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How to analyse noise in the acquired Analog current 4mA-20mA signal?

Hi everyone,

 

I wrote a program to acquire a current signal of 4mA-20mA using cDAQ9185 and NI9227. The acquired siganl contains noise and inoder to eliminate it, I used Filter Express VI and selected Smoothening filter-Triangular-1000. But, I want to design a filter in proper way.

Inoder to do so, I took FFT of the Signal to determine the frequency at which Spikes are Present. I got only Amplitude of the Spikes.

Could you please guide me how to get the frequency at which the Spikes are Present and any other method to Analyse the noise and eliminate them.?

 

Please find the attached VIs and Screenshots

 

Best Regards,

Deepa

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Hello Deepa,

 

I can't open your program, because I use LV 2015.

I have a question : wihich component of your signal do you measure ?

AC component or DC component.

 

I think, you should read to guideline :NI-9227

 

Anyway, I can say this :

  • Concerning the filter, you can redesign it by using the palet Signal Processing>>Filters. You have many models of filters and you can select a type.
  • If you want to find the frequency of your signal and the frequency of the noises, you can use the nodes of the same palet Signal precessing>>Spectral.

 

You will be able to identify the frequeny of your Signal and eliminate the undesired signal by using the filter.

 

Regards,

Fred.

 

Kudos are welcome!Smiley Wink

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Message 2 of 9
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Hi Deepa,

 

don't display the FFT results in charts - use graphs instead!

I also suggest to convert your DDT wires into waveforms (instead of plain 1D arrays). Then use "FFT Powerspectrum and PSD" instead of the plain FFT…

Best regards,
GerdW

using LV2011SP1 + LV2017 (+LV2020 sometimes) on Win10+cRIO
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Hi Fred,

 

Thanks a lot for your reply. Its DC component.

Please find the attached VI in 2015 Format.

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Hi Gerd,

Thanks a lot for your reply.

I made the changes which you have mentioned.

In the graph, I can observe that, the start frequency is Zero and the amplitudes are 14, 14 and 24 for my three acquired signals. Could you please guide what does that mean?

I also added a Frequency and Amplitude search vi. and got the frequency of the Signal to be 50Hz and 49Hz, its varying. Are these frequency the required Signal frequency else the freuency of the noise. Please guide me

 

Thanks in Advance

 

Best Regards

Deepa

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Message 5 of 9
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Hi Deepa,

 

In the graph, I can observe that, the start frequency is Zero and the amplitudes are 14, 14 and 24 for my three acquired signals. Could you please guide what does that mean?

Do ou know anything about FFT and spectral analysis?

The first value of the FFT is always at 0 Hz and describes the DC part (or offset) of your signal. I guess you yre NOT interested in this DC part…

 

Are these frequency the required Signal frequency else the freuency of the noise.

I don't kow what you are expecting to see…

Which frequencies do you expect?

Best regards,
GerdW

using LV2011SP1 + LV2017 (+LV2020 sometimes) on Win10+cRIO
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Message 6 of 9
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Hi Gerd,

The signal which i acquired are noisy.

I just want to get a stable signal. So, trying to find a noise frequency of the signal. Inorder to eliminate them using a filter. 

In the smoothening filter which I used, I am not getting the signal exact value. I am working with these VI for the first time. Sorry, I ll learn about FFTs. 

 

 

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Message 7 of 9
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Hi Deepa,

 

So, trying to find a noise frequency of the signal. Inorder to eliminate them using a filter. 

Usually it's the other way around!

- "Noise" usually has a wide-spread spectrum ("uses a wide range of frequencies")

- "signal" usually has a known/expected limited-range spectrum ("uses only some selected frequencies")

- usually you will filter most of the noise frequencies to keep the signal frequencies (with low-pass or band-pass filters)…

Best regards,
GerdW

using LV2011SP1 + LV2017 (+LV2020 sometimes) on Win10+cRIO
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Hi Herd,

 

Thanks a lot for the explanation. As I found the frequency component of my signal to be 50Hz using Extract signal tone Information VI. I design a lowpass pass filter at fc=50Hz inoder to reject the noise. Am I right.??

 

Best Regards 

Deepa

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Message 9 of 9
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