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RF Electronics Chapter 7: RF Filters Page 217 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. The design principles are outlined by designing a 3 th order low reflection high pass filter with a cut-off frequency of 100 MHz and a 50 Ω impedance. Similar to figures 2.7 and 2.8, the input terminal of both the highpass and lowpass filters making up the low reflection filter must have a high impedance for the unwanted frequencies. Figure 7.10 shows the starting schematic. The ZnXX values are optimised, with the starting values being the normalised Butterworth filter impedance values obtained from equations 7.1 and 7.2 with R = 1 and c = 1. As a result, the highpass filter is symmetrical, with C3H = C1H. To obtain the best return loss, the symmetry restriction on the lowpass filters is removed, to allow L1L and L3L to become different during the optimisation. Figure 7.12. Low reflection 3 rd order high pass filter, optimised values. Figure 7.13. Optimised low reflection 3 rd order high pass filter, frequency response. The optimisation constraints are the passband and stopband attenuation, specified so that the corner frequency does not change during the optimisation, and the return loss. Figure 7.12, shows the component values after optimisation and figure 7.13 shows the resulting transfer function and return loss. The blue curve in figure 7.13 shows that this circuit has more attenuation in the stop band than the basic 3 rd order Butterworth filter, corresponding to the green curve. At the cut off frequency, the red curve shows it has 10 dB more return loss than the return loss of the 3 rd order Butterworth filter shown as the brown curve. Below 70 MHz, the low reflection filter has a return loss more than 30 dB, compared to close to 0 dB, for the conventional filter. That is thus a big improvement, but the low reflection filter requires 3 times the reactive components of a conventional low pass filter in addition to the two resistors to absorb the unwanted signals. RF Electronics: Design and Simulation 217 www.cadence.com/go/awr

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