Siglent 1202X-E runt trigger limit is inaccurate?

[John G433]

My first post, so if I am in the wrong part of the blog, please redirect me.

Persistent problem:  With the runt trigger set up to find a runt pulse, the trigger occurs even though the waveform excursions go beyond the trigger levels.  This occurs with both positive and negative triggers.  Example:  I set the positive trigger levels to 3 V and 1 V, and input 1 kHz square wave of 4 volts.  I decrease the amplitude until a trigger occurs.  The amplitude of the captured trace is 3.44 V.

I can also duplicate the problem by holding the signal level constant while adjusting the trigger limit closer to the signal excursion.

Please see the five screen shots and the chart.

Additional Info:
•   Channel 1 gain.  The error is greater with more volts/division.  See chart.
•   Channel 1 coupling makes no difference.   (AC, DC)
•   Channel 2 is no different.
•   Trigger Coupling:  AC produces the least error.  DC has more (see chart).  High-Frequency Reject is the worst (but I don’t need it).
•   Sine waves have the same problem.
•   Doing the long calibration after a 40-minute warmup does not help.

Do I have a scope problem, or am I doing something wrong?

[EDIT 2023 03 14a]  Probably "this is good as it gets".  Maybe it is good enough for digital work; I was trying for more precise limits for my analog circuit.
Here is the workaround I will use to set limits for MY scope:
1. Increase the gain on the input channel as much as possible and still have the whole signal show on the display
2. Do NOT use Noise Reject on trigger.  This adds hysteresis and makes the trigger deviation greater.
3. Once you set the trigger limit that you want, compensate by move the limit towards the zero voltage about 1/4 of a graticule.  Example: Channel 1 is set to 2 volt/division, so move the runt-trigger limit 2/4 volt towards zero voltage.  This is for setups that have the input channel coupling set to DC, and the trigger coupling set to DC.
[END OF EDIT 2023 03 14a]

[tautech]

Welcome to the forum.

None of the waveforms you capture have a Runt pulse in them so you do nothing wrong other than supplying the scope with a waveform it won't capture a Runt pulse on.

A one off Runt will need a Single trigger setting and the scope with capture and Stop. A regular Runt will show underneath the horizontal trigger point.

This evening when I have more time I can show an example if someone else doesn't.
Oh and BTW, you don't have to Stop the scope to take a screenshot.

[John G433]

I agree the waveform is NOT a runt, so why did the scope trigger on it anyway?  The little triangle at the top of the screenshot shows where the runt trigger occurred.  How can I find a runt if the scope triggers on good pulses?

I am familiar with capturing runt pulses, and I use single trigger.

[tautech]

John, some examples captured with my SDS1104X-E which for this task works just as your SDS1202X-E does.
Waveforms from STB-3, 10x probe.
See P11 https://siglentna.com/wp-content/uploads/dlm_uploads/2018/07/STB3_User-Manual_UM60109_E01B.pdf

First 2 show both pages from the menu and waveform free running triggering on Runt pulses.
Dropped some Cursors in to show the relationship to the Runt duration settings.
3rd is a Single shot with the same settings.

[rf-loop]

I agree the waveform is NOT a runt, so why did the scope trigger on it anyway?  The little triangle at the top of the screenshot shows where the runt trigger occurred.  How can I find a runt if the scope triggers on good pulses?

I am familiar with capturing runt pulses, and I use single trigger.

I do not have anymore this oscilloscope model so I can not make test with it.
With some other Siglent model it is trigger hysteresis what make this. (there need be hysteresis, it is mandatory)

The first impression is that possibly the hysteresis is on the wrong side of the trigger level setting line (either intentionally or accidentally). However, the matter needs to be considered and studied a little more deeply in terms of what its effect is in many different situations and setting options. Quite often, some aspects are compromises with regard to another aspect, and sometimes choices are made in such a way that out of two bad options, one tries to choose the less harmful one. In any case, I still don't take a strong position on whether that is a mistake or a well-considered choice that may involve things I don't know..