Page 223 - Analog and Digital Filter Design

220 Analog and Digital Filter Design

To design a Cauer or Inverse Chebyshev filter, a different circuit topology is
required. The Cauer response has zeroes in the stopband, so a tunable notch
circuit is required. This can be achieved using a circuit that is an extension of
the state variable filter and is known as a biquad. This circuit is illustrated in
Figure 7.17. Note that, in the bandstop biquad, R5 is connected to a different
node. This is dependent on whether the zero is above or below the resonant fre-
quency. If the zero is above the resonant frequency, connect nodes A and C. If
the zero is below the resonant frequency, connect nodes B and C.
-

R3
R1

C
--IF C R

-
In F
i

-
ov

B
A ..
..
..
i’C
IR I IR R I R6
R6
nR5 m
I
T
output

ov

Figure 7.17
The Bandstop Biquad Filter
The following equations give component values.