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Combline / Interdigital Filters

Although very similar to cavity filters, combline filters can be designed to have bandwidths of 1% to 50% of the centre frequency.

Centre frequencies in the range of 100MHz to 20GHz are possible with stopbands extending to 5 times f0. Wider stopbands are achievable with appropriate resonator loading.

Multiplexers can be formed by coupling band pass filters together at a common transformer junction. Both contiguous and non-contiguous types are available. Diplexers, for example, are often used in applications such as Tx/Rx communications systems, where a common antenna is shared.

In some applications requiring wider bandwidths and where flatter group delay is important, we can use interdigital structures. The electrical and mechanical characteristics are similar to combline but the resonators are longer and are alternately inverted. The longer resonators result in a lower upper stopband frequency.