Campbell CR9000, Instruction Manual

Page: 156 / 218

Section 7. Measurement Instructions
7-42
Alaising is not a problem with filter option 1 because any signals in the
transition band up to the stop frequency of 26.8 kHz will be alaised to
frequencies higher than the pass frequency of 23.2 kHz.
The pass frequency ( F
PASS
) is the maximum frequency that is not attenuated by
the filter. Be sure that the selected filter option FiltOption in combination with
FSampRate makes F
PASS
greater than or equal to the maximum frequency of
interest,
high
f
. (i.e., that
pass high
f f
≤
).
One effect of the filter option used is on the number of spectral bins calculated
by the FFT beyond the pass frequency. The pass frequency is defined in terms
of the sampling ratio,
samp
R
, the ratio of the sample rate to the pass frequency :
samp SR pass
R f f /
=
. For the smallest sampling ratio of 2.5, the number of
bins representing frequencies greater than
pass
f
is approximately 20% of the
bins calculated by the FFT. This goes up to 90% of the calculated bins for the
maximum sampling ratio of 20. It is easy to set IHigh to not return bins
beyond
pass
f
. However, the fewer calculations required for the same
maximum frequency,
pass
f f
=
max
, when using a sampling ratio of 2.5 vs a
sampling ratio of 20 may make the difference between seamless and
intermittent FFTs if the FFT length has to be increased at the higher sample
rate to obtain the desired minimum frequency.
Minimum Frequency
Once FsampRate is selected to include the highest frequency of interest,
FFTLen can be set to determine the lowest non-zero frequency.
The lowest frequency AC component of an FFT (bin 1 in the description of the
FFT Spectra above) has a center frequency,
( )
N
f
N
f
f
SR SR
c
= × =
1 1
.
Where
SR
f
is the sample rate ( FsampRate , samples/second) and
N
is the
number of samples ( FFTLen ). This frequency is the reciprocal of the time
required to complete the sampling. In other words, exactly one cycle of this
low frequency is completed in the time it takes to sample the time series for the
FFT. To be sure the spectrum output by the FFT includes the lowest frequency
of interest,
low
f
, set
N
( FFTLen ) so that:
low
SR
f
N
f
≤
.
Frequency Resolution
Frequency resolution goes hand in hand with the minimum frequency. The
difference between the center frequencies of adjacent spectral bins is
N
f
SR
,
and bandwidth of each bin is also
N
f
SR
.