Multiband filters have been studied extensively to meet the increasing demands in areas such as satellite systems and modern communication systems where non-contiguous channels are transmitted to the same geographic area through one beam. There are four typical approaches to realise multi-passband frequency-selective circuits, as shown in figure 1.
One way is to design a number of classical single-band bandpass filters and then connect them together through input/output power splitter/combiner, as illustrated in Figure 1(a). However, this simple approach comes with a cost of overall circuit size and power loss due to the input-single splitting process. Figure 1(b) shows the second approach which introduces an input multiplexer along with an output power adder. However, the wide-band power combiner and the inter-module matching issues remain as shortcomings. Multi-band responses can also be achieved by cascading a bandpass filter and several stopband filters, as depicted in Figure 1(c). For this approach, each filter needs to be designed individually, and after connecting all of them, further optimisation may be required to factor in the interference between individual filters.
The fourth approach, utilised in our research relies on designing a single circuit realising the multiband characteristic. The advantage of the fourth approach is that it uses a single component incurring a lower mass and volume and it also eliminates the need of inter-stage matching required by the other three schemes. The superiority of the multi-band filter of the type in Figure 1(d) has given impetus to the recent development in design techniques, most of which are based on the coupling matrix theory.
Figure 2 shows the topology for a specific multiband filter. It is an 8 resonator filter, this particular layout can be made into a single band filter or a dual band filter depending upon the design of specific values of the couplings (black solid and dotted lines). Figure 3 shows a photograph of the filter made in waveguide when configured as a dual band filter. Figure 4 depicts the performance of the filter showing excellent agreement between measured and simulated results.
Multiband filters have now reached a very sophisticated level of design, where extremely complex transfer functions can be achieved. The most complex responses are achieved by the coupled resonator approach to the design. For example, figure 5 shows the topology of a 16 resonator 4 band filter which has transmission zeros at the normalized frequencies of ±1.5, ±0.495, ±0.45, ±0.405, ±0.075, ±0.035; the response of the filter, derived from the coupling matrix, is shown in figure 6.