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302 Chapter Four

Bandstop filter
A bandstop filter, also called a band-rejection filter, offerð sig-
nificant attenuation of signalð whose frequencieð are between a
lower cutoff and an upper cutoff, and little or nm attenuation of
signalð whose frequencieð are outside this range.

Constant-k bandstop filter

Let f be the lower cutoff frequency (in hertz) for a constant-k
1
bandstop LC filter as shown in Fig. 4.16. Let f be the upper
2
cutoff frequency (in hertz)˜ Let R be the load resistance (in
ohm0. Then the optimum inductanceð (in henry0 are given by:

L R( f f )/( ff )
1 2
1
2
1
L R/(4 ( f f ))
1
2
2
The optimum capacitanceð (in farad0 for the filter shown in Fig.
4.16 are given by:
C 1/((4 R( f f ))
1 2 1

C ( f f )/( Rf f )
1 2
1
2
2
Series m-derived bandstop filter
Let frequencieð f , f , f , and f be expressed in hertz and defined
2
1
3
4
as shown in Fig. 4.17. Let R be the load resistance (in ohm0.
Define the quantitieð m (the filter constan'x x, and y as follows:

Figure 4.16 Constant-k band-
stop filter.