Campbell Scientific CR9000X Measurement and Control System - Manual (Page 273)

Section 7. Measurement Instructions
7-49
FFTFilt (Dest, Reps, Range, Fslot, Channel, FiltOption, Excitation, Mult,
FSampRate, FFTLen, TSWindow, SpectOption, Fref, SBin, ILow,
IHigh)
The CR9052 filter module can perform real-time fast Fourier transform (FFT)
analyses on the voltages measured on its inputs, and then pass the resulting
spectra to the CR9000X CPU for further processing and storage into data
tables. The FFT operation is specified with the FFTFilt instruction.
This instruction cannot be used in a SubScan or Slow Sequence
Scan.
With the VoltFilt instruction the Scan (or SubScan) interval determines the
rate at which individual measurements are passed to the CPU. With FFTFilt
the Scan interval is how often an entire spectrum for each channel is sent to the
CPU. The sample rate for the FFT time-series is set within the instruction.
FFTFilt can provide spectra from “seamless” time-series snapshots if the Scan
interval is set equal to it’s minimum value: the FFT length divided by the time-
series sample rate (i.e., measurements are continuously sampled, an FFT is
calculated each time the required number of measurements are sampled, no
samples are missed.) When the scan interval is greater than this minimum
value there will be gaps between acquiring the FFT time series.
The first eight parameters of the FFTFilt instruction are similar to the first
eight parameters of VoltFilt. The Fslot, FiltOption, FSampRate, and
FFTLen parameters must be the same for all channels of a single CR9052DC
module. The other parameters may be unique for each channel.
Parameter
& Data Type Enter FFTFILT PARAMETERS
Dest
Variable or
Array
The Variable in which to store the results of the instruction. Because FFTFilt returns all or
part of an entire spectrum (see ILow and IHigh) for each Rep, Dest usually must be an array.
Reps
Constant
The number of times to repeat the measurements and subsequent FFTs on consecutive
CR9052DC channels. Spectra from multiple Reps are placed head-to-tail in the Dest array.
Range
Constant
The voltage range for the measurement. The CR9052 normally replaces out-of-range
measurements with not-a-number (NaN) which is displayed in RTDaq's realtime windows as
Range?. Users may choose to have out-of-range measurements to be replaced by the analog-
to-digital converter saturation value with a special code in FiltOption
.
Alpha Code Numeric Code Voltage
Range Module Excitation
Board Supported
mV1000 0 ± 5000 mV CR9052DC, CR9052IEPE
mV1000 1 ± 1000 mV CR9052DC, CR9052IEPE
mV200 4 ± 200 mV CR9052DC
mV50 5 ± 50 mV CR9052DC
mV20 6 ± 20 mV CR9052DC
Fslot
Constant The number of the slot that holds the CR9052 Module to be used for the measurement.
Chan
Constant The CR9052 channel number on which to make the first measurement. When Reps are used,
subsequent measurements will be automatically made on the following differential channels.
NOTE