High frequency signals, such as radio frequency (RF) signals and microwave signals, are often carried over coaxial cables and waveguides between subassemblies, to external connections, and within subassemblies to and from printed circuit (PC) boards, hybrid packages and outside instruments. Signals on boards and packages may be carried over transmission lines or structures such as quasi-coaxial structures, co-planar waveguides, microstrips and striplines, for example. For any RF signal-carrying structure there is a characteristic impedance which should be consistent and controlled throughout such a system. Often this characteristic impedance is set to zo=50Ω. The characteristic impedance is determined by material properties such as conductivity, permittivity and permeability, as well as the geometry of the RF signal carrying structure. An additional factor that may also help to determine how well a structure carries RF signals relates to the dielectric loss of insulator materials.
The signal quality of such a system may be degraded at any place in the system where the characteristic impedance varies from 50Ω. Variations in geometry or material properties can cause changes in impedance, resulting in signal quality degradation of the system. For example, the characteristic impedance may vary locally from 50Ω at a transition between one 50Ω line and another 50Ω line if the structures have different geometries or are made of different materials. The variation in characteristic impedance at a transition between two 50Ω structures may be minimized if the material properties and/or geometries vary gradually in the transition between the two 50Ω structures.
Some systems may include matching connectors on the RF cable and on the sub-assembly or package that can be relatively expensive. For instance, excluding manufacturing costs associated with installation, some assemblies using ceramic hybrid packages may include two RF connectors which together may account for almost 15% of the total sub-assembly or package cost. As connector cost roughly scales with frequency, the costs associated with higher frequency connectors and components operating beyond 50 GHz may be substantially greater.
Moreover, installation of the connectors on the sub-assembly or package may be difficult and error-prone, leading to re-work, low yield, and breakage. The overall cost associated with the connectors may thus be much greater than the cost of the connector itself.
Regardless, these connectors may not provide a gradual transition in geometry and/or materials, leading to deviations from the desired 50Ω signal path. The result may be signal degradation at the connectors.
A connection for use with ceramic hybrids may entail soldering a through connector to the back of the ceramic hybrid. The inner signal wire of the through connector may be wire bonded to the upper surface of the ceramic hybrid. Although such through connectors may be relatively less expensive, a 90-degree bend may be created in the signal path of the inner signal wire. Because RF signals do not follow sharp bends effectively at higher frequencies, due to signal degradation this approach tends to limit frequency performance to about 30 GHz. Moreover, the production costs of processing vias with through connectors are typically high.
Other connections may include printed connector assemblies built with micro-electro-mechanical systems (MEMS)-type processing using sacrificial materials. However, 90 degree bends may be created in the signal path, limiting signal frequency to about 30 GHz. Also, MEMS-type processing is expensive, and building full 3-dimensional structures required to match 3-dimensional RF cables may be difficult. Very little geometrical and/or material transition may be provided by an RF MEMS structure, resulting in signal loss and disruption. Also, the connectors necessary to provide attachment to the cables are expensive, especially for applications at high frequencies.
There is therefore a need to provide a cost effective, high signal-quality connection, and method for building connections, between RF carrying signal structures.