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limit of wavefrom buffers

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limit of wavefrom buffers

Postby gbgb » Thu Jan 19, 2012 1:29 pm

Although I set the number of maximum buffers to 1000, I see that the highest number is lower (depending on timebase) - multiplying the number of samples per buffer with the number of buffers actually captured it seems to be limited at something around 30 megasamples (maybe it is 120 as I use 2 channels and the 3245 is 12 bits).
Am I correct about this limit? Is it possible to increase this limit?
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Re: limit of wavefrom buffers

Postby Martyn » Thu Jan 19, 2012 4:02 pm

Can you explain exactly what value in Picoscope6 is restricted to 1000 and what other settings you have chosen for Collection Time and Number of Samples.

For info if you right click on the display and select View Properties you will have additional information displayed which may show you what is happening.
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Re: limit of wavefrom buffers

Postby gbgb » Sun Jan 22, 2012 9:17 am

I set in preferences "max waveforms" to 1000, so I expect to be able to capture 1000 waveforms.
The "number of samples" at the top of the scope screen is set to 100kS - If I understand correctly this is the maximum
samples per waveform captured (actual number varies - did not yet understand the logic that sets the actual number)
Only one channel is active.
If I select a short timebase - say 200usec, per the properties info the number of samples per waveform is 40,005 and indeed the waveforms buffer counter at the top of the scope screen reaches 1000.
However if I select a timebase of 100 msec (per the properties info the number of samples per waveform is 78,130) the counter stops at 858, and if I increase it to 200 msec (95,244 samples per waveform) the counter stops at 704
If I add another channel (leaving timebase at 200 msec) the counter stops at 352
You will notice that when the limit is reached the multiplication of
number of samples per waveform X number of channels X number of waveforms
is about the same number (around 65meg). This brings me to believe that there is a fixed limit to the space allocated for waveform capture.
So back to my original question - can this limit be changed so that I can capture 1000 waveforms regardless of other settings.
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Re: limit of wavefrom buffers

Postby alan » Sun Jan 22, 2012 8:56 pm

Hi,

I will do my best to explain! With your PicoScope 3425 the waveform buffer is created in main PC memory and its about 100M bytes. As the 3425 is a 12 bit oscilloscope that means that you can store around 67,000,000 samples in the buffer.

If you set the buffer to 1000 waveforms then that means you can collect 1000 waveforms, each of 67,000 samples.

If you allow the scope to collect more samples per waveform (the max for your product is 512,000) then the maximum no of waveforms that can be stored in the buffer is reduced. In the example below you collect 858 waveforms each of 78,130 samples and if you do the sums thats 67,035,540.

The reason the number of samples per waveform varies is due to the timebase. At faster timebases there is not enough time for the scope to collect the max no of waveforms as indicated at the top of the screen.

If you want 1000 waveforms then set the max no of samples to 67,000 or less then you should never run out of memory. If you want you could actually set the max no of waveforms to > 1000 then at timebases that limit the no of samples to less that 67,000 then you will be able to have more than 1000 waveforms.

Note that the above is specific to to products with less than 1M of internal memory. For products with deeper memory the waveform buffer is created in the oscilloscope hardware rather than PC memory as this reduces the load on the PC.
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Re: limit of wavefrom buffers

Postby gbgb » Mon Jan 23, 2012 6:02 am

Thanks,
That confirmed my assumption, and according to my understanding the 100meg limit is hard coded and cannot be changed - correct?
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Re: limit of wavefrom buffers

Postby Martyn » Mon Jan 23, 2012 8:42 am

That's correct.
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