RF Electronics Chapter 7: RF Filters Page 228
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
in these equations, so that if a Butterworth filter is required, those values can easily be
linked to the actual K and Q values used for the filter (q
1a
, q
na
, k
12a
, k
23a
, k
34a
).
Using those variables in the filter schematic gives the filter shown in figure 7.30. Since
the values are calculated in the Global Definitions folder, the resulting variables have
been displayed in figure 7.30 for convenience.
Figure 7.30. Capacitive coupled Bessel LC coupled resonator filter.
Figure 7.31. Frequency response of the filter of figure 7.30.
By comparing figure 7.28 and figure 7.31, it can be seen that capacitive coupling reduces
the attenuation at high frequencies and increases the attenuation at low frequencies. In
addition, the group delay is less flat than for the lowpass to bandpass transformation.
Instead of using capacitive coupling, inductive coupling can be used. For capacitive
coupling, the total resonating capacitance is made up of the capacitor of a resonator plus
the adjacent coupling capacitors according to equation 7.8, which is incorporated in the
LC Bandpass filter design program of figure 7.29. For inductive coupling the capacitance
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