Combline filters, also referred to as cavity filters, are narrowband and used in applications where closely spaced signals need to be separated.
A high Q structure is very important for narrowband applications, particularly when high powers are involved. Low passband, low VSWR and high attenuation are important features of Teledyne Defence & Space machined filters. Teledyne Defence & Space has also investigated the intermodulation performance of high power filter applications and devised special designs to meet these requirements.
The structure consists of a series of Transverse Electromagnetic (TEM) resonators of a circular or rectangular cross section, nominally λ/8 long at the centre frequency, resonated by a lumped capacitance at the open circuit end. The bandwidth and response of the filter is determined by the amount of coupling of each resonator to its immediate neighbour, which in turn is dependent on resonator size, resonator spacing, and ground plane separation. To achieve low mass the normal construction is in aluminium with the application of high grade silver plating to attain the lowest possible passband losses.
Combline filters can be designed for bandwidths of 1% to 50% of the centre frequency in the range of 100MHz to 20GHz. By introducing inductive or capacitive cross-coupling between the appropriate nodes, it is possible to either produce attenuation poles in one or both the rejection skirts, or equalise the group delay with linear phase designs.In applications where flat group-delay narrowband filters are required, linear phase responses of typically 1 to 5 ns variations over 70% of the filter bandwidth can be achieved.
In some applications, the combline designs can be realised in suspended substrate to achieve very compact devices. The printed method is often used where integration with components is involved.
Multiplexers can be formed by coupling together bandpass filters to a common transformer. 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.
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