Via fence

A via fence, also called a picket fence, is a structure used in planar electronic circuit technologies to improve isolation between components which would otherwise be coupled by electromagnetic fields. It consists of a row of via holes which, if spaced close enough together, form a barrier to electromagnetic wave propagation.

Modern electronics have components and sub-units at high densities to achieve small size. Typically, many functions are integrated on to the same board or die. If not properly shielded from each other, many problems can result including poor frequency response, noise performance, and distortion.

Via fences are used to shield microstrip and stripline transmission lines, guard edges of printed circuit boards, shield functional circuit units from each other, and to form the walls of waveguides integrated into a planar format. Via fences are cheap and easy to implement, but use up board space and are not as effective as solid metal walls.

Planar technologies are used at microwave frequencies and make use of printed circuit tracks as transmission lines. As well as interconnections, these lines can be used to form components of functional units such as filters and couplers. Planar lines readily couple to each other when in close proximity, an effect called parasitic coupling. The coupling is due to fringing fields spreading from the edges of the line and intersecting adjacent lines or components. This is a desirable feature within the unit where it is made use of as part of the design. It is not desirable, however, that the fields couple to adjacent units. Modern electronic devices are usually required to be small. That, and the drive to keep down costs, leads to a high degree of integration and circuit units in less than desirable proximity. Via fences are one method that can be used to reduce parasitic coupling between such units.

Amongst the many problems that can be caused by parasitic coupling are reducing bandwidth, degrading passband flatness, reducing amplifier output power, increasing reflections, worsening noise figure, causing amplifier instability, and providing undesirable feedback paths.

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