The introduction of a fractional bandwidth parameter for bandpass filters significantly impacts performance and realizability. Over the years, a number of unique distributed bandpass structures have been developed which provide the best possible performance for certain characteristics at the expense of others. There is no one best solution for all applications. The designer who attempts to apply a favorite structure to all problems will not have the success of those who learn to match filter structures and required specifications. Therefore, this chapter is a study of a range of distributed bandpass structures and the advantages and disadvantages of each. We close with a powerful technique for taming the tricky process of tuning bandpass filters of all types.

8.1 Direct-Coupled Bandpass

The direct-coupled bandpass is formed when the lumped/distributed approximate equivalences of Figure 3-15 are applied to the conventional exact-transform lumped element bandpass structure depicted in Figure 4-1. Consider a 3rd order Butterworth lumped element bandpass with a lower 3.01 dB cutoff frequency of 800 MHz and an upper cutoff frequency of 1200 MHz. The schematic of the lumped element network is given in Figure 8-1a. Lumped-element values are found using expressions in Section 4.2 and are listed in column two of Table 8-1. The element values are symmetric, so L1=L3 and C1=C3. The reactances are listed in column three.