Do I Need Additional Anti Aliasing Before Resampling

[James_McN]

Hi All,

Just thinking about how this resampling works.

Basically this is just a standard resampling with a dynamic frequency. This means that it could alias high-frequency noise down into the region of interest.

The Analog Inputs have anti-aliasing filters that will filter signals to (-3dB) 24.56kHz which works for the IO module because the nyquist frequency is 25kHz.

However we then resample down to (for 50Hz) 12.8kHz nominal. This means a nyquist of 6.4kHz so everything from 6.4kHz - 24.56kHz could alias down.

This can be solved by introducing a digital filter into the FPGA with a better cutoff (have to handle variation so probably want a passband to at least 7.04kHz for 55Hz) but I have not seen this in any of the examples.

Am I wrong, or has no-one ever bothered?

Cheers,

James

[James_McN · Accepted solution]

Decided to take the experimental approach!

Generated a 50Hz wave with a 5% 10kHz imposed on it and it doesn't come through.

Digging deeper into the resampler it appears to have an FIR filter in it which I guess is taking care of it.

Cheers,

James

[johnsold]

James,

I do not have the Electrical Power Suite so I cannot comment on the specifics of VIs in that package.

In general hardware anti-aliasing filters are required before the physical digitizing is done. After that any additional filtering may be done in software so long as the Nyquist criterion is met for the input data and the filter.

For your example a software filter based on the 50 kHz initial sampling rate (derived from your stated 25 kHz Nyquist rate) could be set to 6.4 kHz or lower prior to the resampling.  Be aware that filters do not cut off abruptly at the cutoff frequency so energy slightly above that frequency will still be present at the filter output, although with some attenuation. The specification of a filter often involves trade-offs between cutoff frequency, filter order, the required attenuation at a specific frequency in the stop band, the tolerable attenuation or phase shift at a specfic frequency in the pass band, and other parameters.

Lynn