Page 172 - DSP Integrated Circuits
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4.18 Lattice Wave Digital Filters                                    157


            The reflectance is an allpass function:




        where




        and







            In some cases it may be advantageous to use a series resonance circuit since
        the multiplication by -1 in the wave-flow graph shown in Figure 4.46 is avoided.
            Higher-order reflectances can be obtained by connecting circulators loaded
        with reactances as shown in Figure 4.47. The reflectance corresponding to the
        open-ended unit element in Figure 4.47 is




        where






























                          Figure 4.47 Higher-order circulator structure

            Circulators can be implemented in the microwave range where they are used
        to, for example, direct signals from the transmitter to the antenna and from the
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