RF Electronics Chapter 7: RF Filters Page 223
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
The behaviour of the resulting filter is also calculated using a full electromagnetic (EM)
simulation of the capacitive stub lines. The process of EM simulation using AXIEM is
described in the section EM simulation in this chapter. The frequency response obtained
from the circuit simulation, the EM simulation and the measured performance is shown
in figure 7.23. It can be seen that there is a good agreement for frequencies below 4 GHz
but that the stopband behaviour is still not adequate. The filter hardware realisation is
shown in figure 7.24.
Figure 7.24. 2 GHz Lowpass filter hardware.
Figure 7.25. 2 GHz lowpass filter with stubs added.
The attenuation in the stopband can be improved by adding open circuited stubs, which
create a transmission zero in the stopband at the frequencies where the filter without the
stubs has an inadequate attenuation. In this application, a good stopband performance to
up to 6 GHz was required. The stubs will affect the passband performance and the circuit
elements need to be re-tuned and optimised to restore the passband performance to match
that of figure 7.23. The resulting PCB layout is shown in figure 7.25. The end of each
stub should be a minimum of 3 substrate thicknesses and preferably 5 substrate
thicknesses (4 mm) away from the nearest metal structure to minimise coupling. This
design satisfies that requirement. If needed the right most stub can be placed on the other
side of the transmission line to reduce the distances to other structures.
This filter was constructed and figure 7.26 shows the simulated and measured
performance. The simulated performance has a very good stopband performance. The
RF Electronics: Design and Simulation
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