CS IRGASON, Instruction Manual

Page: 57 / 74

A-1
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, 20, and 25 Hz. FIGURE A-1
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 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 (datalogger 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.
Fortunately, the covariance calculation itself relaxes the need for the ideal
bandwidth. First, the time-averaged (typically thirty-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
(datalogger 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 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.
The following examples show how to use TABLE A-1 . To synchronize EC100
data to other datalogger measurements when the datalogger scan rate is 25 Hz
and the EC100 bandwidth is set to 20 Hz (a 200 ms delay from TABLE A-1 ),
delay the non-EC100 data by five datalogger scans. Similarly, for a 10 Hz
datalogger scan rate and the same 20 Hz EC100 bandwidth, delay the non-
EC100 data by two datalogger scans to match the EC100 data. For the best
synchronicity, choose a datalogger scan interval that is an integer multiple of
the EC100 filter delay.