oscilloscope waveform update rate wfms/s

[modrobert]

What does "waveform update rate" (wfms/s) mean exactly regarding oscilloscopes?

I'm assuming that the screen update rate of the LCD showing the waveform is still 60Hz or similar. Eg. if you put a high speed camera next to an oscilloscope capable of 1 million wfms/s, it will not be 1 million screen updates per second, right?

[mikeselectricstuff]

Intensity grading.
E.g. if lcd refresh is 60hz and update rate is 60k. Each display frame is a representation of 1000 acquisutions, where intensity represents how often the waveform occupies each pixel.

[tautech]

It's somewhat related to the pure grunt (or not) of the acquisition HW and internal processor selection in a DSO.
2 similar models from the Siglent stable: SDS1000X ~60k wfms, SDS1000X-E ~100k wfms.

The higher the better to see and capture rare events. Often the more specialized Trigger modes are used or Persistence settings to make rare events more visible.

As Mike says it might be linked to display refresh rate but might not be either.

[modrobert]

mikeselectricstuff,

OK, thanks. In other words short pulses can be displayed for several screen update frames then to be detectable by the human eye?


tautech,

I've seen the reasoning many times, why you should care about waveform update rate etc., but not any clear info how it works in relation to the actual screen update rate of the LCD. The marketing talk is all about catching runt pulses, but with 60Hz screen update rate you are going to miss a lot as a human observer unless the screen is "persistent" as Mike mentioned.

[tautech]

tautech,

I've seen the reasoning many times, why you should care about waveform update rate etc., but not any clear info how it works in relation to the actual screen update rate of the LCD. The marketing talk is all about catching runt pulses, but with 60Hz screen update rate you are going to miss a lot as a human observer unless the screen is "persistent" as Mike mentioned.

There's a few ways to display infrequent events......presuming the scope is fast enough to see them.
Persistence can done in a # of ways, depending on the scope.

Infinite or n# seconds of Persistence; simplest and supported by most scopes.
Intensity grading; common these days in most scopes.
Coloured Intensity grading; where brighter colours represent the highest incidence of a waveform.

These all can be mixed and matched to suit requirements in the DSO's that support such functions.

Much can be done with smart use of Trigger suites however this presumes you know what your infrequent event parameters might be and how to get the trigger to find them.

[mikeselectricstuff]

If intensity is set to max, any event in the capture will appear with equal brightness, so a 1-in-1000 glitch will easily be seen. You can also add persistance to keep infrequent things onscreen for longer.

[bson]

OK, thanks. In other words short pulses can be displayed for several screen update frames then to be detectable by the human eye?

It's really about how many waveforms per second it can capture.  Things like measurements and decoding that can't be done in real time at the higher acquisition speeds will create dead time before the trigger can be rearmed since it can't overwrite the record while it's being processed.  If you're counting events for instance such dead time makes the result useless.  You will end up miscounting back-to-back events.  The wfm/s spec is typically stated with processing set to an absolute minimum, meaning no rms or peak voltage measurements, no decoding, no mask testing, and probably no complex triggers.  But a scope in this mode is rather useless, and where the rubber hits the road is the acquisition rate you can get while making measurements like counting mask test misses.  Things like segmenting also impose blind periods.  This is a much more complex problem, some very high acquisition rate scopes have poor processing, or vice versa and in reality the wfm/s spec is pointless since as you point out it's too fast to process with a Mk1 Eyeball.  The best way to determine the actually useful acquisition rate - which depends on so many factors, including any auto-zero time for active probes and M&D implementation - is to hook up a counter to the ext trg out on the scope.  This is the ONLY way to determine what the blind time is, if any, and the impact of various settings like enabling measurements or the DVM on it.  (Yes, even the free DVM every vendor throws in these days can impose a blind time.)

To be honest, I wish scopes would state on screen the blind time between the end of a record and the next trigger.  This can either be straight-up measured using the timebase, or calculated from the time processing time and record duration.  (Maybe some scopes do provide this?)

[danadak]

Some ref material -


http://www.bitscope.com/blog/DJ/?p=DJ06A


http://literature.cdn.keysight.com/litweb/pdf/5989-7885EN.pdf


http://www.tek.com/learning/oscilloscope-tutorial


Regards, Dana.