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Picoscope 3204B timebase confusion

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Picoscope 3204B timebase confusion

Postby MarkPotter » Fri Nov 16, 2012 3:59 pm

Hi;

I have been doing development with a Picoscope 3204B for a while now (writing Labview software for it). Up until recently I have only used timebase settings of 1, 2, 3 and 4.

According to the manual these should give timebases of 0.004, 0.008, 0.016 and 0.032 microseconds per sample, with subsequent time bases doubling this. All was well until I took some readings on timebase 5, which didn't seem to show the waveforms arriving at the right time.

Further investigation showed the timebase was not 0.064 as expected, but 0.048 microseconds per point (approximately - all I could do was measure a known signal).

Since then I have checked the longer timebases and come up with the following (microseconds per point):

Timebase 6: 0.065 us
Timebase 7: 0.082 us
Timebase 8: 0.1 us
Timebase 9: 0.118 us
Timebase 10: 0.14 us

(all are a bit approximate based on some measurements)

This is a bit confusing as they don't seem to follow any rational pattern. Is my picoscope / software working correctly, and importantly will these timebases be consistent across all picoscopes as the software I am writing will need to give the same result on another Picoscope of the same make and model? Has anyone else had similar problems??

Mark.
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Re: Picoscope 3204B timebase confusion

Postby Martyn » Mon Nov 19, 2012 2:32 pm

Exactly how are you calculating the sample interval of the data that is returned to your application by the scope ?
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Re: Picoscope 3204B timebase confusion

Postby MarkPotter » Mon Mar 04, 2013 9:31 am

Hi, martyn;

sorry for the slow response. I kind of buried this problem in my mind but now it has resurfaced. I measured the sampling interval by generating a waveform with a 3rd party function generator and splitting the signal between the Picoscope and a 3rd party conventional oscilloscope. By noting the spacing between features on the waveform I worked out what the real delta-t must be on the scope data. this is crude and messy but I can't see how to make the picoscope tell me what its delta-t or sampling rate actually is (other than as a reference number 1 ... x, which doesn't give the right answer when referred to the look-up table. Oddly the Picoscope virtual oscilloscope does give the right answers so it probably isn't hardware...

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Re: Picoscope 3204B timebase confusion

Postby Martyn » Mon Mar 04, 2013 1:56 pm

For timebases 0,1 and 2 the following formula is used
Code: Select all
2 to the power of timebase / 500,000,000

For timebases from 3 upwards this formula is used
Code: Select all
(timebase - 2) / 62,500,000


Which gives the values you are seeing, give or take measurement error
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Re: Picoscope 3204B timebase confusion

Postby MarkPotter » Mon Mar 04, 2013 9:49 pm

Thanks for the information - is this in the manual somewhere and I just missed it?

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Re: Picoscope 3204B timebase confusion

Postby Martyn » Tue Mar 05, 2013 7:15 am

Yes it is, although it doesn't jump out at you so not surprised it can be missed.
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