Campbell CR9000, Инструкция по эксплуатации

Страница: 66 / 218

Section 3. CR9000 Measurement Details
3-22
3.3 CR9052 Filter Module Measurements
Each CR9052 module has six differential analog measurement channels with
programmable input ranges from ±20 mV to ±5 V. Each channel has its own
programmable-gain instrumentation amplifier, pre-sampling analog filter, and
sigma-delta analog-to-digital converter. All CR9052 channels in a single
CR9000X chassis are sampled simultaneously.
The CR9052 implements anti-aliasing with 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 100,000 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.
The digital implementation of the CR9052 FIR filters maintains a group delay
that is independent of frequency (linear phase response). In addition, the
digital filter performance does not change with time, temperature, or
component tolerances. The on-board DSP automatically chooses the