RF Electronics Chapter 7: RF Filters Page 260
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 7.79 shows the frequency response of the four filters at the harmonics of the 1 GHz
passband. For the direct-coupled resonator filter the lengths of the coupling lines was
12.5% of a wavelength, which is half of the length of the coupling lines used for the filter
in figure 7.73. This resulted in a lower harmonic attenuation. The direct-coupled filter has
the best close-in stopband attenuation, but the worst stopband attenuation above 2 times
the centre frequency. The second harmonic response from the hairpin filter is significant,
but can be removed using harmonic stubs. The interdigital filter does not have a
significant harmonic response, but it does not have a very good stopband attenuation
either. The stopband attenuation for interdigital filters does not significantly improve with
filter order. The Combline filter is the smallest, occupying 41 x 60 mm. The interdigital
filter is slightly larger occupying 42 x 60 mm. The direct-coupled resonator filter,
occupies 75 x 60 mm and the hairpin filter is the largest, occupying 78 x 68 mm.
Figures 7.80 to 7.81 show the hardware of the four filters, approximately to the same
scale. The Combline filter is slightly smaller than the interdigital filter, but in most
instances, there is not enough space saving to warrant the worse stopband attenuation.
Figure 7.80. Interdigital filter hardware. Figure 7.81. Combline filter hardware.
(42x60mm) (41x60mm)
Figure 7.82. Direct-coupled filter hardware. (75x60mm).
RF Electronics: Design and Simulation
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