INTEGRATION OF MICROWAVE FILTERS AND

MICROWAVE CAVITY STRUCTURES

XSSS MICROWftVE, goϋglirøs

TECHNICAL FIELD

The present invention relates to microwave packaging of microwave assemblies and, in particular, to the construction of internal launch apertures for microwave filters and cavity structures in microwave housings.

95

-2-

BACKGROUND OF THE INVENTION

Microwave assemblies are generally constructed of a housing or enclosure that envelops the electronics and/or microwave structures. These microwave electronics/structures are enclosed for a number of reasons, among others, to prevent undesired electromagnetic waves from being received into, or emitted from, the microwave assembly. A typical microwave assembly may contain numerous and distinct microwave elements or structures such as amplifiers, filters, mixers, waveguide combiners, other microwave cavity structures, etc. Generally, these elements or structures are inserted into the microwave assembly housing and secured or connected to the housing, and are commonly referred to as "drop-in" elements or structures. For example, a typical microwave assembly may contain both a microwave amplifier and a standard "drop-in" comb-line filter.

Utilization of "drop-in" microwave structures or elements in the microwave housing creates spurious "blow by" above and around such structures. Further, at microwave frequencies, "drop-in" filter and cavity structures develop ground discontinuities caused by the bolted (connected) interface along the ground plane. As machining and milling techniques have improved and developed, microwave assembly construction allowed for the

integration or construction of "machined-in" filter and cavity structures in the microwave assembly housing. Replacement of "drop-in" microwave structures with integrated structures has many distinct advantages. These advantages include the reduction of spurious "blow-by" effects and ground discontinuities. The reduction of spurious "blow-by" significantly improves suppression characteristics. Reduction of ground discontinuities improves the voltage standing wave ratio (VSWR) in microwave filters. Integration of filters and cavity structures into the microwave housing further helps to reduce the size and weight of the overall microwave assembly.

As discussed in the foregoing, it is very advantageous to construct and integrate microwave structures directly into the assembly housing. The major drawback with this type of microwave packaging is the placement and creation of internal launch holes or apertures. Internal launch holes provide an input or output path for the electromagnetic signal to enter or exit the integrated microwave structure. These internal launch holes are required, in most cases, to be formed through the internal side walls (the cavity walls of the microwave structure) of the microwave assembly housing and accurately positioned. As such, the desired positioning and orientation of such

95

-4- holes presents a difficult problem during construction. Physical dimensions and geometries of the microwave assembly housing and the desired orientation and location of the internal launch apertures preclude the use of normal drilling techniques to form the apertures.

The current method for formation of the internal launch holes uses drill bits. In particular, access holes are drilled through the outer housing walls to allow access for the drilling of the internal launch holes. Since the distance from the outer housing walls to the location of the internal launch hole site is usually substantial, long drill bits are inserted through the access holes to drill the internal launch holes. After the launch holes are drilled, the access holes in the outer housing walls are either plugged or used to accommodate a signal feedthrough for an external connector.

The problems with the present method used for forming internal launch holes are the electrical and mechanical limitations associated with the higher microwave frequencies that preclude the use of high performance microwave/millimeter wave external connectors. As stated earlier, the access holes are generally used as a signal feedthrough for an external connector. However, at microwave frequencies, use of such external connectors requires special drilling bits that do not allow enough

-5- σlearance for use of the long drill bits needed to drill the internal launch holes. Further, the plugging of drilled access holes is undesirable and expensive. Additionally, use of a long drill bit causes unwanted chatter that results in less than desirable positional tolerances of the internal launch holes at microwave frequencies.

Accordingly, there exists a need for a method of forming internal launch holes for integral or "machined-in" filters, waveguide cavities, and othermicrowave structures without drilling access holes in the microwave assembly outer housing. Further, there is needed a method that eliminates the use of drill bits to form the internal launch holes. Additionally, a method of forming launch holes is needed that produces the positional accuracy required at microwave frequencies.

-6- SUMMARY OF THE INVENTION

In accordance with the present invention, a microwave assembly having internal launch apertures and method of forming the internal launch apertures is provided. The microwave assembly includes an integrated microwave structure integrally formed into an assembly housing. One or more internal launch apertures interconnect and provide a signal path between the integrated microwave structure and another microwave structure located within the microwave assembly. The internal launch apertures are formed by an electric discharge machine.

-7- DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is made to the following detailed description taken in conjunction with the accompanying drawings wherein:

FIGURE 1 shows a microwave assembly and an assembly housing illustrating the formation of an internal launch aperture using an electro-discharge machining probe?

FIGURE 2 shows a different configuration of an assembly housing having an internal launch aperture; and FIGURE 3 illustrates a feedthrough inserted in the internal launch aperture and used to conduct an electric signal through the internal launch aperture.

395

-8- DETAILED DESCRIPTION

With reference to the drawings, like reference characters designate like or similar parts throughout the drawings. Now referring to FIGURE 1, there is illustrated a microwave assembly 10 with an electro-discharge machining probe 20 positioned to form an internal launch aperture 18, as shown. The microwave assembly 10 comprises an assembly housing 12 having a comb-line filter 14 and a comb-line filter 16 integrally formed into the assembly housing 12. The comb-line filters 14 and 16 are integrated microwave structures.

The assembly housing 12 further comprises a cavity area 22, a first internal housing wall 24 and a second internal housing wall 26. The internal housing wall 24 forms part of the comb-line filter 14 and separates a comb- line filter cavity 28 from the cavity area 22. The internal housing wall 26 forms part of the comb-line filter 16 and separates a comb-line filter cavity 30 from the cavity area 22.

The comb-line filters 14 and 16 are integrated, or integrally formed, (i.e. machined-in) in the assembly housing 12. The assembly housing 12 is typically made of metal, such as aluminum or steel, and is machined or milled to meet the desired specifications. After construction of

-9- the assembly housing 12 as shown in FIGURE 1, one or more internal launch apertures 18 are required to be formed in order to provide a signal path between the comb-line filters 14 and 16 and the cavity area 22. Typically, additional microwave electronic structures or elements (such as MESFET amplifiers, filters, couplers, transmission lines, etc.) are inserted into the cavity area 22 to eventually form the complete microwave assembly 10. As will be appreciated, once the assembly housing 12 is constructed with the integral component comb-line filters 14 and 16, a path is needed to allow electric signals to pass between the comb-line filters 14 and 16 and the cavity area 22 (containing other microwave structures or elements) . As will be understood, the comb-line filters 14 and 16 could also be other types of microwave structures, such as cavity or waveguide structures, that require internal launch apertures to provide a signal path between the integrated microwave structure and other structures in the microwave assembly. The electro-discharge machining probe 20 extends from an electric discharge machine 21. The electro-discharge machining probe 20 forms the internal launch apertures 18 at a predetermined location to provide a signal path through the internal housing walls 24 and 26. In FIGURE l, the probe 20 is shown positioned at the predetermined

95

-10- location to form the internal launch aperture 18 through the internal housing wall 24.

The electric discharge machine (EDM) uses an arc of electricity to perform the cutting function. Traditionally, EDMs have been used in the mold industry for cutting and shaping molds. Generally, two basic types of EDMs exist. One type has a cutting element consisting of a wire or wires while the other type has a cutting element referred to as an electrode or a "machining probe". The present invention uses an EDM having an electrode or a machining probe to create the internal launch holes. These electrodes or machining probes are custom made to the desired hole diameter. EDMs of the type used in the present invention are available from manufacturers such as Elox-Agie, Charmilles Technologies or Fanuc, while the electrodes or machining probes are generally available from machine shops that own or operate EDMs. The electrode or machining probe is typically made of graphite or copper. The electrode or machining probe is the element that the EDM uses to create "spark-erosion." This "spark- erosion" disintegrates the material. During the erosion operation by the EDM, the workpiece (i.e. microwave housing wall) and the electrode are submerged in a dielectric fluid that washes out the eroded material and controls the temperature of the area of erosion. The electrode is

-11- physically separated by a gap of <0.0005" during the entire process and never touches the workpiece. The workpiece can be positively charged and the electrode can be negatively charged, or the converse is applicable depending on the metals involved. Both manual and computer numerically controlled (CNC) versions of EDMs are applicable to the present invention.

Now referring to FIGURE 2, there is illustrated another microwave assembly 60 comprising an assembly housing 62 having an integrated microwave structure 64. As will be appreciated, the integrated microwave structure 64 is a waveguide combiner or some other microwave structure requiring an internal launch aperture. The electro- discharge machining probe 20 is shown extending through an internal launch aperture 66 (after completion of formation of the internal launch aperture 66) . The internal launch aperture 66 provides a signal path between the integrated microwave structure 64 and an area 68 in the assembly housing 62. Now referring to FIGURE 3, there is shown an internal launch aperture 80 in an assembly housing wall 82 having a feedthrough 84 positioned within the internal launch aperture 80. The feedthrough 84 also insulates the signal path through the internal launch aperture 80 from the assembly housing wall 82. The feedthrough 84 is generally

395

-12- held in place in the internal launch aperture 80 by use of epoacy solder or some similar means. Typically, the feedthrough is a low-loss element for conducting an electromagnetic signal through the internal launch aperture 80. In the preferred embodiment, the feedthrough 84 is a low-loss, glass-to-metal feedthrough having an impedance of about fifty ohms.

Although several embodiments of the present invention have been described in the foregoing detailed description and illustrated in the accompanying drawings, it will be understood by those skilled in the art that the invention is not limited to the embodiments disclosed but is capable of numerous rearrangements, substitutions andmodifications without departing from the spirit of the invention.