Could someone please explain to me what's the functionality of the trigger coupling in an oscilloscope (the difference between ac trigger coupling and the dc trigger coupling) ?

Thank you .

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    \$\begingroup\$ Try observing 50mV ripple on a 100V signal in DC coupling \$\endgroup\$ – PlasmaHH Aug 27 '18 at 11:16
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    \$\begingroup\$ Same as the difference between AC and DC coupling on the regular inputs. \$\endgroup\$ – JRE Aug 27 '18 at 11:37
  • \$\begingroup\$ @JRE I do know that the AC input coupling removes the DC component whereas the DC coupling doesn't , but I don't understand what the triggering has to do with the DC component being present or not ,we can see some videos on YouTube ( ex : youtube.com/watch?v=OFGm-Pel4Hg#t=5m40s ) ,when he actually enables the DC trigger coupling the wave form starts to slide in diagonal fashion across the screen , why is that ? \$\endgroup\$ – Hilbert Aug 27 '18 at 12:01
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    \$\begingroup\$ The trigger is some fraction of the signal voltage. If the signal includes some high DC voltage, then it will be very difficult to trigger on a small change. If you remove the DC, then you can trigger on small changes of the AC part of the signal. Just like how you can't see a 100mVAC signal riding on top of 100VDC. \$\endgroup\$ – JRE Aug 27 '18 at 13:10
  • \$\begingroup\$ Thank's a lot it certainly became more clearer , but I still can't relate it to the reason of the diagonal translation ,in the video (link above) , when he turned from ac to dc and started turning the trigger level knob ? \$\endgroup\$ – Hilbert Aug 27 '18 at 16:04

Let's say you have a scope with an "intrinsic" sensitivity of 10 volts full scale. That is, with no amplification it shows a range of 0 to 10 volts. Consider a 0.1 volt AC signal riding on a 100 volt baseline. How would you display it? Using DC coupling, by scaling down to 10 volts (DC) you could show it, but the AC component would be reduced by a factor of 10 as well. This would have you trying to see a .01 volt signal on a 10 volt signal, or approximately 0.1% of the screen. You would find that very difficult to see. Instead, if you use AC coupling, you can then eliminate the DC, then add a gain of 100 to the result, and see the 0.1 volts occupying the entire vertical range of the screen.

Now let's think about triggering. With the triggering run from the same input signal as the display, it is reasonable to think of the triggering circuit as being in essentially the same position as the display. So it should come as no surprise that the trigger will have a hard time reliably picking up a 10 mV level riding on 10 volts. Instead, by using AC coupling and gain, the trigger level can be precisely adjusted to give a stable display.


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