How oscilliscope trigger level knob works

Thread Starter

jaydnul

Joined Apr 2, 2015
152
As far as I understand it, the trigger knob on the oscilloscope just adjusts at what voltage the scope will trigger the horizontal sweep (whether is be on the falling or rising edge using the +/- slope switch). Therefore I would assume it would just shift the waveform left or right on the screen, but the trigger level knob actually allows a messy looking signal to be adjusted to a single clean waveform to be displayed on the screen. How does it do that?
 

crutschow

Joined Mar 14, 2008
24,403
Without a trigger, the horizontal sweep will free run at an arbitrary frequency (if the sweep is set to free run "Auto") which is unrelated to the signal frequency.
The signal start will thus appear at a different point for each sweep and thus looks jumbled.
The trigger synchronizes the start of the sweep to the same point on the signal so the waveform now appears stable.
The trigger level determines at what waveform voltage the sweep will start.
 

ian field

Joined Oct 27, 2012
6,539
As far as I understand it, the trigger knob on the oscilloscope just adjusts at what voltage the scope will trigger the horizontal sweep (whether is be on the falling or rising edge using the +/- slope switch). Therefore I would assume it would just shift the waveform left or right on the screen, but the trigger level knob actually allows a messy looking signal to be adjusted to a single clean waveform to be displayed on the screen. How does it do that?
It sets a variable Vref for a comparator that initiates a sync pulse for the horizontal sweep.
 

Thread Starter

jaydnul

Joined Apr 2, 2015
152
But even if there is a trigger the signal still sometimes looks jumbled. If you adjust the trigger knob to some other value, it will clean up. I'm confused how this cleans it up because I would think that if there is any sort of trigger, the sweep will start at the same point on the signal every time and you'll have a clean looking signal no matter WHERE the trigger point is at. Then just turning the trigger knob moves the signal left or right on the screen. But it doesn't work like that, thus my confusion.

Thanks
 

#12

Joined Nov 30, 2010
18,190
I would think that if there is any sort of trigger, the sweep will start at the same point on the signal every time and you'll have a clean looking signal no matter WHERE the trigger point is at.
If that were true, there wouldn't be any trigger knob. The factory would just set the trigger voltage at the right level for all signals that could ever exist. So it seems that your confusion stems from the idea that some signals are different from other signals. If we just make all signals the same, we won't need a trigger level adjustment. Your telephone will sound just like your electric blender, your light bulbs, and your car ignition. Problem solved.

But, seriously, the signal to your ceiling light is a lovely smooth sine wave that never changes frequency or amplitude. A car ignition is a very spiky thing that can't hold still on the screen if you rev the engine. A kitchen blender is almost impossible to synchronize because it's almost all a great hash of noise. The audio in your phone is another difficult to synchronize signal because it's always changing frequency and amplitude. There is no such thing as a one trigger level that suits everything and there are some very difficult wave forms that refuse to be synchronized.
 
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Generally, you have level, slope, source, and auto.

First auto trigger, it's essentially uses some pre-determined creiteria, with one addition. If it doesn't see what it expects, it triggers anyway. The purpose is to get a trace unless it's off the screen. This is where the beam finder comes into play.

Something that may be overlooked is the trigger source. Sometimes you can only trigger on Channel 2.
There can be TV line, TV Field, External etc.

If you have a sine wave for input, when you change the slope, the waveform will invert. Level will appear to shift the waveform because it is now starting at a different point.

So, for trigger to work reliably, you need a periodic signal or an external signal related to what your looking at.
 
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