RF Electronics Chapter 7: RF Filters Page 272
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
These filters are tuned by capacitive top loading on each resonator. Each of the resonators
is thus a fraction shorter than a quarter wavelength, with the required top-loading
capacitance making up this shortfall.
Figure 7.101. 1.8 GHz Coaxial Cavity Filter.
Ceramic Filters
For many consumer applications IF filters are required at only a few specific frequencies.
For example, all FM radios have an IF of 10.7 MHz. All AM radios use a 455 kHz IF. As
a result, commercial filter manufacturers like Murata have a wide variety of low cost
filters available for those frequencies. To achieve small size, low cost and freedom from
microphony, ceramic materials are used for the resonators. Two typical 10.7 MHz
ceramic IF filters are shown in figure 7.102. Many of the latest versions are surface mount
instead of the through-hole filters shown in figure 7.102. The right hand filter contains
two filters like the left hand one, inside the one casing and is used to provide more filtering
of unwanted signals. Most of the FM IF filters are designed to provide a flat group delay
over the passband. This minimised the distortion caused by the IF filter to the
demodulated audio signals.
Figure 7.102. 10.7MHz ceramic IF filters [19].
RF Electronics: Design and Simulation
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