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| Standardised Way To Test Oscilloscope Screen Update Rate |
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| joeqsmith:
--- Quote from: EEVblog on April 29, 2024, 06:07:03 am ---I'm thinking about a way to come up with the best and/or most easily test screen update rate (NOT waveform update rate). High speed camera and frame anlaysis is one way obviously. Comments invited. --- End quote --- This is why I had asked to see those musical X-Y graphing tracks. --- Quote ---I guess it wasn't clear but my goal wasn't to evaluate the X-Y mode of a DSO. Rather I am trying to get a feel for how they update their screen. Granted, its about as revealing as using an AM broadcast band radio to determine how much noise is on your supply rail. Still, I suspect it would tell me something... Maybe not.. --- End quote --- Some of the new DSO's a very impressive. My old scopes are worse than the worse ones shown in those threads. Hard to say if it is a useful metric. https://www.eevblog.com/forum/testgear/magnova-oscilloscope/msg5446370/#msg5446370 https://www.eevblog.com/forum/testgear/oscilloscope-music-on-dsos-post-your-results/msg5446613/#msg5446613 |
| moerm:
Possibly an approach that turns out to be not smart, but here's how I'd approach it. Analysis: - any optical approach (sensors, cameras, ...) Nope, because that just shifts the problem into a different area instead of solving it sufficiently well (the sensors info must be processed and displayed, etc). - we already have a display available. Let's use it. - if there's one factor even a cheap scope can measure sufficiently well it's time/frequency, so let's make use of that. After all the order of what we're (mainly) looking for is ridiculously low (< 1kHz). - we also have or can get very cheaply another strong (in terms of time) player, namely a (dirt cheap) mcu board thingy. - It seems we actually want to get two pieces of information, a) the effective screen (aka TFT or similar) update rate, 'effective' as in "no matter who's slow, processing, the display itself, or whatever, we just want to see what we effectively get out of it". And b) the time delta between 'signal in' and 'signal shown'. Implementation: I'd express the results on the DUT display in the form of what a scope can measure and show quite well, frequency. How? Have a cheap MCU generate pulses that a) provide timing information ("what's the current test frequency?") as well as b) pulse trains which "contain" the currently tried frequency information with a sufficiently long pause in between "shots" (> scope's blind time). So if my current "shot" is at say, 42 HZ the pulse train would contain '42' in binary. Assuming an increasing frequency the last one I see well (clearly) is the upper display update boundary. As for (a) a sufficiently precise "ping" of some frequency, but integer fractions or multiples of 10 MHz may come in handy, should do the trick. And again I'd express some info in a pulse train, e.g. a "marker" every so often. Note however that generating pulse trains of hundreds of MHz will require a fast (~ more expensive) MCU or a FPGA, so probably my first approach at a solution of (a) isn't the smartest. Please note that those two measurements are made separately as is the kind of information we're looking to gather. |
| XiMMiX:
That sounds like using the screen's refresh rate which may not be the actual update rate of the information on the screen. As a comparison, the screen I'm using on my computer runs at 120HZ but if I'm watching a fullscreen jpg the actual screen update rate is 0. |
| m k:
How very slow time is updated, like a heart monitor? I guess I'm wondering how partial pixel is appearing and can it be manipulated by a test signal. At least badly scaled screen resolution is clearly visible. Something like a triangle wave where a vertical step is either bigger or smaller depending how frame start correlates the wave amplitude. One other, a Moire pattern. Can the angle of extra raster tell something? |
| joeqsmith:
Using a camera to compare them: Based on the X-Y music plotting, I wouldn't be surprised if my DSO was not slower than the Rigol shown in this video. *** I have no idea how he has these two scopes configured. Maybe it's not a good comparison. Searching EEVBLOG, seems the topic has come up before: https://www.eevblog.com/forum/testgear/dso-waveform-update-rate-importance-in-practice/ *** Google search: Chapters and Articles Test Equipment Principles Morgan Jones, in Building Valve Amplifiers (Second Edition), 2014 Refresh rate The screen’s refresh rate has to be fast enough (>80 Hz) for flicker not to be visible, but if the oscilloscope’s memory was only refreshed at this rate we might miss momentary glitches. Ideally, we would like every waveform capture to be followed by another at the first occurrence of the user’s trigger conditions being satisfied, and for all of this data to be written to display memory. In practice, some triggers and subsequent acquisitions may be missed because the oscilloscope’s memory system is not ready to accept data. Although refresh rate can be measured easily enough in terms of waveforms per second, it is hard to specify on a data sheet because it depends on time base setting, record length, triggering, and waveform shape, so vague descriptions like “up to” and “typical” are used. It is rare for an oscilloscope to reveal its refresh rate in use, which is a shame because indication allows the user to iteratively adjust settings to maximise it if necessary. Be assured that a cheaper oscilloscope is likely to save cost by having slower memory and busses that degrade its refresh rate. Although difficult to specify, a good subjective assessment of a basic oscilloscope’s refresh rate may be quickly made using orchestral music recorded in stereo without data compression. Using analogue audio, connect L to Ch1 and R to Ch2 (or vice versa), select XY mode, select AC auto-triggering, and observe the resulting Lissajous figure. A perfect display would be identical to that produced by an analogue oscilloscope, resembling a continuous ball of wool smoothly pulsing in size and intensity without any jumps, glitches, or discontinuities. Unlike an analogue oscilloscope, a digital oscilloscope is forced to trigger and capture input waveforms before displaying them sequentially as discrete Lissajous figures, and because music is essentially random to an oscilloscope, really basic oscilloscopes fare very badly on this test due to their poor refresh rate. |
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