EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: laney on November 12, 2015, 11:30:43 pm
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I was recently playing with one of these WS2812b LED strips. These take 24bits per "pixel" at 800khz over a single data line, with the length of the duty cycle of each pulse indicating 0/1 (spec (https://www.adafruit.com/datasheets/WS2812B.pdf))
So pushing 3 of these should show me 72 pulses in the scope - which is exactly what I captured (first image). Btw, notice how "pulses" in the left vertical menu becomes "pluses" in the horizontal measurement area.
However, as I change the timescale from 20us to 50us and then to 100us, the pulse count drops to 68 and 31, respectively (second image). It looks as though the pulse count is determined not from the actual sample memory but its screen buffer rendering. Is this normal? If so, what's the reason behind it? There is no actual data loss here, changing the time scale back will show me the correct count again.
Many thanks.
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Pretty sure on the 1000z the measurements only use the on-screen data. If you adjust the time base you alias the data and hence why the number of pulses counted goes down.
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Thanks Tim. That matches my observations - but why would they do it that way and not go back to the original data? Does this work differently on more expensive scopes?
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It's a lot faster to only deal with the on-screen data. You've only got a few hundred sample points displayed at any one time, while there could be several million in memory. Some of the measurements can be particularly processing intensive, e.g. rms amplitude.
The downside is that many measurement functions are going to become very sensitive to the timebase used. E.g. rms amplitude will only be accurate if an exact number of wavelengths are displayed on the screen - e.g. if there are 5.5 wavelengths across the display the rms value won't be accurate as it will be too high or low due to the extra half-cycle. If you were zoomed in on just the peak of the waveform it will give you an rms value that is very high and not representative of the rms value of the entire signal.
Similarly there are limitations with software time domain measurements - for example, if you want an accurate rise time you'll have to expand the rising part of the signal to fill the display. For frequency, if there is less than one wavelength on the display it won't be able to calculate the frequency. If there are too many wavelengths on the display, aliasing occurs and the calculated frequency becomes inaccurate. The hardware frequency counter will be accurate regardless of the timebase, as long as the scope is triggering properly.
At least a few other scopes seem to base their measurements on a larger amount of data than is displayed - i've noticed some Agilent/Keysight scopes maintain an accurate software frequency measurement when at a very small or very large timebase relative to the wavelength.
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hello,
i was playing around with my ds2072A and was looking for an option to count pulses.
after looking on google i came across this topic.
here i see a 2 pictures with options for pulse counting on the horizontal display.
can anyone tel me how i can get this option?
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hello,
i was playing around with my ds2072A and was looking for an option to count pulses.
after looking on google i came across this topic.
here i see a 2 pictures with options for pulse counting on the horizontal display.
can anyone tel me how i can get this option?
By buying a DS1000Z series scope? ;)
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can anyone tel me how i can get this option?
Pulse counting is a feature added in the latest 2-3 versions of firmware.
You need to update to the latest firmware version for your DS1000Z oscilloscope in order to see the count pulses options.
Count pulses can work either with the data displayed on screen, or the data selected between the cursor markers.
It is not suppose to work with the whole raw memory data.
I don't know if DS2000 series have this option.
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Thanks Tim. That matches my observations - but why would they do it that way and not go back to the original data? Does this work differently on more expensive scopes?
It works differently on older DSOs also. My guess is that Rigol did it this way to get a high number of waveforms per second at the expense of making automatic measurements on the long record.
The downside is that many measurement functions are going to become very sensitive to the timebase used. E.g. rms amplitude will only be accurate if an exact number of wavelengths are displayed on the screen - e.g. if there are 5.5 wavelengths across the display the rms value won't be accurate as it will be too high or low due to the extra half-cycle. If you were zoomed in on just the peak of the waveform it will give you an rms value that is very high and not representative of the rms value of the entire signal.
Or in the case of the Rigol, RMS measurements do not work at all on noise or when undersampling is present. (1)
At least a few other scopes seem to base their measurements on a larger amount of data than is displayed - i've noticed some Agilent/Keysight scopes maintain an accurate software frequency measurement when at a very small or very large timebase relative to the wavelength.
Oscilloscopes basing their measurements on the processing record was common before the cheap DSOs showed up. I never encountered it before Rigol. This also means that measurements were made on the high resolution record produced by averaging or high resolution mode.
(1) Aliasing does not effect RMS measurements but Rigol's display processing does.