Noise Bandwidth/Res BW for Pico4262

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MichaelEaton
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Joined: Thu Aug 22, 2019 7:57 am

Noise Bandwidth/Res BW for Pico4262

Post by MichaelEaton » Thu Aug 22, 2019 8:08 am

We are using the Pico 4262 on +/- 10mV FS and the Mac SW. For the spectrum function, we don't see a Resolution BW knob, nor a "Video BW" knob (post-detection BW, or power-spectrum averaging).

Need to be able to calibrate your "dBu" scale to energy, or W/Hz for continuous spectra. What is the noise bandwidth? Does Bn change with vertical scale used? How do we adjust the SW to select the sample rate that we want to use, and the # of periodograms to average (i.e. post-detection filter to reduce variance in the spectrum estimate)?

Need guidance ASAP. Pls point to the documentation on controlling the spectral analysis function. Looks like the scope has plenty of memory, and plenty of vertical res, we just need to understand how to divide the memory into the periodogram length and the # of periodograms to average. Making measurements on signals with continuous spectra, and noise signals. Also need noise BW.

Gerry
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Re: Noise Bandwidth/Res BW for Pico4262

Post by Gerry » Fri Aug 23, 2019 9:53 am

Hi Michael,

The Spectrum Mode provided in our PicoScope 6 software is performing a Fast Fourier Transform of the Digitized captured data, so it is not a Spectrum Analyzer, and therefore doesn't give you a Resolution BW control to vary the BW of the swept filter, so that you can control the frequency resolution of the displayed Spectrum. However, when using FFT's to convert the signal samples to frequency values, you specify the Number of Bins to use for the conversion, which will vary the frequency resolution of the plotted spectrum. You can find the available values for the number Number of Bins in the Spectrum options window (which you can access by clicking on the 'Spectrum Options' Icon, just the right of the 'Spectrum Mode' Icon). If you also open a Properties Pane (which you can do by going to the Menu options 'Views->View Properties') you will see that increasing the Number of Bins increases the frequency resolution (as shown in 'Number of bins.png' below - the Bin-width is the smallest difference between frequency values that you can resolve in Spectrum Mode).
Number of bins.png
We also don't have a video bandwidth control when using FFT's instead of Spectrum Analyzers. Instead we can average Spectrums over time, by averaging across waveform buffers (Periodograms) to display the equivalent of a video averaged signal by going to the Spectrum Options window, clicking on the drop down list for the Y-axis Display Options, and selecting Average. We can also display the maximum values across time by selecting 'Peak Hold'.

Unfortunately, FFT's can only be applied to Linear Spectrums (showing values converted from volts) or Power/Energy Spectrums (showing values for volts squared), and at this time we only convert for a Linear Spectrum (there is an article here that explains why you can only have one or the other at this link: https://community.plm.automation.siemens.com/t5/Testing-Knowledge-Base/What-is-a-Power-Spectral-Density-PSD/ta-p/360969). We already have a request in to our development team to also convert for a Power Spectrum.

When you say "What is the noise bandwidth?" what are you referring to (i.e. the noise bandwidth of what?).

Again, not sure what 'Bn' is but Bandwidth and Bin-width don't change with vertical scale used.

You adjust the Sample Rate by selecting a different bandwidth (when Spectrum Mode is you're primary view), because Sample Rate is always = 2 * Bandwidth.

To divide the memory into Buffers for multiple waveforms you just need to set the Trigger Mode to anything other than 'Single', and then go to the Menu option 'Tools->Preferences', set the Maximum Waveforms value under Waveform Buffer to the number of waveforms that you want (as shown in 'Number of waveform buffers.png' below - the default is 32).
Number of waveform buffers.png
To restrict the number of Waveforms (Periodograms) that you average over you need to do the following:
  • 1. Set the number of waveforms to stop at (i.e. set the number of waveforms to be captured, as described above)
  • 2. Set an alarm to stop PicoScope 6 from capturing any further waveforms, by going to the menu option 'Tools->Alarms', and then click the drop down list for 'Event' and both select 'Buffers Full', and click the checkbox for 'Buffers Full' (as shown in 'Set alarm event.png below)
Set alarm event.png
  • 3. Click on 'Add' to add the action for when the Buffers are full, and from the drop down list of actions select 'Stop Capture' (as shown in 'Set alarm action.png' below):
Set alarm action.png

The documentation on controlling the Spectral Analysis Function is the context sensitive help (clicking the help button in the software) and starting from page 25 (labelled as 19) in our PicoScope 6 User Guide here: https://www.picotech.com/download/manuals/picoscope-6-users-guide.pdf. There are also many posts on the forum discussing different aspects of using Spectrum Mode.

Regards,

Gerry
Gerry
Technical Specialist

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