National Instruments 4551 User Manual page 82

Input Time
Domain Signal,
Sampled at fs
Sampling
frequency = fs/2
Sampling
frequency = fs/(2
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BPF
BPF
BPF
LPF
2:1 Decimator
BPF
BPF
BPF
LPF
2:1 Decimator
(N-1)
)
BPF
BPF
BPF
Figure 6-8. Fractional-Octave Analyzer Architecture
The input time-domain signal is sampled at a sampling frequency f
Band Pass Filters (BPFs) in stage 1, corresponding to the highest octave,
filter the signal directly at the sampling frequency. The signal is then passed
through a digital low pass filter (LPF) to remove higher frequencies, then
to a 2:1 decimator, which effectively reduces the sampling frequency to
half (f /2). That is, for every two samples input, one sample is output. The
s
lowpass-filtered and decimated signal is then passed to the BPF(s)
corresponding to the next highest octave in stage 2. The process is repeated
for a maximum of 10 stages of BPF(s), or a maximum of nine stages of
lowpass filtering and decimation.
Averaged Power
Average
Averaged Power
Average
Averaged Power
Average
Averaged Power
Average
Averaged Power
Average
Averaged Power
Average
Averaged Power
Average
Averaged Power
Average
Averaged Power
Average
6-25
Chapter 6 Advanced Concepts
Stage 1
Stage 2
Stage N
. The
s
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