Different waveform display while/after measuring

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hilde276
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Different waveform display while/after measuring

Post by hilde276 »

I have observed the following behaviour multiple times. I'd like to understand what is going on. I'm using the PicoScope 3406DMSO togheter with PicoScope 6. I record an analog signal (which is not periodic). I set the (desired) sampling rate to some high level (e.g. 2MS) and the time base to 1s/div. Sometimes the signal amplitude (as it is displayed) during measurement is much higher than the signal amplitude displayed after the measurement was stopped. I understand that PicoScope adjusts the sampling rate automatically. So in my case the actual sampling rate (as shown in the properties window after measurement stop) may only be 250kS/s. I can hardly imagine that this reduction in sampling rate is the reason for the different waveform displays during/after measurement. 250kS/s should still be more than enough for my signal. Sometimes during measurement I even get "Channel over range"-warnings, but when I then stop the measurement the signal levels are far away from the end of the measurement range. I observed that this also affects the measurements added to the scope window: The "Maximum"-value from the measurement can not be seen in the waveform display. Can somebody please explain?

Martyn
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Re: Different waveform display while/after measuring

Post by Martyn »

This sounds as if you are describing the downsampling of data which is used when drawing a live trace on the screen whilst the scope is running, and the drawing of the complete trace using the collected raw data as soon as you stop the recording.
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hilde276
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Re: Different waveform display while/after measuring

Post by hilde276 »

Martyn wrote:This sounds as if you are describing the downsampling of data which is used when drawing a live trace on the screen whilst the scope is running, and the drawing of the complete trace using the collected raw data as soon as you stop the recording.
Ok. But in that case I don't understand why this downsampling is happening. My desired sampling rate was 2MS or even higher. Why is the signal downsampled to 250kS/s after stopping the measurement?

Martyn
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Re: Different waveform display while/after measuring

Post by Martyn »

The 2MS at the top of the screen is not the sampling rate, this is the number of samples you would like to collect for the screen time you have set. As this is 1S/div the screen time is 10 seconds so a sampling rate of around 2000kS/sec would be correct.

The way PicoScope works is that you set the screen time, x secs/div, you then request the number of samples you would like to collect in that time, xMS, and the software will work out what sampling interval will be needed. Due to the internal clocks within the device not all sampling intervals are possible so the software will get as close as possible to the requested number of samples. If you use the properties panel by selecting Views->ViewProperties you will see at the top of the panel on the right exactly how the scope has been set by the software.
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hilde276
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Re: Different waveform display while/after measuring

Post by hilde276 »

This is understood so far. But why is the live trace drawn with a higher sampling rate? In other words: If the SW is able to draw the live trace with a high sampling rate, why is this downsampled at measurement stop?

Martyn
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Re: Different waveform display while/after measuring

Post by Martyn »

The live trace isn't drawn with a higher sampling rate, it will be constructed by downsampling the raw data that is being streamed to the drivers buffer. Once the the stream is stopped the raw data from the drivers buffer is drawn. The value in the properties window is the actual sampling rate in use.
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hilde276
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Re: Different waveform display while/after measuring

Post by hilde276 »

Ok. The way you describe it is logical to me. Unfortunately this does not explain my observations. Do active math channels have an influence on this? Any other ideas?

Martyn
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Re: Different waveform display while/after measuring

Post by Martyn »

Can you post a video showing exactly what is happening, making sure the properties window is open. I think we are missing something in the descriptions and explanation.
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hilde276
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Re: Different waveform display while/after measuring

Post by hilde276 »

Hi,

I was able to reproduce the described behaviour at my desk (see video under the link below). It is not as dramatic as I experienced it last time. But still you can clearly see that the spikes that are visible during the measurement are shrinking in the moment the measurement is stopped. Why is this happening?

https://vimeo.com/279262718

Martyn
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Re: Different waveform display while/after measuring

Post by Martyn »

The reason you are seeing this effect is that you are using Resolution Enhancement which can only be applied once the capture has completed and the raw data has been downloaded into the application. The scope will capture and draw the raw data at 8 bit resolution, then as soon as you stop the trace, or the trace reaches the end 12 bit resolution enhancement will apply a 256 sample moving average filter to the signals. This will remove high frequency noise, and reduce the peak levels of spikes, resulting in a cleaner trace.

Code: Select all

newsample1 = (sample1 + sample2 + .... + sample256)/256
newsample2 = (sample2 + sample3 + .... + sample557)/256
....
newsample9001= = (sample9001 + sample9002 + .... + sample9256)/256
etc
Martyn
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hilde276
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Re: Different waveform display while/after measuring

Post by hilde276 »

Thank you for your explanation! Is there a way to turn this feature off? As far as I can see, I can only reduce the impact of that moving average filter by choosing the minimum number of samples used for the moving average (8 Bits), or by using a much higher sampling rate...

Martyn
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Re: Different waveform display while/after measuring

Post by Martyn »

Setting 8 bit turns off the feature, you are then using just the hardware resolution for the device, any other value enables it.
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