RF Electronics Chapter 7: RF Filters Page 261
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 7.83. Hairpin filter hardware. (78x68mm).
Figures 7.84 to 7.87 show the measured frequency responses of the filters. Comparing
these with figures 7.78 and 7.79, shows the remarkable agreement between the calculated
and measured results. The passband of the interdigital, combline, and direct-coupled
filters are a few percent lower than calculated. The difference matches the length of the
pins used in connecting the grounded end of the resonators to the ground-plane on the
bottom of the PCB. This can be taken into consideration in the second iteration of the
filter designs if needed.
The hairpin filter has a measured centre frequency that is very close to the desired 1 GHz
centre frequency. This is due to no ground connections being required, thus removing an
uncertainty of pin or via placement from the design. From the measured passband
responses, hairpin filters have a better close in stopband attenuation and a more accurate
centre frequency are thus a good choice for one off designs, where the performance has
to be correct first time. The frequency error between the measured and calculated centre
frequency for the interdigital, combline and direct-coupled filters, closely responds to the
resonator length equal to the thickness of the substrate.
Figure 7.84. Measured frequency response of interdigital filter.
RF Electronics: Design and Simulation
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