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FFT and 60 Notch filter software

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loop123:

--- Quote from: WatchfulEye on March 13, 2024, 11:37:25 am ---
--- Quote from: loop123 on March 13, 2024, 09:46:05 am ---First, please tell me what exactly are these noises? In the 2nd image, they don't look like harmonics:

If they are harmonics. They should be resultant in the waveform. But my audacity waveforms don't show any resultant. The noises are riding in each part of the sine wave. How do you differentiate between EMI, RFI, amplifier noise, and quantization ADC noises? How sure are you they are powerline noises or harmonics? Please tell me what exactly those are first.

--- End quote ---

You seem to be looking for an individiual frequency as the source of your noise/distortion. It actually appears that this is broadband noise/noise floor that you are seeing. Also, a hint when using the audacity spectrum viewer - you need to amplify the signal first, as the viewer has a fixed minimum on the Y axis. You can then magnify as required in the spectrum viewer.

This is the spectrum of the last waveform you posted. I've also calculated normalised amplitudes for narrow band noise, on the basis that the signal is 10 uV rms.

You can see the 50 Hz signal, and small 2nd and 3rd harmonics. You can also see a small amount of 60 Hz noise (1 uV), but no harmonics.
However, there are 2 unknown and more important noise sources - 78 Hz (1.7 uV) as well as harmonics of 78 Hz, and broadband noise (approx 100 nV/sqrt Hz).

The 60 Hz is likely coming from power line noise, and could be mitigated with better shielding, or the use of a notch filter (depending on the frequencies of interest in your signal), or differential measurement.
The 78 Hz is a mystery, and may be noise generated by the simulator.
The broadband noise may be the noise floor of your setup, and because of it's 1 kHz bandwidth, it has a total contribution of approx 3 uV.

I suggest repeating the measurements with other waveforms and amplitudes, to see what happens to the unknown noise sources. It may be that some of the unknown noise is from your waveform generator.

--- End quote ---



Oh. I missed this message. Is the waveform you downloaded the one with lots of noises, like above? That's what I wanted to know, whether it is just powerline harmonics or the noise floor already. But the noise of the AMP01 used in BMA-200 is 5nV/sqrt (Hz). So at 1000Hz bandwidth, the noise is actually 0.158uV. Where did you get the 100nV/Sqrt (Hz)? 

loop123:

--- Quote from: WatchfulEye on March 13, 2024, 03:34:36 pm ---Most of the noise is between 50 Hz and 1000 Hz - it is broad band noise, not at any specific frequency.
Apart from the 60 and 78 Hz spurs, the only way to reduce the noise is reduce the bandwidth. You cannot keep up to 1000 Hz and reduce the noise any further with any simple filter process.

--- End quote ---

What was what I wanted to know since message 1 of this thread. Whether this is powerline noises or noise floor already. Did you ran them at Audacity? How did you choose the LPF in the last 3 images?   But the AMP01 has 5nV/sqrt (Hz) noise or 0.158uV at 1000Hz. So with 10uV signal, the noise floor is very small fraction.

WatchfulEye:
Only the 60 Hz noise is powerline.

There is a mystery 78 Hz noise.

The main noise is broadband noise - which looks like noise floor, but by my calculations it is about 100-150 nV/sqrt Hz - much higher than specified noise of the AMP01. It could be noise from the waveform generator.

However, note that BMA-200 specifies 7uV noise - and your recording shows less noise than this.

You could try measuring the noise directly by using a 1 kohm resistor instead of the waveform generator. It would be interesting to have a series of recordings with different waveforms and 1 with just a resistor - all settings left exactly the same.

loop123:

--- Quote from: WatchfulEye on March 13, 2024, 03:50:12 pm ---Only the 60 Hz noise is powerline.

There is a mystery 78 Hz noise. The main noise is broadband noise - which looks like noise floor, but by my calculations it is about 100-150 nV/sqrt Hz - much higher than specified noise of the AMP01.

However, note that BMA-200 specifies 7uV noise - and your recording shows less noise than this.

--- End quote ---

I don't know how the 0.158uV rms noise at 1000Hz at the AMP01 became so bad at the BMA-200. But is it really possible to make an amplifier that can deal with clean 10uV, 1000Hz bandwidth signal? Is there a known device that can do this already? Using AMP01, why can't one make this?

I'll try the resistor measurement tomorrow.. please let me know what can imitate 10uV too instead of using the Netech simulator.

gf:
Here is the accumulated spectral power up to 1kHz. The big jump is the wanted signal. Even the spurs result only in small jumps. Everything else is wideband noise as you see. It makes sense to lowpass filter at 1kHz to get rid of the top 5% beyond 1kHz.

EDIT: I should not have labeled the plot with "noise power", but rather spectral power.

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