Does Rigol DS4000 Osciloscope Trace Persistance Work Correctly? - Page 1
Forum section: electronics / testgear

author: martys
author_rank: Regular Contributor
timestamp: July 21, 2017, 05:06:26 pm
content: If I am using my DS4000 series scope to view a periodic pulse in Normal trigger mode, shouldn't the Persistence Time setting to "min" cause the display to clear the display after a few milliseconds?Setup:No Acquire or Single Sweep function used.Normal triggering,edge, level is set to properly trigger withing the amplitude window of the expected pulse.The Normal trigger functions correctly, every time a pulse occurs, it triggers a trace.Trace Persistence is set to "min" (Trace Persistence can be set over a large range from "min" to "infinity" duration.)Time/Div: 1-mSec/DivDelayed Sweep OffThe problem is that the most recent pulse that triggered a sweep remains forever displayed, showing no regard to Trace Persistence settings.If I am monitoring a pulse from a Human or from a device, I must know,at a glance, when the pulses have stopped occurring, and while high pulse repetition rates will display thousands of overlapping pulses, the scope fails to blank a stale display, even when no storage function is activated.On a conventional analog oscilloscope, the persistence of a display is strictly controlled by the phosphor itself, on older storage scope, the trace is remembered and kept displayed until cleared, but on this digital scope, one doesn't know at a glance if the pulse train has ceased.How could this be normal operation?Would an Agilent or Tek scope behave the same?.

author: H.O
author_rank: Frequent Contributor
timestamp: July 21, 2017, 05:14:06 pm
content: That's how the Normal triggering mode works, it keeps the last acquisition on the screen. If you want it to automatically disappear then use the Auto triggering mode, it will trigg on the event and then, when no new events occur within a certain ammount of time it will trig anyway causing a new "sweep". To me, it sounds like your DS4000 is operating as it should.

author: martys
author_rank: Regular Contributor
timestamp: July 21, 2017, 05:27:07 pm
content: The idea here, that at a glance at the display monitoring a pulse, once could easily have the false impression that the patient is still kicking, when, in fact, the patient has long since passed away.Auto triggering mode is useless for the most part as it tends to display a slow repetitive pulse bouncing around all possible display positions, making it extremely hard to see a pulse clearly or if there is any pulse at all, while single-sweep operation fails to show real-time events, only historical events, and no continuous viewing is possible, the display is limited to the single sweep time interval.Obviously, an analog scope would have no trouble showing at a glance, when pulses are occurring and when they've stopped.Why can't the Trace Persistence setting be used to set trace persistence in Normal triggering mode?Does anyone owning a Tek or Agilent or HP or Keysight or LeCroy digital scope notice this same type of functionality with Normal triggering?

author: H.O
author_rank: Frequent Contributor
timestamp: July 22, 2017, 12:30:37 am
content: You need to look at the state of the trigger system, it'll show you something like , , .If it's in  then what you see is on the screen is old, as is microseconds or days - you don't know.If it's in  (or whatever it's called) then a new aquisition is ongoing and will be displayed on the screen ASAP.If it's in  then what you see on the screen is old, usually the last acquisition and the trigger system is disabled.In your example, when the trigger status shows  for a period of time longer than the patient can handle he, or she, is gone. The screen will show the patients last hearbeatI don't have a LeCroy or Keysight but I'm fairly sure they'll behave exactly the same - it's one of differences between analog and digital scopes.

author: martys
author_rank: Regular Contributor
timestamp: July 22, 2017, 02:25:11 am
content: Thanks H20, your reply was helpful but failed to identify the scope you hare usingl.Maybe my post should instead cause scope uses towonder why digital scopes don't include an option to set trace persistence in Normal triggering mode.It seems to me it could be much easier from a user's distant view to see when the display (and also maybe a patient) has become stale.Anyone with a Tek or Agilent scope would be welcome to pen any differences in operation of their digital scope with Normal triggering?

author: JPortici
author_rank: Super Contributor
timestamp: July 22, 2017, 09:18:59 am
content: i am trying to understand your question but i keep failing at it. can you try and rephrase?but i'll try:your scope could show the time when last trigger happened (my lecroy from the 80s does that) i am sure that every scope with segmented memory and a RTC will do thatyou may set persistence to infinte so every acquisition is drawn on the previous one.. or at least this happens in all the scopes i've used, from siglent to rigol to tek to lecroy to keysight

author: martys
author_rank: Regular Contributor
timestamp: July 22, 2017, 02:02:32 pm
content: JPortici, I am just wondering why it is not possible to use the Trace Persistence settings to automatically clear a stale sweep display after the trace persistence time has elapsed.In this way, a quick glance, even from a long distance from the scope display could easily see that a pulse or no new pulse has again triggered the scope after the last displayed pulse and so the possibly critical stop of an expected repetitive event is easily noticed.In other words, it should be easily made an option on a digital scope to behave like a glass CRT scope and be able to choose to not use any acquire and store mode to view signals and so tell instantly and at a fast glance when and when not a trigger occurs.

author: nctnico
author_rank: Super Contributor
timestamp: July 22, 2017, 04:17:29 pm
content: Quote from: martys on July 21, 2017, 05:27:07 pmTry and see whether the trigger hold-off time also affects the auto trigger interval.

author: David Hess
author_rank: Super Contributor
timestamp: July 23, 2017, 07:47:36 pm
content: I had never given any thought to it but persistence on a DSO assumes regular triggers unlike on an analog oscilloscope; persistence on an analog oscilloscope is related to time while persistence on a DSO is related to triggers. I am not sure how you could make a DSO duplicate the behavior of an analog oscilloscope in this respect and I am unaware of any DSOs which support it.Quote from: nctnico on July 22, 2017, 04:17:29 pmIt does on my oscilloscopes but if the trigger frequency is low enough for the automatic horizontal mode to time out, then the holdoff is unlikely to have enough range to help.