EEVblog #1320 - Premature Oscilloscope Triggering - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: July 17, 2020, 02:09:01 pm
content: Don't be caught out by embarrassing premature triggering!Dave demonstrates how and why your oscilloscope may trigger before it's supposed to, and how to solve it.

author: jbaribeault
author_rank: Contributor
timestamp: July 17, 2020, 04:24:50 pm
content: So I've got the Siglent SDS1104X-E that Dave showed doesn't have the HF Reject; what are my options when I see this? Some kind of filtering on the probe?

author: 2N3055
author_rank: Super Contributor
timestamp: July 17, 2020, 04:38:19 pm
content: There are two problems here: fact that you can have pulses that are so fast that you cannot see them at your current timebase, and fact that scope will trigger on them nevertheless.That is exactly why Nnctnico and myself are advocating use of peak detect mode all the time when you're "scoping" around completely unknown signal. If you use peak detect mode, you will see the very short pulses and it will be obvious on what is scope triggering. It is not a solution but removes confusion.Second thing is fact that many scopes today have advanced triggering modes in addition to simple edge trigger. Things like slope, window, timeout, pulse width trigger are there to help with these situations.Or simple edge trigger with HF reject if that does the job.But first thing is that you have to see all the details. In this video Dave nicely illustrated the problem...

author: StillTrying
author_rank: Super Contributor
timestamp: July 17, 2020, 04:38:31 pm
content: Some of us think the SDS1104X-E's trigger is all digital so it can only interpolate between the captured1nsADC samples.So when storing at 10MSa/s or 100MSa/s it's quite possible the trigger level will see some 1ns ADC values that count as a trigger level, but don't make it into the memory or screen trace.- probably.https://youtu.be/GZHnrGIK9V8?t=261Edit:What 2N3055 said, peak-detect should always show the level that caused the trigger.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 17, 2020, 05:45:30 pm
content: Quote from: jbaribeault on July 17, 2020, 04:24:50 pmYou could try a different trigger type than edge detect - for example, pulse width, if your scope supports that.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: July 17, 2020, 06:04:02 pm
content: We've all suffered from premature triggering at some point. No need to be ashamed about it.

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2020, 12:57:09 am
content: Quote from: StillTrying on July 17, 2020, 04:38:31 pmThe limited bandwidth before the digitizer spreads and lowers any pulse so *something* should be visible, but peak detection is the easy solution to not miss it.The trigger has to operate during decimation but that is usually not the whole story. Usually asecond "trigger qualifier" executes after the acquisition to see if the trigger conditions were actually met, and if not, the acquisition is discarded. This is also the point where interpolation of the trigger point aligns the incoming acquisition with the display record.

author: tautech
author_rank: Super Contributor
timestamp: July 18, 2020, 02:09:52 am
content: Quote from: jbaribeault on July 17, 2020, 04:24:50 pmWelcome to the forum.HF and LF are in theeach channelsTrigger Coupling menu.There are several lessons to be learnt from Daves video other than the obvious need for Peak select to be ON if you need to see what was actually triggered on.Obviously mem depth that the acquisition system has to work with is different than what the trigger works with so what we see on the display may have hidden parts if mem depth settings are low.Dave as this video shows was sidetracked into investigating the misalignment of the waveform of interest WRT the actual trigger point which at this point plainly indicates there's some other factors triggering the scope when in fact all he was needing to see was the PSU output rail rising edge.Lifting the trigger level closer to the PSU output level may have it totally miss the glitches and trigger correctly on the waveform of primary interest.The 20 MHz BW limit would also help control triggering of parts of the waveform that weren't of interest.Lots of tools in these little X-E scopes to get the info you need once you understand clearly what that may be.

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2020, 04:00:14 pm
content: Quote from: Dave at 10:07 in his videoOlder DSOs worked that way but newer ones with some exceptions going back at least a decade implement triggering in the digital domain after the analog-to-digital converter but before decimation which has the effect you described where it is possible to trigger on an event which is not displayed, as you showed in your video. The solution is to use a longer record length, faster time/div setting, a trigger qualifier, or peak detection so that every sample point contributes to the display.So there is no separate analog trigger path and this is easy to verify; for DSOs which share a digitizer between channels, set the trigger source to an unused vertical input which would normally share the digitizer, and watch to see if the sample rate for the displayed channel is reduced.High frequency reject is a good solution but comes with a problem, at least with oscilloscopes that actually do implement analog triggering. The low pass filter which implements high frequency reject introduces group delay so the trigger point no longer aligns with the unfiltered signal, which did *not* happen in your example. At slower sweep speeds this would never be noticed but at higher sweep speeds, it can move the trigger point a significant distance or completely off of the display and introduce jitter.This issue also comes up when the oscilloscope's bandwidth filter is not applied to the trigger signal path. The early Tektronix 7A13 vertical amplifier had this problem but Tektronix fixed it in the late model by adding a separate bandwidth limiter to the trigger circuit path. Usually the bandwidth filter could be implemented before the trigger pickoff but not in the case of the 7A13 because it would have interfered with the fast overload recovery feedback circuit. For simplicity many analog oscilloscopes implemented the bandwidth filter after the channel switch but this means it does not affect triggering.

author: TK
author_rank: Super Contributor
timestamp: July 18, 2020, 10:10:42 pm
content: Quote from: jbaribeault on July 17, 2020, 04:24:50 pmJust raise the trigger level

author: 2N3055
author_rank: Super Contributor
timestamp: July 18, 2020, 10:58:08 pm
content: Quote from: TK on July 18, 2020, 10:10:42 pmIt DOES have HF reject... Just not where Keysight has it. It's not in the same menu..Manuals are available, shame nobody reads them..

author: StillTrying
author_rank: Super Contributor
timestamp: July 18, 2020, 11:38:12 pm
content: What happens to the sample rate on a 2 channel 1GSa/s scope when you use the EXT Trig, or even AC LINE as the trigger source?Quote from: 2N3055 on July 18, 2020, 10:58:08 pmOf the 'affordable'scopes I've read them all over the years.Quote from: tautech on July 18, 2020, 02:09:52 amThe trigger's HF & LF Reject is on the Trigger menu.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 19, 2020, 01:52:18 am
content: Quote from: 2N3055 on July 18, 2020, 10:58:08 pmIs HF Reject for the displayed signal the same as HF Reject for the trigger, though? (Perhaps if the scope gets the trigger from the displayed signal...)

author: 2N3055
author_rank: Super Contributor
timestamp: July 19, 2020, 09:22:37 am
content: Quote from: SilverSolder on July 19, 2020, 01:52:18 amYou have selectable "BW Limit" (a 20 MHz low pass filter) for input channel and in Trigger Coupling you can choose: DC: allow DC and AC components into the trigger path. AC: block all the DC components and attenuate signals lower than 8 Hz. Use ACcoupling to get a stable edge trigger when your waveform has a large DC offset. LF Reject: block the DC components and reject the low frequency components lowerthan 2 MHz. Low frequency reject removes any unwanted low frequency componentsfrom a trigger waveform, such as power line frequencies, etc. that can interfere withproper triggering. Use LF Reject coupling to get a stable edge trigger when yourwaveform has low frequency noise. HF Reject: reject the high frequency components higher 1.2 MHz)You would use "BW Limit" to intentionally make your scope 20 MHz scope and filter out higher frequencies because you don't even want to see them on the screen. You wan't to ignore them completely. You would use that, if, for instance, you have a known interference source, you know it's not your problem and you just want to concentrate on your signal.You would Trigger coupling filtering (both HF and LF reject) to keep looking at full signal with all components and full frequency range, but want to trigger on a specific part of it.As I said many times before, even the cheapest scopes have many triggering options that only 10 years ago you had to buy mid range scope to get it. Cheap Rigols, Siglents, MicSigs, Gw Insteks are good enough for large percent of usual scope work. Their more advanced models are even much more than that..Biggest problem I see with people using B brands is expectation they will work exactly the same as their favorite A-brand ( i.e. they expect Siglent or Rigol will have exactly the same naming and have exactly the same menu layout, which they won't) so instead of learning new ways, they proclaim cheap instruments "stupid" and insist that, for instance, "Keysight has best layout" while it is only familiarity of using their equipment for 20 years... Or simply make false statements like "this instrument doesn't have XYZ" because it's not where they expect it and don't even bother to check if that is true. And then, when corrected, they attack back with " if it weren't stupid it would be here where it should be..", and back to square one.I've seen this with musical instruments for years, where musicians would wax poetic about all kinds of keyboards, guitars, effects, amplifiers.. Instead of practicing playing and making music, which might make them better musicians, they waste time talking.Take Chick Corea and give him cheapest, tuned, functional upright piano, and take mediocre player and give him the most expensive Yamaha Grand Piano.. Guess which one will make more beautiful music...

author: tautech
author_rank: Super Contributor
timestamp: July 19, 2020, 09:29:17 am
content: Quote from: StillTrying on July 18, 2020, 11:38:12 pmYes silly me, brain fart !You're quite right as these screenshots from a SDS1104X-E show the channel menu has only the 20 MHz BW limit.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 19, 2020, 10:56:43 am
content: Quote from: SilverSolder on July 19, 2020, 01:52:18 amThe whole point of the video is that the trigger is not allways from the displayed signal, especially if the sampling rate is lower than the maximum.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 19, 2020, 12:22:47 pm
content: Quote from: Kleinstein on July 19, 2020, 10:56:43 amEven if the scope doesn't have a separate analog trigger. Got it -

author: TK
author_rank: Super Contributor
timestamp: July 19, 2020, 03:13:41 pm
content: Quote from: 2N3055 on July 19, 2020, 09:22:37 amDoes it mean that scope is BW limiting to 1.2MHz? Will it trigger on a 50MHz low to high edge signal?

author: jbaribeault
author_rank: Contributor
timestamp: July 19, 2020, 03:26:07 pm
content: Turns out I hadn't updated the firmware on the scope; options are thereLoving this scope - so many features for the cost; simply incredible.

author: 2N3055
author_rank: Super Contributor
timestamp: July 19, 2020, 05:24:38 pm
content: Quote from: TK on July 19, 2020, 03:13:41 pmI don't know how sharp the filter is. But I guess that 50MHz should be attenuated enough to achieve stable trigger on slower parts of waveform..1.2 MHz is interesting choice.. Keysight 3000T has it at 50 kHz (both LF and HF)

author: StillTrying
author_rank: Super Contributor
timestamp: July 19, 2020, 06:03:01 pm
content: Quote from: TK on July 19, 2020, 03:13:41 pmThe trigger's LF Reject only filters the HF noise and spikes off the signal used by the trigger, the BW of the scope doesn't change, and the screen still shows the same original signal + the HF noise. A channel's 20MHz BW limit would also effect the trigger if that channel is used for the trigger.If the trigger's HF Reject is on you'd notice the trigger start to not work so well on >2MHz sine waves. It's very easy to forget that you've been using the trigger's HF Reject or Noise Reject, and then find that the scope's trigger barely works at high frequencies or small amplitudes.

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2020, 07:07:48 pm
content: Quote from: StillTrying on July 18, 2020, 11:38:12 pmTest it to find out; it depends on how the external trigger was implemented. Some DSOs have a separate and simplified analog trigger only for the external in; you might be able to tell if only level triggering is supported. And some include a separate digitizer but no or very limited sample memory.Quote from: 2N3055 on July 19, 2020, 09:22:37 amExactly, nominally the bandwidth limit applies to the displayed signal and the trigger although I gave some examples were that was not true in older instruments.The vertical input AC coupling (high pass, 6 to 120 Hz) and bandwidth limit (low pass, 20MHz and higher) are optimized for different conditions than the trigger's LF and HF reject which are typically around 50 kHz.Quote from: TK on July 19, 2020, 03:13:41 pmIt could completely miss it, just like LF reject could completely miss triggering on a line frequency waveform.

author: StillTrying
author_rank: Super Contributor
timestamp: July 19, 2020, 07:40:43 pm
content: Quote from: David Hess on July 19, 2020, 07:07:48 pmI would if I could but I've only got the CML+ here, and that's different,it can certainly keep 1GSa/s RT (or even 10s of GSa/s ETS) on one channel while triggering off something else, - such as the channel that's switched off.

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2020, 10:59:18 pm
content: Quote from: StillTrying on July 19, 2020, 07:40:43 pmETS implies that it does support analog triggering but that is not absolute and there are some alternative triggering implementation which have not been discussed here. Low and even medium cost oscilloscopes for more than a decade now have largely lacked analog triggering completely simply because of economics; with increasing digital integration, digital triggering becomes essentially free.

author: TheNewLab
author_rank: Frequent Contributor
timestamp: July 21, 2020, 12:12:30 pm
content: Quote from: 2N3055 on July 18, 2020, 10:58:08 pmUsed to read them. Now, a Nikon DSLR manual is over 500pgs! The New Rav 4 SUV has TWO manuals, 1st about 800 pgs the 2nd about 200pgs LOLI fear in the near future reading the entire 3 volumes of Gibbon's work Decline and Fall of the Roman Empire will a quicker read than a product manual.That said, the Siglent is kinda short

author: TheNewLab
author_rank: Frequent Contributor
timestamp: July 21, 2020, 12:15:32 pm
content: Quote from: jbaribeault on July 19, 2020, 03:26:07 pmI am terrible at keeping firmware up-to-date. Maybe dave could do a video explaining the importance of doing the updates..or add on to another related video

author: SilverSolder
author_rank: Super Contributor
timestamp: July 21, 2020, 03:27:16 pm
content: Quote from: TheNewLab on July 21, 2020, 12:15:32 pmIf the equipment is obsolete, I usually install the latest firmware.For current equipment, I normally only update the firmware if I have a problem that is covered by an update... on the theory that usually, no software is ever fixed without breaking or at least changing something about it that you liked!

author: SilverSolder
author_rank: Super Contributor
timestamp: July 21, 2020, 05:17:34 pm
content: Quote from: David Hess on July 20, 2020, 10:59:18 pmI'm having trouble envisioning how external trigger could be supported by "hijacking" one of the data channels - the "hijacked" channel would have to lose some performance if you do that (interleaving, or something like that)?

author: StillTrying
author_rank: Super Contributor
timestamp: July 21, 2020, 07:44:41 pm
content: Quote from: SilverSolder on July 21, 2020, 05:17:34 pmYes we'll have to wait for someone with a DS1202Z-E or SDS1202X-E who knows what they're doingto test the EXT Trig's quality andanyeffects it has on the 2 channel's operation.

author: 2N3055
author_rank: Super Contributor
timestamp: July 21, 2020, 08:04:40 pm
content: Quote from: StillTrying on July 21, 2020, 07:44:41 pmWhy would it have any effect?? EXT is simply comparator going directly to trigger engine..It has nothing with channels data stream..

author: StillTrying
author_rank: Super Contributor
timestamp: July 21, 2020, 08:29:09 pm
content: Quote from: 2N3055 on July 21, 2020, 08:04:40 pmI'm beginning to think even the main channel(s) triggering is done with a fast comparator.

author: 2N3055
author_rank: Super Contributor
timestamp: July 21, 2020, 10:06:43 pm
content: Quote from: StillTrying on July 21, 2020, 08:29:09 pmFor crying out loud, how hard is to understand this, really...Is this antivaxer site ??Scopes today have digital trigger for analog channels and those that have separate trigger in have a separate comparator going directly into triggering engine...All the information is there on manufacturers web sites..

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2020, 10:38:47 pm
content: Quote from: SilverSolder on July 21, 2020, 05:17:34 pmThe external trigger is either analog, with its limitations of trigger modes and this leads to the problem of how to align the trigger to sample clock unless a time delay counter is implemented, or it is a simplified digitizer input which can take advantage of the same digital trigger logic used for the vertical inputs; the digitizer can be simplified because no acquisition memory is required for digital triggering. The digitizer may or may not be shared with the digitizer used for the vertical inputs as would be the case with a 4 channel instrument.What the external input cannot be is a comparator driving a logic input without some way to measure trigger to sample clock delay. What happens when the logic inputs of a mixed signal DSO are used as an external trigger for the vertical inputs?

author: 2N3055
author_rank: Super Contributor
timestamp: July 22, 2020, 07:15:33 am
content: Quote from: David Hess on July 21, 2020, 10:38:47 pmYou are thinking too simplistically. There is no triggering "chip" with physical "trigger now" input. It is all a logical block in acquisition engine.Ext In is simply another variable in. Triggering engine digital block takes care of timing (and trigger point interpolation)Keysight has Ext In implemented same as a digital MSO input, on logic side. On analog side it has 1M input impedance (like standard scope input, but unspecified capacity), provisions for variable probe attenuation and variable trigger level that is freely setable to +-8V, HF filter, DC/AC coupling, hold-off.For a user it provides most of the usual controls that an analog ch triggering would have.Some simply have +-0,6 V (and +-3v with x5 attenuation) like Siglent for instance. Pico has 1M/14pF input, with +-5V range...So every manufacturer will have different ways of doing it. For Keysight it is part of their Megazoom. For rest it is simply a logic block in FPGA bitstream, assisted with some analog parts to interface with real world. I guess every single one will be slightly different, depending on what was intrinsically easy to do on their architecture and priority how important it was to make it simple or not..

author: maginnovision
author_rank: Super Contributor
timestamp: July 22, 2020, 04:02:30 pm
content: I guess the lower end R&S scopes have a leg up there. Limits for Ext In are the same as the analog channels themselves.

author: David Hess
author_rank: Super Contributor
timestamp: July 22, 2020, 06:48:15 pm
content: Quote from: 2N3055 on July 22, 2020, 07:15:33 amIf you have a digital input, then how is the trigger point interpolation done? Are they really going to implement a tapped delay line time delay counter in an FPGA just for this? This is a pretty specialized structure and good performance is difficult to achieve compared to the alternatives.This is why I asked about triggering on the logic inputs because we know they are digital inputs. What happens then? I have yet to see someone run that test but it would be very easy to do. The other test I suggested earlier is whether the external trigger supports all of the same trigger modes as the vertical inputs; if it does, then it cannot be a digital input.The other method I mentioned was used in early DPOs where a digital trigger input went through a pulse shaping network and was then digitized allowing transition midpoint or centroid timing. The advantage of this method over earlier time delay counting methods is that it has a very short rearm time making it suitable for high record acquisition rates but it does not support advanced triggering modes. With a little more cheap logic, the extra analog signal conditioning can be discarded and you end up with a simplified vertical input and no acquisition memory like I described and then it *can* support all of the same trigger modes as the vertical inputs.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 22, 2020, 09:52:25 pm
content: Quote from: David Hess on July 22, 2020, 06:48:15 pmJust had a look at my older Megazoom MSO, a 54622D, and can confirm that it does not support anything other than edge triggering on the external input (with adjustable level, and various filters). The external input is less capable than triggering off one of the digital lines D0-D15 as the external trigger does not support any pulse or pattern modes, for example.

author: David Hess
author_rank: Super Contributor
timestamp: July 22, 2020, 11:40:25 pm
content: Quote from: SilverSolder on July 22, 2020, 09:52:25 pmThe HP 54622D is old enough though that it probably does use analog triggering separate from dedicated advanced triggering hardware which is common for DSOs of that era. The question is how modern DSOs which implement digital triggering for the vertical inputs after the digitizer, typically in an FPGA now, handle triggering for their external and logic inputs.On your HP 54622D, what happens when one of the digital lines is used as the trigger source for a vertical input? I guess what I would do is multiplex all of the digital inputs to a single digital output, which is cheap and easy, to feed an external trigger input so only a single analog trigger path is required and all of the analog triggering hardware serves double duty. That just leaves how modern MSOs implement triggering for their external trigger inputs if they do the same thing; I am dubious that they implement an analog time delay counter and this should be easy to verify. The easy way for them is to digitize the external trigger signal and treat it as a vertical input with no or a limited acquisition record.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 23, 2020, 01:47:46 am
content: Quote from: David Hess on July 22, 2020, 11:40:25 pmI suspect there is a separate trigger circuit for digital 0..7 and 8..15 on this scope, because these two groups can be set to their own trigger levels independently.Your explanation makes sense, that all the trigger comparators lead to the same point inside the Megazoom brain, which takes control after a trigger event and looks at what happened across all the data streams and determines if the trigger is allowed to proceed or not (and adds support triggering on patterns, sequences, pulse width, and all the rest of the advanced triggers).[Edit: this is not correct, see diagram(s) below]

author: 2N3055
author_rank: Super Contributor
timestamp: July 23, 2020, 06:31:16 am
content: Quote from: David Hess on July 22, 2020, 11:40:25 pmErr... no, sorry. Like I said, it is not how it's done. And I'm not speculating or inventing how it might or should be done.EXT input trigger doesn't take any resources from analog ch and adding separate A/D is expensive...And that is Keysight who could have added something in Megazoom while they were at it... Other scope manufacturers have to buy A/D converters chip by chip and it costs real money..Also, there is no need to overthink. I it is not a 5 or 21 ch scope,it is 4ch and Trig in, or 4+16 MSO+ext trig in. Ext trig in is very limited and digital channels will have their horizontal resolution aligned to sampling clock. Whatever you're seeing on digital channels is quantized to scope sampling clock. It is not synchronous with input signal. There is not even attempt to realign digital edges with interpolated trigger point.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 23, 2020, 07:41:57 pm
content: There's a good article that covers the design of the MSO trigger circuits in the attached HP Journal from '97.The analog trigger path seems to just parallel all the possible trigger sources:Then, the digital trigger system appears to integrate analog and digital sources (that presumably feeds into a trigger mux similar to the above diagram)?So, it looks like the External Trigger is completely "out of the loop" on the MSO and acts as an independent entity, and does not use any CPU resources here.

author: thm_w
author_rank: Super Contributor
timestamp: July 23, 2020, 08:54:01 pm
content: Quote from: SilverSolder on July 23, 2020, 01:47:46 amThat should just be the high speed comparator reference voltage. Useful if you wanted to monitor say 1.8V and 3.3V digital signals at the same time.You are only limited by the number of DAC channels present, so two DACs in that case.

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2020, 04:35:45 am
content: Quote from: 2N3055 on July 23, 2020, 06:31:16 amThose diagrams are often simplified, and those HP DSOs are significantly more sophisticated than the entry level DSOs we are discussing. Of course I expect HP to get it right in their custom Megazoom ASIC no matter how they implemented it.QuoteBut an external input trigger which is not digitized requires a delay time counter. So where is it? Your HP example shows it, or at least a block for it, but not details of its operation which is also important because it will limit trigger rearm rate. Besides taking advantage of digital integration, digital triggering can operate in real time and some time delay counters operate in exactly this way.QuoteBut they don't have to buy acquisition memory for it.QuoteThat is why I suggested that someone with one of these DSOs test it!If the digital inputs work like you suggest, then they cannot be used to accurately trigger the oscilloscope. And if the external trigger works that way, then it cannot either. Does anybody remember the triggering problem with Rigol when AC coupling was used? That is exactly the problem I would expect.In my previous post, I pointed out a very cheap way requiring only digital multiplexing for the digital inputs to be funneled into the external trigger and actually work just fine as external trigger inputs. Incidentally, old analog oscilloscopes and DSOs which include word recognizers do exactly this and have no trouble triggering on their logic inputs.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 24, 2020, 02:37:03 pm
content: Quote from: David Hess on July 24, 2020, 04:35:45 amDoes the trigger have to be more accurate than the horizontal resolution being displayed on the screen, in order for triggering to work "well enough"?

author: David Hess
author_rank: Super Contributor
timestamp: July 25, 2020, 12:48:15 am
content: Quote from: SilverSolder on July 24, 2020, 02:37:03 pmNo, but on a DSO, the horizontal resolution is normally much higher than the sample rate because of equivalent time sampling or interpolation. Logic analyzer inputs typically have a resolution of a couple nanoseconds (100s of MSamples/second) (1) but oscilloscope trigger resolution is an order of magnitude higher than the sample rate of a DSO. A 100 MHz (3.5 nanosecond) bandwidth oscilloscope cannot measure a 350 picosecond signal but it can measure the delay between two such signals. Achieving 500 picosecond resolution which is reasonable on a 100 MHz instrument would require a 2 GSample/second sampling clock and there are just better ways to do it.(1) I think some high performance logic analyzers have implemented time stamping with higher resolution than the sample clock similar to segmented memory on a DSO but this is not common and an interesting design puzzle. (2) And as I have pointed out above, it is fairly easy to forward a single or combination of digital inputs to an oscilloscope's trigger circuit to achieve high resolution. I would like to know if various low end MSOs do this; the last time I messed with one, the question did not come up.(2) I know how to do it in the analog domain but I am not sure how to do it in the digital domain.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 25, 2020, 01:40:18 pm
content: Quote from: David Hess on July 25, 2020, 12:48:15 amAaaah.... a flickering light appears in the darkness!So if an example 100MHz scope screen has 1000 pixels horizontal resolution, and the time base is set to 5ns/div, it will display 50ns of data spread over 1000 pixels, or 50ns/1000 = 500 picoseconds per pixel. Since one pixel is the minimum shift between two signals that can actually be displayed on the screen, it means the trigger has to reliably resolve a little better than 500 picoseconds.Wouldn't the digital channels have to resolve phase exactly the same way for the MSO to function properly?Going back to the discussion of cheap scopes: If triggering is based purely off the digitized data stream, you could get reasonably close to the same performance (given a 100Mhz scope) with 1GSa/s by having less pixels horizontal resolution across the screen? [Edit: in the sense that the display is the limitation, not the trigger, so the package as a whole "works as it should". The performance overall is of course still lower than having a 500ps resolution trigger on a higher resolution screen.]

author: David Hess
author_rank: Super Contributor
timestamp: July 26, 2020, 03:07:28 am
content: Quote from: SilverSolder on July 25, 2020, 01:40:18 pmOddly enough that is often exactly how it works at least for oscilloscopes where the processing record and display record are identical but see below about time delay counter resolution. Sometimes the specifications do not give the timing resolution but it can be worked out from the number of horizontal pixels per division. If the automatic measurements use histograms, then the actual measurement resolution is even higher.QuoteIf they did not, then there would be considerable jitter between the MSO inputs and the oscilloscope trigger, and that is usually the case. Usually the MSO timing resolution is limited to the logic input sample rate. In the past, implementing a time delay counter on every logic input would be prohibitively expensive except for specialized applications but maybe someone manages it now with increased digital integration. Most applications still do not need it though.That is why I asked about triggering off of the MSO inputs to display a vertical input; they might have made special provisions to avoid jitter like I described earlier. But working in the opposite direction by triggering off of the oscilloscope to display the MSO inputs should always reveal the relatively coarse sampling of the MSO inputs.QuoteYou could, but there is no need to, and it would look pretty bad. If the sample rate is high enough, which it has to be to allow digital triggering to be used, then after the trigger occurs and the acquisition is captured, a second stage of triggering can be done on the interpolated waveform to align the acquisition on the display. Or it can be done during triggering with some extra logic.On older DSOs which do not support digital triggering, usually the time delay counter provides all of the alignment information. The actual resolution of the time delay counter is much higher than the resulting horizontal resolution which allows for automatic calibration for each timebase setting as needed. The time delay measurement circuit can be pretty crude yet deliver accurate results after calibration for zero, span, and linearization which is surprisingly easy.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 26, 2020, 08:26:17 pm
content: Quote from: David Hess on July 26, 2020, 03:07:28 amI did some experiments on the 54622D, comparing the analog trigger with triggering from a Digital channel.As you predicted, the jitter on the digital signal is much worse than analog - 5ns exactly - which corresponds exactly to the 200MSa/s sample rate:However... the trigger doesn't seem to be to blame! As an experiment, I switched triggering between the digital channel and the analog channel, and the jitter looks exactly the same.In a further experiment, I looked at the delay between the leading edge of the test signal and the trigger output (external connector on the back of the scope) with a HP 5335a Counter. It seems that when the trigger is set to the same voltage on both Digital and Analog, the trigger performs exactly the same, and there is no difference in the jitter on the trigger, whether using the analog or digital channel as the source.It seems that unlike what happens with the Analog channels, no attempt is made to re-align the displayed signal with the actual trigger point - instead, it is always aligned to the sample clock. So the trigger itself appears to always a "precise" trigger... it's just that the scope doesn't make use of the information, for whatever reason. There are probably good reasons for it (e.g. can you trust all the pulses in a train to need the same adjustment as the one you happened to trigger on). By switching off one of the analog channels, the scope switched to an interleaved 400MSa/s mode on the single digital channel, cutting the jitter in half to 2.5ns. All other behavior was the same.

author: David Hess
author_rank: Super Contributor
timestamp: July 27, 2020, 10:22:38 pm
content: Quote from: SilverSolder on July 26, 2020, 08:26:17 pmBy default you cannot rely on the trigger output when triggering is digital because it is quantized with the same sampling clock as the digital inputs. On oscilloscopes which use analog triggering, the trigger output is tied directly to the trigger comparator in one way or another.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 27, 2020, 11:57:10 pm
content: Quote from: David Hess on July 27, 2020, 10:22:38 pmLooks like we may have a good test? Inspect the jitter between a square wave input signal and the trigger output.If the jitter is always a multiple of the sample rate on the analog inputs, the scope uses digital triggering. Don't try this on the digital channels, though, since they may be sample aligned even with analog triggering.

author: David Hess
author_rank: Super Contributor
timestamp: July 28, 2020, 06:42:58 pm
content: Quote from: SilverSolder on July 27, 2020, 11:57:10 pmIt should not be necessary to rely on the "trigger" output unless the trigger output is what you want to test. Comparing the display of the digital in and vertical in with triggering provided by the digital in, vertical in, and external trigger in if present should reveal as much as possible. The edge should be fast enough to approach the rise time of the vertical input but that is not difficult to achieve.The difference in available trigger modes between the three inputs also reveals something of the design.In practice, the difference between triggering of the digital inputs and vertical inputs should never come up. If it does, then a faster logic analyzer should be used.