I'm sure this belongs to the "every once in a while asked again" questions, but I've read several posts on this site and also on other sites and haven't found an answer to two of my questions, yet.


The LINE-Trigger is supposed to trigger whenever the signal has 50 Hz as frequency. It makes sense to me that 100 Hz, 200 Hz, and so on are triggered, since instead of one period it does 2. However, when I put in an 75 Hz signal into the oscilloscope it still triggered and I dont' understand why. The points where the 50 Hz signal is 0 are also points where the 75 Hz signal is 0, but while the 50 Hz one is increasing (if it's a sine-signal) the 75 Hz isn't. So does it really only matter that the signal is 0 at time 0 and then after 1/50 seconds it's zero again?


The second question I have is about external triggering. According to the sources I read "the external signal is used to trigger". For me, this sounds as if the oscilloscope triggers, when the input signal is exactly the same as the external trigger signal. However, I used a function generator and used the external trigger output and also let the oscilloscope display the signal, it was a square voltage. I then gave a sine input (which I wanted to use the external trigger to trigger for).

(For me) surprisingly, it still triggerd even tho the sine signal had a different form than the external trigger (square voltage). The sine signal was increasing when the square voltage was positive and the sine signal was decresing when the square voltage was negative. So not only the shape was different but also when the sine signal reached it's peak, the external trigger jumped from negative to positive, so the 0 points didn't match. The only way I could explain this to myself is that the oscilloscope only triggers when the frequency of external trigger and input signal are the same.

Sorry for that much text, I just wanted to show you that I have already thought about this and I'm not just to lazy to read other posts.

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    \$\begingroup\$ you asked only one question and it is unclear what you are asking ..... the trigger signal is not necessarily 0V at time 0 ..... it will be at the voltage of the trigger level setting \$\endgroup\$ – jsotola Mar 31 at 22:00
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    \$\begingroup\$ Questions here really need to be singular. Your second is based on a complete misunderstanding of external trigger mode; the only comparison is of the external signal to trigger criteria, not the channel signal(s). \$\endgroup\$ – Chris Stratton Mar 31 at 22:27

If you use the line trigger or the external trigger then the frequency and form of the displayed input channel does not matter. The scope doesn't trigger when the line or external signal are the same as the displayed signal, it triggers when the chosen trigger signal passes through your selected trigger level with your selected slope. If you are looking at a signal that has a different frequency from the line or external signal then the scope will seem to be triggering at random. You should only use these modes when the signal you want to observe is derived from or strongly related to the line or some external signal.

  • \$\begingroup\$ So for the line-trigger it always triggered the same no matter what frequency i used on the input and the signal image didnt move at 50,75,100...Hz because after each sweep (which was initiated by the line-signal crossing the trigger level) the input signal looked the same/was at the same position of its period. when the input signal was for example at 60 hz the period was already over when the line-signal startet again and a new sweep startet. so while the line image stayed the same, the input signal looked different at this sweep - is this right? \$\endgroup\$ – Kekks Mar 31 at 22:44
  • \$\begingroup\$ I'm having a little trouble understanding your meaning but I think you are on the right track. If your input signal is harmonically related to the trigger signal frequency then you may see a stable display, but if the frequencies are not related (like 50Hz and 60Hz) then it will appear as though the scope triggered at random. \$\endgroup\$ – Elliot Alderson Mar 31 at 23:59

The scope has a special box inside: the Triggering.

The Trigger box has Trigger-Source selection, usually just a switch (rotary switch, to accommodate the numerous Trigger Modes).

Once a strong enough input signal arrives at the selected Trigger Sources

----- internal trigger (from one or both of your channels you might be displaying)

----- line trigger (from the Scope's power supply)

---- external Trigger (usually a BNC coaxial input connector)

the Scope then SWEEPS across the screen ONCE.


A Scope uses a V threshold, polarity with options for HPF (AC), LPF, video or BW limit of 20MHz to eliminate long probe ground coax resonance and similar noise and in some cases variable hold-off delay to next trigger. It will not retrace in manual mode unless the same conditions are met. Thus is the S/N ratio is good, a stable display is achieved.

Since the trace is edge triggered, it behaves like a harmonic PLL such that trace-sync is achieved with any harmonic of the input trigger frequency.

It will not retrigger unless the conditions repeat. Normally for AC waveforms, AC couple with some filter options are best with the threshold set near zero crossing.

It is not triggered by level but by the edge of crossing the threshold with the correct polarity selected.

  • \$\begingroup\$ anyone who votes down this answer without a comment is something wrong with them. \$\endgroup\$ – Sunnyskyguy EE75 Apr 1 at 16:02

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