Trigger
I’ve already mentioned that the trigger system on the SDS2000 is superb in my book, and now I want to back up this claim with some detailed test results.
The basic settings for the test to follow are:
Only CH. 4 active with DC Coupling, Full Bandwidth, Probe 1x, Impedance 50 Ohms.
Display type dots, persistence 1s.
Acquisition Normal, 28M Memory, Fast Acquisition.
Trigger Type Edge, Source Ch. 4, Slope Rising, Holdoff Close, DC Coupling, Noise Reject Off.
Unless stated otherwise, trigger level is auto-adjusted by pushing the trigger level knob once.
Trigger Bandwidth:
This used to be an important parameter on analog scopes, as it determined the absolute frequency limit for a stable waveform display. The same is to expect for DSOs, when analog trigger systems are used.
Like many modern DSOs, the SDS2000 has a digital trigger system, hence no explicit trigger bandwidth anymore. As a rule of thumb, just about any signal can be triggered regardless the frequency, as long as it appears on the screen with a peak to peak amplitude (minus noise) that exceeds one vertical division. This is true for any digital trigger system and the exact minimum trigger amplitude varies a little between different scopes, we’ll find out what it is for the SDS2000 right now.
Trigger sensitivity:
As stated before, trigger sensitivity is independent of frequency, but trigger hysteresis obviously has some impact. So we start the test with Noise Reject On, which supposedly increases the hysteresis to some fixed (and unknown) value. So there is no adjustment for the hysteresis, but only two settings, Noise Reject On/Off. As mentioned before, we start with Noise Reject On, hence high hysteresis and use a rather low frequency of 200kHz for this test. Reason is – apart from the fact that signal frequency should not matter – that I’m still able to accurately measure the actual signal amplitude at this frequency (rather than just relying on the signal generator setting).
Vertical gain setting for this test is 200mV/div. I don’t go any lower as I want to see the sensitivity of the trigger system alone, where noise of the analog frontend should have as little impact as possible.
With auto trigger level and noise reject, we get reliable triggering down to 310mVpp, which is about 1.5 divisions, see screenshot ‘Trig_NR_min_auto’:
![](https://www.eevblog.com/forum/testgear/siglent-sds2000-new-v2-firmware/?action=dlattach;attach=186898)
Note that the automatic measurements suggests a level of 336mVpp, which clearly isn’t correct (+8.4%).
There is also the ‘Amplitude’ measurement, which should be essentially the same for an undistorted sinewave as this, but it reads only 296mV (-4.5%).
It seems that quantization noise takes its toll here.
But then again, an 8-bit scope is not a precision instrument, all the more so with a signal peak to peak amplitude that is less than 19% of full screen (and less than 15% of full scale for the 8-bit ADC).
Also note the frequency display, which reads complete nonsense (remember, tsignal frequency is exactly 200kHz). AT the same time, automatic period measurement shows the correct value of 5µs.
With manual tweaking of the trigger level we can get down to just 150mVpp, equivalent to 0.75 divisions (Trig_NR_min_man).
![](https://www.eevblog.com/forum/testgear/siglent-sds2000-new-v2-firmware/?action=dlattach;attach=186900)
Automatic amplitude measurements are off again and period shows a ridiculous number. Instead, now suddenly the frequency display has at least a remote similarity with the real value.
Now let’s repeat these tests with Noise Reject switched Off:
With auto trigger level, we get reliable triggering down to 120mVpp, equivalent to 0.6 divisions (Trig_min_auto).
![](https://www.eevblog.com/forum/testgear/siglent-sds2000-new-v2-firmware/?action=dlattach;attach=186902)
Amplitude measurement appears correct here (other than Pk-Pk) and this is also true for the period, while the frequency display shows all kinds of street numbers, just not the true frequency of the input signal.
With manual tweaking of the trigger level we can get down to just 50mVpp, equivalent to 0.25 divisions (Trig_min_man).
![](https://www.eevblog.com/forum/testgear/siglent-sds2000-new-v2-firmware/?action=dlattach;attach=186904)
Automatic measurements are totally off now, which indicates a major offset error. Despite the tiny amplitude, Period appears almost correct and even the frequency display is in the same ballpark at least.
Since we got inaccurate measurements as a side effect of this test, I’ll just verify that they do indeed work as expected, as long as the signal amplitude is high enough. It appears that we need to have a peak-peak voltage level of more than 2 div in order to get reasonable accurate measurements. This is also true for the frequency display (Trig_display_verification):
![](https://www.eevblog.com/forum/testgear/siglent-sds2000-new-v2-firmware/?action=dlattach;attach=186906)
I’ll end this post to draw a conclusion on trigger sensitivity. And I still think it is just superb – clearly as good as it can get.
The automatic measurements and all the more so the frequency display on the other hand let us down, if the peak to peak amplitude of the measured signal drops below two divisions.