Filter type of LabVIEW FPGA Rational Resampler

[gabriel.brunheira]

Hi!

I would like to have more information regarding the filter designed by the Rational Resampler Express VI. 

• Is it a FIR or IIR?
• Which type of filter is used? Butterworth, Chebyschev, Equi-ripple, etc?
• In case of FIR, does it have linear phase (constant group delay)?

We pretend to use this VI to downsample signals acquired with different sampling rates to the same output frequency, then write them to a single acquisiton file for post-analysis. By knowing the characteristics of these filters, we would be able to compensante their group delays.(which will be different for each case) and guarantee proper synchronization in time.

Best Regards,

Gabriel Brunheira

gabriel.brunheira@lnls.br

LNLS/CNPEM

[JÞB]

Well, let's look at that picture of the Express VI configuration page just a bit more closely.

On the lower right corner of that page are THREE Buttons. Did you see the rightmost button? It is Labled "Help".  What happens when you actually PRESS THAT BUTTON!? 

Hint: the explanation provided by the Help would be better than my own post could be.

[gabriel.brunheira]

Hi Jay,

We indeed looked up in the help page and as you can see below, the required information is not present:

We also searched for information in NI website and foruns, and nothing. 

Best Regards,

Gabriel

[JÞB]

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@gabriel.brunheira wrote:

Hi Jay,

We indeed looked up in the help page and as you can see below, the required information is not present:

We also searched for information in NI website and foruns, and nothing. 

Best Regards,

Gabriel

---

Great! Then you found out that the Express VI is not a filter at all.   It simply is a resample of the data input when L =1 and a linear interpolation where L>1.  

[gabriel.brunheira]

Hi Jay,

With all the respect, I have the feeling you don't know what you're talking about, neither read the Help page yourself:

In any process of downsampling, you should limit the signal bandwidth (using a digital filter) before resampling to avoid aliasing: Downsampling (signal processing) - Wikipedia

I'm just asking for more information regarding this filter, which frequency response clearly appears in the Express VI shown above.

Best Regards,

Gabriel

[gabriel.brunheira · Accepted solution]

Hi! In case anyone is interested, here's the response we got from a NI Engineer in a requested support regarding this question:

• Filter type: low-pass filter
• Design method: Kaiser window, which helps to design a linear phase Nyquist FIR filter
• Default pass-band edge frequency is 80% of half of the highest sample rate
• Default stop-band edge frequency is 120% of half of the highest sample rate
• Pass-band ripple: Determined by stopband attenuation