Page 82 - Analog and Digital Filter Design

Time and Frequency Response 79

Figure 2.27
Fourth-Order Lowpass

L2 L4

c2 c4
Input r II II output

Figure 2.28
Fifth-Order Lowpass

L2 L4
L6
c2 ' c4
Input II II output
Figure 2.29
Sixth-Order
Lowpass

The pattern of circuit design as the filter order increases can be seen from the
examples. A seventh-order filter will have an extra two capacitors; C6 will be
connected in parallel with L6, and C7 will be between the output and the
common rail. Odd-order filters are symmetrical, but even-order filters have a
single series inductor to the load.

Minimum capacitor designs are also possible, although less likely to be
used since inductors are much harder to produce than capacitors. In this design,
inductors replace capacitors and capacitors replace inductors. Figure 2.30
gives a circuit schematic for this filter topology. In this design series inductors
replace shunt capacitors, and a series resonant shunt arm replaces the
parallel resonant series arm. The value of the shunt capacitor is equal to the
series inductor in the minimum inductance design. Similarly, the value of
the shunt inductor is equal to that of the series capacitor in the minimum
inductance design.