"...  you can usually get an older device up and running, including remote operation via IEE488 using simple text commands - all within a few hours after unboxing it, even if you've never seen or used one before. 

The manuals for them are usually task focused as well - introduce the instrument, explain its accuracy and limitations, and show how to use it in a basic way, all within the first few pages.

Newer instruments, in contrast, have a thick layer of software on top of their basic sensing features that takes time to learn, and sometimes ends up getting in the way if you want to try something outside the design parameters.  Remote operation usually requires installing software on your PC which also has to be learned before you can do much with it (forget about simple ASCII commands).

Consequently, the manuals have to go through an enormous amount of "preparatory work" before you get to use the instrument, even in a basic way.
 

Online Tom45

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Re: oscilloscope auto trigger explained
« Reply #12 on: October 10, 2018, 03:25:46 am »
I've attached the scope's table of contents.

After the getting started section that a few trivial and probably obvious preliminaries and the Functional Overview ditto, it goes on for many pages of how to do a number of possible tasks (General Purpose Application Examples).  Note the 7 page Understanding the front panel section. Because most digital scope features aren't found on the front panel, and especially for a handheld digital scope with a tiny "front panel", this front panel section just isn't going to get into the kind of detail that analog scope manuals covered in the equivalent section.

Finally a Reference Section deals with all features including those that aren't found on the front panel.

I got to the auto trigger information by looking at the table of contents where I found the trigger controls section. Just as easily someone could have looked in the index where auto mode triggering is shown to be on page 3-56.

The auto trigger info is easy to find using either the table of contents or the index. That information could easily be missed doing a linear search through the 230 page manual.
 

Offline Rohde_TestWalker

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Re: oscilloscope auto trigger explained
« Reply #13 on: November 10, 2018, 09:34:59 am »
Hey stern0m1, here's how the manual explains auto and normal triggering:
•   Auto: in this trigger mode, the oscilloscope will force a trigger if the specified trigger condition is not found.
•   Normal: in this trigger mode, the oscilloscope only triggers when the specified trigger condition is found.

Of course, which mode you should use depends on the type of signal you're trying to measure. If you're trying to measure a DC signal then selecting auto trigger mode will always display the signal. If you're trying to measure an AC signal, though, then you probably want to select normal trigger mode and set the trigger level and slope. Adjusting these levels will give you a stable display, which you can then use to make your measurements.

One of the cool features of modern digital oscilloscopes is the AUTOSET feature. When you press the AUTOSET button on the scope, it will analyze the signal, and adjusts the horizontal, vertical, and trigger settings to display a stable waveform. You can then make your measurement. Hope this helps and you're up and running quickly. Rohde_TestWalker
 

Offline alsetalokin4017

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Re: oscilloscope auto trigger explained
« Reply #14 on: November 10, 2018, 02:10:15 pm »
Autoset may be a cool feature of modern DSOs but it is a crutch, and can really mess your scoposcopy mind up if you depend on it too much.  Some scopes even come with a way to _disable_ the Autoset feature so that student technicians don't learn to rely on it, but rather learn how to set up the scope themselves for their desired measurements. The Rigol DS1054z is one such scope which allows disabling the Autoset feature. (in Utility>Auto Options). Careful, if you disable it here in this menu you will have to use the SCPI command over LAN or USB to re-enable it.

Also, the Auto trigger mode is clearly accessible on the front panel of the DS1054z, simply by pressing the MODE button in the Trigger section of the front panel on the right side center. The current Trigger Mode is indicated both by an LED above the button and also in the top left corner of the screen display. Here, the "AUTO" word flashes when the scope is "free running", and changes to "T'D" when the scope senses an external signal that meets the Trigger parameters set in the Trigger menu. The default is rising edge, voltage set with knob. Since a DC voltage won't have a rising edge, it won't satisfy the trigger condition and the scope will continue to flash "AUTO" at top left, and the trace level will show the voltage.


The easiest person to fool is yourself. -- Richard Feynman
 


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