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


I'am using Butterworth filter in FPGA Code labview with PXI7841R.

I wana put a frequency less than 50mHz but i can not introduce i modified Sample rate parameters wchich allows to put less than 50mHz frequency but the behavior is not the same.

 i want to introduce cutoff frequency Fc=2mHz Fc=10mHz with 1order filter in FPGA math. 


Thanks for future advice.


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Message 1 of 21

You'd probably be able to get some advice from others if you attached a VI that demonstrates the problem you are having.


Perhaps you hit a limitation of LabVIEW.  Perhaps it is a mistake you made in your cdoe.  But we can't tell unless you provide something we can work with.

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Message 2 of 21

 Code is very simple i have simply input signal and out put signal and i wana modifie cutoff frequency.


i found an exemple of IIR filter i chosse butterweorth filtre and i modifie cutoff frequency with desired value like 2mHz and 5mHz and  it work perfectly as mentionned in figure but i can not use it in FPGA code.


i must use FPGA math library with butterworth filtre which is limited at 50mHz. 


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Message 3 of 21

Those are just screen shots.  They aren't VI's.


And please provide more description besides it "it doesn't work".  In what way does it not work.  Do you get an error message?  Do you get data, but it is wrong?  Does your computer blow up?

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Message 4 of 21

it means that the output signal is not like desired output signal as founded in IIR filter (which work perfectly) and in theorical calculation of 1ordre filter desired signal is known by maths.

the problems is in the configuration of sample rate and cutoff frenquency.  

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Message 5 of 21
What is your sample rate and cutoff frequency? Your sample rate needs to be at least 2x the cutoff frequency (Nyquist).

You are correct that the first filter VI is not supported on the FPGA - the FPGA does not support DBLs.

How do you have the FPGA filter configured?

LabVIEW Champion, CLA, CLED, CTD
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Message 6 of 21

Of course 5mHz is an awfully low frequency (200 seconds per cycle), so why would you need an FPGA for that in the first place? Maybe some counter is rolling over twice.

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Message 7 of 21

i configured it many many times but there is no solution:

First i considered default configuration of labview  with :

cutof Frequency : 50mHz

Sample rate: 50K S/s

here with 50 K S/s its limited to 50mHz

Then i putet : 

cutof Frequency : 20mHz

Sample rate: 2 S/s  ==> ouput signal its zero.

and then

cutof Frequency : 5mHz

Sample rate: 50m S/s  ==>  ouput signal its zero.

Same problem

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Message 8 of 21

My physic system i wana have this curve with Fc=5mHz.

i must have this result its no logical to not programe this in FPGA !!!!


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Message 9 of 21

@Rayden wrote:

My physic system i wana have this curve with Fc=5mHz.

i must have this result its no logical to not programe this in FPGA !!! 

What the logic behind your "not logical"? Please explain. Your frequency is so low that the RT system would have no problem dealing with it accurately. Maybe you can look inside the filter VI and rewrite it to your desired specifications, hopefully without running out of FPGA real estate..

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Message 10 of 21