BRUEL & KJAER 1615 - Page 11

<>--1
Input
c 201
<>--:-1
Input
L201
Ground
Fig.2.2 Schematic diagram of a single 1/3 octave active filter
network
L 202
p 201 c 202 p 203
L 203
c 203
Ground
Fig.2.3 Schematic diagram of a single 1/3 octave passive filter
network
The overall characteristics conform to the existing standards for 1/3 octave
filters. Typical examples
of filter characteristics are shown in Fig.2.4, 2.5;.
Peak-valley ripple in the pass band is less than 0.5 dB and rejection is greater
than 75 dB at frequencies of five times and one fifth of the centre frequen-
cy.
A typical set of adjacent filter characteristics is shown in Fig. 2.6 The freq-
uency characteristics cannot be expected to intersect exactly at the 3. 7 dB
point because of the steep slope at this point. A very small displacement of
the filter centre frequency will cause a large displacement of the point of
intersection.
A buffer amplifier is placed directly after the 1/3 octave filters. This is
necessary because the filter characteristics are very dependent on the
terminating impedance.
7