FPGA butterworth filter problem on Labview 8.6

[Heikki]

Hi,

I was testing the reconfigurable lowpass butterworth filters on FPGA found from the Example Finder (Using Reconfigurable Butterworth Filter - R-series.lvproj). This VI generates a sine signal and passes it through a Butterworth filter. When I set the lowpass cutoff frequency to a low value (~2 Hz) and generate a 0.5 Hz sine signal the filttered output doesn't follow the input so well anymore (see figure). The filter is 2nd order, so is this a feature of the filter? And what should I do if I want to sample with high frequency and use a low cutoff value?

I'm using Labview FPGA Module version 8.6 and PCI-7833R -card. On Labview 8.5 I remember that the Butterworth filters for cutoff frequencies above and below 2 kHz were actually different VIs but on LV 8.6 they seem to be the same.

-Heikki

[JLewis · Accepted solution]

Hi Heikki,

The reconfigurable version (ie, with "Show configuration terminal" checked) does not support the modified low-frequency implementation. This is because we can't change filter architectures at run-time. The nonreconfigurable version, however, should use the same modified implementation you saw in LabVIEW 8.5 for low cutoff frequencies, defined here as frequencies less than .01 * sample rate.

Your example of 100 kS/s sample rate and 2 Hz cutoff is pretty extreme, even for the modified implementation. The usual technique for situations like that is to do an anti-aliased downsampling to a rate that is better-suited to the cutoff you need. The Rational Resampler from the FPGA Math & Analysis palette is designed for this purpose.

Regards,

Jim