SIRIUS®
One way that this can easily happen, is when you set a custom sampling rate on a single channel in the analogue
channel setup:
Illustration 189: Analogue Channel Setup – custom sample rate
In the example above you can see that the dynamic acquisition rate () is set to 100kHz. The sample rate of the first
analogue input channel (AI 1) is set to only 10kHz (). This means, that the Sirius® hardware will send the
measurement data of all channels at a rate of 100kHz via USB to DEWESoft® and DEWESoft® will then reduce the
data for channel AI 1 to 10kHz.
This reduction could be done by simply using only every 10
th
input sample that we get from Sirius® (and thus skipping
9 out of 10 samples): we already know from the previous chapters that this will introduce aliasing!
DEWESoft® provides a convenient way to select how to tackle this problem: the custom Low-pass filter setting ( in
Illustration 189) in the analogue channel setup:
OFF: when you deactivate the Low-pass filter, the samples are skipped as described above and you will
experience aliasing effects.
Alias free: this setting, will automatically adjust the low-pass filter frequency, so that your measurement data is
always aliasing free.
Details: DEWESoft® will automatically set the filter-frequency to 0.4 of the reduced rate and use a Bessel
filter of 8
th
order
Note: when you do not set a custom sampling rate for the channel (i.e. and are the same in Illustration
189), then no filtering is required. Thus Alias free is the same as OFF in this case.
Custom frequency (e.g. 100Hz, 300Hz, 1kHz, …): will set the filter frequency accordingly.
IMPORTANT
The custom low-pass filter is an IIR filter, so it will introduce a phase shift – similar to an
analogue filter.
Using a FIR filter for many channels is often not possible, because it requires considerably
higher computation power (which cannot be provided by current general purpose CPUs).
Note: if you want to use a FIR filter for your channel/s anyway, then you can disable the
custom filter (set it to OFF) and add FIR filter Math-channels instead.
Page 136/166 www.dewesoft.com Doc-Version: 1.4.2
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