Campbell IRGASON, Руководство по эксплуатации продукта, Страница: 62 / 91

Appendix A. Filter bandwidth
and time delay
The EC100 measures CO
2
, H
2
O, 3-D wind components, and sonic temperature from the
IRGASON at 60 Hz and then applies a user-selectable low-pass filter. The available filter
bandwidths are 5, 10, 12.5, and 20 Hz.
FIGURE A-1 (p. 59) shows the amplitude response of these
filters. The EC100 filters provide a flat pass band, a steep transition from pass band to stop band,
and a well-attenuated stop band.
FIGURE A-2 (p. 60) compares the EC100 10 Hz filter to a 50 ms
moving average filter with approximately the same bandwidth.
The ideal eddy-covariance filter is one that is wide enough to preserve the low-frequency signal
variations that transport flux and narrow enough to attenuate high-frequency noise. In addition,
to minimize aliasing (the misinterpretation of high-frequency variation as lower-frequency
variation), the measurement bandwidth must be less than half of the sample rate (data logger
scan rate). Two factors complicate choosing the ideal eddy-covariance bandwidth. First, the flux
signal bandwidth varies from one installation to another, and the flux signal bandwidth varies
with mean wind speed at a given installation. Second, the fast sample rate required to anti-alias a
desired signal bandwidth may result in large, unwieldy data sets.
The covariance calculation itself relaxes the need for the ideal bandwidth. First, the time-
averaged (typically 30-minute) covariance calculations inherently reduce noise, and second,
aliasing does not degrade the accuracy of covariance calculations. Therefore, the factory default
for the EC100 bandwidth (20 Hz) is rather wide to preserve the signal variations that transport
flux, and that bandwidth is suitable for most flux applications. Additional bandwidths are
available for experimenters desiring to match the EC100 filter bandwidth to their data acquisition
sample rate to avoid aliasing. In this case, the selected bandwidth should be one-half of the
sample rate (data logger scan rate), and experimenters should be careful to avoid attenuation of
flux-carrying signals.
The EC100 electronics synchronously sample the gas analyzer and sonic anemometer of the
IRGASON. However, experimenters wishing to synchronize their EC100 data with other
measurements (e.g., energy balance sensors) in the data acquisition system must account for the
time delay of the EC100 filter.
Table A-1 (p. 60) shows the delay for each of the filter bandwidths.
The EC100 provides a constant time delay for all spectral components within each filter’s pass
band.
IRGASON® Integrated CO
2
/H
2
O Open-Path Gas Analyzer and 3D Sonic Anemometer 58