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ButterWorth Filter FPGA d'ont work with Cuttoff frequency less then 50mHz !!!

What about IIR filter which work perfectly  ! how can i use it in FPGA i'am looking for solution here ! if you understand what do you sugess !!

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Message 11 of 21

Someone from NI may need to join in on this message thread and comment.  I looked at that Express VI.  Interesting thing is that I can't do Altenbach's recommendation of looking into the Express VI.  It seems to have a different shortcut menu than other Express VI's and I can't do an Open Front Panel to turn it into regular LabVIEW Vi's to poke around in.


I think he has a point about the scaling factor between those numbers.  I tried entering 50 mHz in the Express VI and saw that the sample graph went empty.  I decreased the sampling rate to 5 KHz.and the line came back.  Than decreased the filter to 5 mHz and it went away again.  It seems that the ratio of sampling rate to frequence has to be about 145,000:1 or smaller (based on a few data points I checked) for the line to show up in the Express VI.  Probably a limitation built into the express vi so that it can work on the FPGA.

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Message 12 of 21

OK, I have to ask, your frequency band is 2 milliHertz to 10 milliHertz. Is this correct milli (10^-3)? Or do you mean MegaHertz (10^6) or kilo (10^3).


You won't be able to sample much at mHz freqs but MHz is more realistic physical value.


Rich J

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Message 13 of 21

mHz means (10e-3)  physicaly i want to have big constant filter Taux.

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Message 14 of 21

Tau - time constant. T=RC=1/2pi*fc = at 50mHz makes tau a big big number. OK.

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Message 15 of 21

No Sorry . is not enough for my physicaly system.


i wana to have Tau=10s;50s;150s

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Message 16 of 21

Then you will need to lower your sampling rate.


At 1 kHz, 150 seconds is 150,000 samples.  That seems like a lot of data for an FPGA to handle.

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Message 17 of 21

someone can explain Why when i put 

cutooffrequency = 50mHz

sample rate = 50k S/s  physicaly i obtain Cutoff frequency = 0.11Hz  !!!!!!!! when i wana to lower physically cutof frequency what should i do ?!!!

i don't understand anything with butterworth filtre configuration. thanks for help.

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Message 18 of 21

I think some of the early comments about posting your code would be good!  Let us take a crack at it!




Ben Johnson
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Message 19 of 21

the code its very simple, its enough to use filtre butterworth bloc in the "FPGA math" library of labview. with PXI7841R. the code contain 2 loops :

1**the First loop (loopSCTL) to generate square signal.

2** the second its to do the task of filtration and see the filtred signal. 


as its mentionned in the figure linked here.

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Message 20 of 21