Campbell Scientific CR9000X Measurement and Control System - Manual (Page 119)

Section 3. CR9000X Measurement Details
All CR9052 channels in a single CR9000X chassis are sampled
simultaneously (channel to channel sampling simultaneity of less than 100
nanoseconds).
The CR9052 takes measurement samples at 3.2 MHz and implements anti-
aliasing, using programmable, real-time, low-pass, finite impulse response
(FIR) filters. An on-board digital signal processor (DSP) collects alias-free,
50-kHz samples from each of the module's sigma-delta converters, and then
applies real-time, programmable low-pass filtering and decimation to anti-alias
and down-sample the data to the selected measurement rate, selectable from 5
Hz to 50 kHz.
The CR9052 can also accumulate snapshots of anti-aliased time-series, Fourier
transform them into frequency spectra, and send the resulting real-time spectra
to the CR9000X's main processor.
The CR9052 can burst measurements to its on-board, 8-million sample buffer
at 50,000 measurements per second per channel. Using the FFT spectrum
analyzer mode, the module's DSP can provide real-time spectra from
"seamless", anti-aliased, 50-kHz, 2048-point time-series snapshots for each of
its six analog input channels. The decimated data can be downloaded to an
appropriate PC card at an aggregate rate of 300,000 measurements per second.
The CR9052 filter's pass-band ripple is less than ±0.01 dB (0.1 percent), and
the stop-band attenuation exceeds 90 dB (1/32,000). The FIR filter's transition
band has a steep roll-off, with the stop-band frequency starting a factor of 1.24
above the pass-band frequency. In comparison, the stop-band frequency of an
ideal eight-pole Butterworth filter with the same ripple and attenuation starts a
factor of 5.81 above its pass-band frequency. See Chart 3.3-1 for comparison.
Chart 3.3-1 FIR Filter versus 8-Pole Butterworth Roll-off
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