RF Electronics Chapter 7: RF Filters Page 221
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 7.19. Frequency response of the LC filter and the corresponding Microstrip design.
Figure 7.19 compares the frequency response of the ideal LC filter (red) with the PCB
filter of figure 7.19 (blue) and it shows that the PCB filter behaves well in the passband,
but that the stopband performance is not very good. That is due to the elements becoming
comparable to a quarter or half wavelength long at certain frequencies.
Figure 7.20. Replacement of capacitors using shunt Microstrip transmission lines.
By replacing the capacitors with open circuited shunt transmission lines as shown in
figure 7.20, it is possible to obtain a better stopband performance. By making the lengths
of the stubs connected at each cross-point slightly different, the effect of the transmission
lines being one quarter or one-half wavelength is minimised. In figure 7.20, the
transmission line has a high impedance of 2.48 kΩ at 2.48 GHz. For figure 7.20, by using
stubs, the impedance is 1.82 kΩ at 9.2 GHz. By using radial stubs as shown in figure 7.21,
the resonance can be extended even further to give a peak impedance of 1.59 kΩ at a
frequency of 12.5 GHz. The shape of the radial stub is shown in figure 7.21. The angle
for that stub is set to 60 degrees. A wider-angle stub will push the high impedance
frequency even further. It is possible in a filter to vary the radii and angles to ensure that
the resonances do not coincide.
RF Electronics: Design and Simulation
221 www.cadence.com/go/awr
