FILTER
CENTRE FREQUENCY
The centre frequency
of
a pass-band filter
is
the
geometric mean
of
the
band-edge frequencies.
where ·
fc=~
fc = Filter centre frequency
and
f1, f2 are
the
band-edge frequencies.
FILTER
BAND-EDGE FREQUENCIES
The band-edge frequencies are
the
frequencies
at
which
the
response
of
the
filter
is
a certain value lower
than
the
maximum response.
Usually this value
is
chosen
to
be 3 dB
but
it can be defined as required. In
the
case of the 1614/15 Filter Set for instance;-3.7
dB
is
quoted, so
that
the
bandwidth of each filter
is
1/3 octave
at
the-3.7
dB points and this
is
also
then
the
effective bandwidth.
BANDWIDTH
The bandwidth
of
a
band-pass
filter
is
the
value (in
Hz
of
octaves)
the
difference between
the
upper and lower band-edge frequencies.
i.e. B
=
f2-f1
or
b
log2
:~
octaves
Bandwidth
is
sometimes also expressed as a percentage
of
the
passband
centre frequency.
SPECTRUM
The spectrum of a function of time
is
a description
of
its resolution into
components, each of different frequency.
Continuous Spectrum
A continuous spectrum
is
a spectrum
in
which
the
components are continuously distributed over a fre-
quency region.
White Noise
Spectrum A white noise spectrum
is
a continuous spectrum
with a
constant
spectrum density (mean square
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