RF Electronics Chapter 7: RF Filters Page 225
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
2.0 mH and a 4.4 mH inductor is required. These are very large inductors and will be
difficult to make with acceptable Q values. Figure 7.28 shows the frequency response of
this filter. The low-pass to band-pass transformation has transformed the amplitude
response correctly, but the group delay is no longer flat.
Figure 7.27. Filter from lowpass to bandpass transformation.
Figure 7.28. Frequency response of filter of figure 7.25.
LC Coupled Resonator Filter
In coupled resonator filters, the filter consists of a series of resonators, which are coupled
using capacitive or inductive impedances, coupling loops, as shown connected to the
BNC connectors in figure 7.58, or coupling apertures, as shown by the gap between the
resonators in figures 7.57 and the slots cut between the resonators in figure 7.101. For
many Microstrip filters like the one shown in example 7.1, the coupling between the
resonators are achieved by varying the spacing between the MCFIL lines making up the
filter. Example 7.3 shows how the correct coupling between resonators can be
determined.
The centre frequency of the filter is determined by the frequency of the resonator and the
bandwidth of the filter is determined by the coupling between the resonators. The input
impedance of the filter is determined by the coupling into the first resonator and
RF Electronics: Design and Simulation
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