# How does external trigger work in an oscilloscope?

I'm having hard time to find any easy explanation to understand how triggering and especially external triggering works.

And it is even more confusing when a function generator's SYNC/TTL output is used for that purpose.

It is not easy to illustrate in mind what is happening when using a function generator to external trigger the oscilloscope just by reading a text.

I also tried find if there is similar question which could be a duplicate but I couldn't find.

Is there an easy way to grasp or illustrate the idea behind external trigger?

• Do you want to know how it functions (hint: it functions just like any other normal trigger, so first learn about those) or what you could possibly use it for? Commented Jun 10, 2016 at 10:19
• I want to know how sync pulses are used by the scope to detect the period of an arbitrary signal. Commented Jun 10, 2016 at 11:05
• Let's go back in time in an era where there were only analog scopes with no memory. Suppose you want to see what happens to a certain voltage when you power up your circuit. The whole transient is maybe 2 ms long. You could not see it on the scree, it's too fast. If only you could make it repeat over and over again, you could get a persistent waveform on the screen. But you also want the transient waveform to be superposed starting from the same point. So, you use a square wave to power up/down your circuit and as an external trigger. You end up with repetitive transient waveforms all in sync. Commented Jun 10, 2016 at 11:35
• @user16307: They are not used to detect anything, they are used to trigger, that is to start display in a CRO or to start capture in a DSO Commented Jun 10, 2016 at 11:39

Think what would happen if the scope didn't trigger at all - it would display several cycles of a sinewave (e.g.) across the screen and then continue to display a 2nd series of sinewaves but they would not be superimposed on top of the first set of sinewaves because the chances of the sinewave frequency exactly matching the timebase scan time is very, very small. At best you would see a moving sinewave on the screen and this is useless for inspecting the waveform. Here's one slowed down: -

How would you go about trying to estimate the period of this sinewave as its roll-rate speeded up?

You need a stationary waveform so, trigger works by displaying a series of sinewaves then waiting until the sinewave amplitude is correct before updating the display: -

I've cheated a bit with the picture but the same basic thing happens. After the 1st acquisition period (aka first scope scan) there is some dead time while the scope waits for the sinewave to be exactly in phase with the original scan thus, the 2nd quisition period (aka 2nd scope scan) overlays the waveform on top of the original waveform. In a good scope the "dead time" would be very small but it can never be instantaneous because it has to wait for the waveform to be in the right position.

Here's an example of two scans of a triggered and untriggered waveform on a scope screen: -

An external input just triggers the scan.

And it is even more confusing when a function generator's SYNC/TTL output is used for that purpose.

Well, for a regular periodic waveform there is no need to use a sync output because the scope will be more than capable of self triggering and displaying a stationary waveform. However an arbitrary waveform that repeats periodically is a problem: -

Clearly, if the above waveform repeats and you want to display it on your scope you need something like an external trigger event that can ensure the waveform doesn't continually roll across the screen. That's where the sync pulses (in red) come in. Because the generator "knows" the start position of the waveform it can generate an unambiguous marker that can retrigger the scope.

• "However an arbitrary waveform that repeats periodically is a problem" What do you mean by "arbitrary periodic waveform" Is that because it crosses zeros several times in a period? Commented Jun 10, 2016 at 10:55
• It's pretty much that and also can't be reliably triggered on some unique amplitude event. See the last picture. Commented Jun 10, 2016 at 10:56
• so do you mean sync's frequency should be exactly same with the arbitrary signal's? Commented Jun 10, 2016 at 10:57
• I don't understand how that relates to a signal generator producing an arbitrary signal. How does the 2nd signal generator "lock-in" sync with the first generator? How can it? Commented Jun 10, 2016 at 11:16
• It's an easy feature to give and can be used as a clock source in a digital circuit. I've hardly ever used that type of output! Commented Jun 10, 2016 at 13:47

Do you understand how the internal triggering works ? It triggers (starts the timebase) when the trigger level voltage exceeds a certain value (usual default setting, you can change that of course).

The external triggering does the same but on an external voltage.

There's a sync output on a function generator, it generates a square wave that is synchronized (same frequency, same phase) with the (analog) output signal of the function generator. This square wave usually has a large (5 Vpp) amplitude, that is easy for triggering :-)

You can use this signal to start the trigger on the oscilloscope, connect the sync output of the function generator to the external trigger input of the scope and let it trigger on that. Now if you observe the output signal of the function generator, the scope will always trigger even for extremely small voltages making it easier to see these.

Try looking at a 10 mV, 1 kHz sinewave signal from the function generator with and without external triggering and you will get the idea.