of Microprocessors

SOURCE-SIGNAL ANALYSIS 553
SIGNAL
BANDPASS
TO BE
FILTER
ANALYZE- D
RECTIFIER
LOW-PASS
TIME-VARYING
FILTER r-- SPECTRAL
COMPONENT
Fig. 16-5. One channel from a filterbank spectrum analyzer
structure of each channel of such an analyzer is shown in Fig. 16-5. First the
signal is bandpass filtered with a center frequency corresponding to the
channel under consideration. The output of the filter, which is still an ac
signal but with a limited frequency range, is rectified as the first step in
determining its amplitude. This is best accomplished with a full-wave rectifier
so that the ripple frequency will be high. A final low-pass filter removes
the ripple, giving the short time average of the bandpass filter output. For the
lowest couple of bands, the design of this filter is critical, since roo much
filtering means a slow response, while inadequate filtering lets the ripple
through, effectively adding noise to the channel output.
A Digital Filterbank Spectrum Analyzer
Let's now discuss a digital implementation of the analog filterbank
analyzer. The main advantage of the filterbank method over the Fourier
transform method that will be described later is its simplicity and ease of
understanding. Because of its simplicity, dedicated hardware implementa-
SAMPLED
INPUT
SIGNAL
SPECTRUM I
SAMPLE RATE
(100 Hz)
I I CHANNEL
OUTPUT
• •
N
• •
• ••
• ••
N
SPECTRUM
SAMPLE
CLOCK
Fig. 16-6. Digital filterbank spectrum analyzer