TABLE OF CONTENTS The generalised direct-coupled cavity filter described in the previous section has certain limitations. The structure is ideal for linear phase filters with monotonic stopbands where all the couplings are of the same sign. However, there are difficulties in realising filters with real frequency transmission zeros as these require both signs of couplings, which are difficult to achieve in simple waveguide structures. Furthermore, consider the case where real frequency transmission zeros are required to be symmetrically located on either side of the passband. In the fourth-degree case a pair of zeros would be associated with a single cross-coupling around all four resonators. Thus the pair of transmission zeros are not independently tunable. This can cause problems in manufacturing associated with sensitivity. It is more desirable to be able to extract individual transmission zeros above or below the passband in an independent manner.
This may be achieved by cross-coupled networks in which the cross-coupling is realised by a coupling around three resonators, as described in Chapter 3. This is a useful technique for coaxial resonator and dielectric resonator filters. It is not always suitable for waveguide filters because of the difficulties in coupling around three resonators which requires coupling from both narrow and broad walls of the waveguide. More importantly, when the transmission zeros are located very close to the passband-edge the values of the required couplings may be physically unrealisable.
It is often more desirable to synthesise the transmission zeros using bandstop resonators or extracted poles.
