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MSO8104A - Triggering on digital channel.

Question asked by cwosnitza on Jul 24, 2007
Latest reply on Aug 12, 2007 by cwosnitza

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In my example the rising edge of the digital channel D1 was used for triggering. My question is: Why the trigger point and the rising edge of channel D1 do not coincide (see Figure)? Did I make a mistake?

Regards,
Christoph Wosnitza

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Brig Asay Jul 25, 2007 4:41 PM
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This is an excellent question. You may want to check out your threshold level on the digital channels and make sure that they are the same as the analog channels.

The other difference that you might be seeing is the 8 bit ADC on the analog channels versus the 1 bit ADC on the digital channels. This can cause a slight delay on the digital channels versus the analog channels.
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cwosnitza Jul 31, 2007 6:17 AM
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I agree that the might be a delay between analogue (8 bit ADC) and digital (1 bit ADC) channels, but I'm using the digital channel D1 mainly. The analogue channel is of no interest and has only a passive part. The scope was triggered on the digital channel D1 and there is a delay between the trigger point and the displayed digital channel D1. The threshold for the digital channel D1 and the threshold for the trigger at the digital channel D1 were identical. The trigger and the displayed channel were acquired on the same 1 bit ADC - they have to be at the same position, isn't it?

Regards,
Christoph
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nbrush Aug 1, 2007 1:41 PM
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Christoph,
This is my first attempt at answering a scope question, so I will do my best to make myself clear. As discussed previously in this thread, the digital signals only have a single bit of vertical resolution. The digital signals are sampled using an asynchronously clocked timing analyzer, similar to a logic analyzer. I could be wrong, but I believe for the MSO8104A that this sample rate is 1GSa/s. As a result, triggering off of a digital signal will vary within the period of the timing analyzer clock, one nanosecond (ns) in this case. I hope this helps. Please let me know if that doesn't make sense and I will try to explain further.

Ned
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edgardog Aug 2, 2007 12:56 PM
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Fundamentally, the trigger system is MORE accurate than the display system in this case. It can determine the trigger point to within a few tens of ps.

The display system fundamentally has 1ns of jitter. This is due to the limited VERTICAL resolution of the digital data. The d0 rising edge is within 1 sample of the actual trigger. Unfortunately, that is all that can be expected of the 1ns sampler on the digital channel.
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stewart_lee Aug 2, 2007 5:53 PM
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The actual trigger threshold voltage is different from the digital channel 1bit quantization voltage, if these two voltage are identical, the digital transient will occur at the 0s point.
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cwosnitza Aug 12, 2007 11:35 AM
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Dear all,
thank you for your answers.

Best regards,
Christoph
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MSO8104A - Triggering on digital channel.

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