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2 MHz spectral area option in FFT mode with 4262

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2 MHz spectral area option in FFT mode with 4262

Postby Techland » Fri Apr 21, 2017 4:28 am

4262 with latest Picoscope 6.12.7. Choosing a spectral range of 2 MHz the x axis is labelled 1.6 MHz, with a max of 1.65 MHz to the right. I'm a new user but heard that in older versions there was no 2 MHz option at all, but a 1.7 MHz. So it looks like this choice is wrong and should be 1.7 MHz?
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Re: 2 MHz spectral area option in FFT mode with 4262

Postby Gerry » Mon Apr 24, 2017 2:50 pm

Hi Techland,

The selected 'Spectrum Range' is often a rounded up approximation of the frequency span on the x-axis, and when selected determines the sample rate used. So, for example, if you select a bandwidth of 2MHz for the PicScope 4262, the sample rate used is the nearest sample rate available that would be 2 times the Bandwidth which is 3.333MS/s (the only sample rates available are based on the maximum sample rate for the PicoScope model divided by factors of 2). So for this actual sample rate used the actual bandwidth available, satisifying the Nyquist criteria is 1.666MHz.

In the older versions of PicoScope 6 these actual bandwidths were only rounded up by decimal places to give the selectable 'Spectrum Range', but in the newer versions the Spectrum Ranges are fully rounded up to provide consistency across the Models.

Regards,

Gerry
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Re: 2 MHz spectral area option in FFT mode with 4262

Postby Techland » Tue Apr 25, 2017 1:05 pm

Gerry wrote:So, for example, if you select a bandwidth of 2MHz for the PicScope 4262, the sample rate used is the nearest sample rate available that would be 2 times the Bandwidth which is 3.333MS/s (the only sample rates available are based on the maximum sample rate for the PicoScope model divided by factors of 2). So for this actual sample rate used the actual bandwidth available, satisifying the Nyquist criteria is 1.666MHz.


The 4262 is advertised as 10 MS/s DSO. I have no clue how 10 divided by factors of 2 can end up in 3.333MS/s.
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Re: 2 MHz spectral area option in FFT mode with 4262

Postby Martyn » Wed Apr 26, 2017 6:25 am

The timebase /sampling rate equation for the 4262 is
Code: Select all
(n+1) / 10,000,000
n=0: 100 ns
n=1: 200 ns
n=2: 300 ns
......
n=2^30 -1

giving

Timebase 0 - 10MS/s
Timebase 1 - 5MS/s
Timebase 2 - 3.33MS/s
Timebase 3 - 2.5MS/s
etc
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Re: 2 MHz spectral area option in FFT mode with 4262

Postby Techland » Sun Apr 30, 2017 7:09 pm

Well, I really don't know what to say. Is this a serious measurement instrument costing 1,200 €? Or a toy for my child to play? What else did you 'round up' in this software to make it look nicer? Is there anything I can still trust, or is this an example for the accuracy I have to expect from Picotech products in general?

I bought an expensive aliasing filter, corner frequency 1.7 MHz, that would have fitted perfectly to the 2 MHz spectrum range. I had no reason to mistrust the clear labelling in your software. But with a real 1.66 MHz Nyquist border the filter is too high (reaches effective attenuation around 2 MHz) and useless. Thank you so much!
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Re: 2 MHz spectral area option in FFT mode with 4262

Postby Martyn » Wed May 03, 2017 9:02 am

The hardware is still the same, and provides the same measurement accuracy as it always has done.

Having discussed this in house, the software engineers have chosen to select the lower range in the frequency domain, as happens in the time domain when requesting number of samples, when what should happen is that it should select the nearest possible option above the requested value, so in this case 2.5MHz. This would allow you to use your aliasing filter, and zoom as necessary to see the data of interest. Until now no one had noticed this oversight.

Unfortunately it is not possible to manually select 2.5MHz, even though it is supported by the hardware, so currently you would need to manually select 5MHz range and zoom in.

I have raised this as an issue with our development team.

If you would like to discuss this in more detail please email support@picotech.com
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Re: 2 MHz spectral area option in FFT mode with 4262

Postby Techland » Thu May 04, 2017 7:34 am

An additional 2.5 MHz setting would indeed not only solve my problem, but be appreciated from anyone using this DSO as FFT spectrum analyzer as serious lab tool. I do, and selected it because there is no competition for what the hardware can do. I don't use the DSO part at all.

Martyn, how long you think do I (we) have to wait until there is a way to use that 2.5 MHz option? I am aware of the 5 MHz workaround, but I am using the FFT very extensively and down to -155 dBu (successfully!). That requires 1Ms with the smallest possible spectrum range. More wide and I loose resolution depth...
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Re: 2 MHz spectral area option in FFT mode with 4262

Postby Martyn » Fri May 05, 2017 10:53 am

Once I have some feedback from the development team I will let you know.
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