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366 Other Direct Filter Design Methods
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Figure 9.30.
Three·pole bandpass equivalent network using L I2 and CD'
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Figure 9.31. Three~pole bandpass equivalent network using C l2 and L 23 •
9.4.2. Trap Approximations. Taylor series were described in Section 5.] A. A
first-order approximation of susceptance by value and slope at a frequency
was employed for resonators in Section 8.304. The difference here is that the
susceptance at the frequency of interest will not be zero. In the following, it
will be assumed that trap notch frequencies are greater than any reference
frequency. Pole-zero branches having two, three, or four elements, similar to
those discussed earlier in Section 904.1, will be considered.
It is informative to note that two elements cannot replace one in a
first-order equivalence. Consider the two branches shown in Figure 9.32. The