Analogue Alias Filter Required?

[signal1]

I'm using DASYLab to acquire accelerometer data. 

I'm using a IOtech Wavebook 516 scanning at 1000 Hz to be on the safe side for a data range 0.5 to 80 Hz.

Output is to the frequency domain via FFT. 

I always thought an analogue filter was required for anti-aliasing. 

However I'm told that analogue filters are not required because a digital filter can do the same thing. 

I learned that a digital filter works by converting time data to the frequency domain and so using a digital filter would be a case of GIGO. 

Who's right and is there other options? 

For example, can the raw data be written to disk and then post-processed through a digital filter for anti-aliasing? 

Or is the raw data already compromised by not starting an anti-aliasing filter? 

Thanks,

Bob

[Travis G.]

Hello Bob,

To truly eliminate aliasing of high frequency signals in your measurements, you must use an analog low-pass filter to eliminate the high-frequency components in the analog signal.  After the signal is digitized, there is no possible way to determine if the frequency components in your data represent actual signals, or aliases of higher frequencies. 

There are ways to use digital filters to implement anti-aliasing.  This requires oversampling of your data in order to extend the Nyquist frequency of your measurements, and applying digital low-pass filtering and decimation in order to get your signal back down to a comparable sampling rate.  However this method only extends the Nyquist frequency of your measurements, and doesn't totally eliminate aliased signals.  Most high-precision data acquisition devices that use this method actually incorporate a combination of analog and digital filters to get the best measurements possible.

In your case, without a device capable of oversampling your data, you'll need to incorporate an analog low-pass filter into your system to truly eliminate any possible aliasing.

I hope this helps,

Travis G.
Applications Engineering
National Instruments
www.ni.com/support

[stephan.gerhards]

Bob

I worked with a WaveBook some years ago. When I remeber right, there are analog high and lowpass filters integratetd into the device. But I don't know, if you can activate them through the dasylab driver.
Maybe you can try to activate the filter with the WaveBook software, and than do your measuemnets with dasylab.

Stephan Gerhards
measX GmbH

[signal1]

Thank you both, Stephan and Travis. 

I appreciate your help.

Stephan, the Wavebook 516 has a 20 kHz 5 pole low pass Butterworth filter.  Because my needed range (0.5 to 80 Hz) is well below this 20 kHz cutoff it will not be effective. 

Besides, I'd prefer to work with smaller data files - the needed 40 to 80 kHz scanning rate would mean much bulkier files. 

Travis, are there any guides/tutorials/links that you can suggest so I can digitally post process the raw data? 

Or any suggestions for how to digitally optimize the data and its processing to minimize aliasing? 

Thanks,

Bob McCarville

Hitachi Trucks

[Travis G.]

Hello Bob,

There are some good resources on the National Instruments website for designing digital filters in LabVIEW using the Digital Filter Design Toolkit.  

For DASYLab technical support, I would recommend contacting technical support at www.dasylab.com.  If you are in Europe or Asia, additional support can be found www.measx.com.

I hope this helps,

Travis G.
Applications Engineering
National Instruments
www.ni.com/support