Figure 3.13: High Q notch filter circuit. f 0 =
2πR1C1 , R1=R2=2R3,C1=
C2= C 2
Figure 3.14: Adjustable Q notch filter circuit.
For a system where digital controller is not used or when it is required to
use analog notch filter for other reasons, passive notch filterssuchasBridged-
T networks  or twin-T notch filter circuit or active notch filters shown in
Figures 3.13 and 3.14 can be used. These active notch filters have relatively
higher Q factors. Active notch filters can also be implemented using switched
capacitor filters such as MF10, or LMF100 † .
We need several components such as resistors, capacitors, operational am-
plifiers etc dedicated to the realization of filter only when an analog filter is
used. A discrete realization, however, does not require dedicated components.
Thecomponentsusedtorealizeadiscretefilter, e.g., ADC, processor etc can
be shared by many other functions. For a mass produced product like HDD,
reduction in the component count is one of the possible ways of reducing cost
of production. In addition, the hardwared notch filters, once designed, can
not be easily modified. Hence, in mass produced HDD's, where the frequency
of resonance may vary up to 5% from one drive to another and in which
feedback controllers are implemented using powerful digital signal processors,
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