Suspended substrate stripline is similar to standard stripline circuits with the exception that most of the dielectric surrounding the circuit is removed to increase the structure's unloaded Q. This increase in unloaded Q results in lower passband loss than conventional stripline filters. In addition, the lack of dielectric surrounding the circuit makes the filter less sensitive to ambient temperature variations.
A basic suspended substrate stripline filter consists of an etched copper clad dielectric board secured in air between silverplated, machined housings. To ensure proper support and suppress RF leakage, internal walls are also used to secure the board a various intervals within the housing. Suspended substrate filters can be supplied with either coaxial connectors or axial/radial feedthroughs for installation into a circuit board or for drop-in MIC applications.
The photo-etched circuitry ensures consistency and repeatability from unit to unit. This topology is suitable for lowpass, highpass, bandpass, bandstop and multiplexing applications in both broadband and narrowband filters. It is currently used in many active EW systems involving ELINT, ECM and ECCM.
The broad range of achievable impedance values make suspended substrate particularly suitable for highpass and lowpass filters. Broadband bandpass filters and multiplexers can be constructed by cascading highpass and lowpass filters together. The simple, repeatable printed circuit design and small number of parts result in an easily producible unit well suited for high volume manufacturing.
The Cauer/pole-placed transfer functions used in many of the suspended substrate designs result in highly selective, low insertion loss devices. In addition, the compact structure and the flexibility of the printed circuit technology make suspended substrate designs particularly well suited for integrated assemblies.
Couplers, power dividers, and amplitude equalizers can be readily realized in suspended substrate. The printed circuit board configuration naturally lends itself to integration with other topologies, as well as to the direct integration of active components.
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