Openbci FFT

Input :sin(x); Vpp=400mV F=10Hz . Signal generator input
The FFT analysis on OPENBCI shows that the frequency component is 10hz and30Hz .
I don't know why there's a 30Hz signal, where does it come from.

I used OpenBCI data and performed FFT in MATLAB, and found that it also contains a frequency component of 30Hz. I use a signal generator as input, and the input signal does not include 30Hz.
I donnot know why?

Comments

  • wjcroftwjcroft Mount Shasta, CA

    Do you have a notch filter installed at your mains frequency? Those EMF radiations are so strong they generate sub-harmonics. Your generator is likely mains powered. But internal shielding does not extend to the boards, which are microvolt sensitive.

    You did not mention which board.

  • fanfanfdxfanfanfdx LiaoNing

    We are using Cyton Board .
    I use dry batteries and 3.7V lithium batteries for power supply, and the effect is the same. After filtering out 50Hz power frequency interference through a notch filter, there is still 30Hz interference. May I ask if there is a solution to this interference

  • wjcroftwjcroft Mount Shasta, CA

    So let me restate, you are seeing the 30 Hz (and 40 Hz, 50 Hz, etc.) peaks in your FFT. Using the OpenBCI GUI and you have turned on the notch filter for your mains frequency.

    Are you aware of this idea of 'harmonics' induced by non-linearities or distortions in your incoming signal? That is what appears to be going on to me. Have you looked at your signal generator with ANOTHER type of instrumentation? For example, oscilloscope or signal analyzer / FFT device? Your generator might also be overloading the Cyton input which is designed for biosignals, not injections from electronic instruments.

    The fact that your FFT above shows repeating patterns at 10 Hz intervals is very suspicious. It indicates to me that there is some distortion going on and harmonics are generating these 10 Hz interval peaks.

    400mV, which is .4V, is a HUGE signal that is not seen in biological systems. Do you know how to construct a voltage divider network to bring it down to say 10's of mV? As you know, EEG is in the range of 1 to say 80 microvolts.

    https://ohmslawcalculator.com/voltage-divider-calculator

    Regards, William

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