Microwave Circuit Having Grounding Structure

Effective and practical methods for providing the required short circuits for waveguide and coaxial transmission systems have existed for many years; however, for microwave printed circuit or stripline systems using dielectric substrate support for the printed circuit, totally satisfactory methods are not as yet in general use.

Basically the stripline circuit in its simplest configuration consists of two layers of dielectric substrate material having copper ground planes on their nonadjacent sides, the dielectric substrate materials being passed tightly together with a printed circuit or stripline transmission circuit interposed between. To short such a transmission line at microwave frequencies, it is necessary to completely ground the transmission line to each of the spaced ground planes fastened to the dielectric substrates.

A number of techniques have been or are being used to ground the transmission line to the ground planes or to a metallic housing. One common techique is to solder a tab onto a printed circuit or stripline conductor which is placed between two dielectric substrates having ground planes on their opposite sides. This tab, which extends beyond the substrate members, is clamped under a shorting bar and screwed down or otherwise fastened tightly to the conductive housing. The shorting bar is then held down by the housing cover, it being of such height that the cover exerts a substantial force against it as it is secured. The difficulty with this arrangement is that air gaps tend to be formed adjacent the tab which is soldered to the stripline or printed circuit and also between the ground plane and the cover adjacent the shorting bar. Another technique which has been used is to solder a somewhat elongted tab to the transmission line, wrapping it around and soldering it down to one of the ground planes on the opposite sides of the dielectric substrate. An additional copper layer is then soldered to this wrap and also to the opposite dielectric substrate. Again, this arrangement ususally results in an undesirable air gaps between the substrates caused by the thickness of the tab soldered to the conductor. A further problem with this arrangement is that there is no means associated with this arrangement for positively controlling the position of the substrate and conductor within the support chassis. An additional problem is that since this results in a bonded or sealed assembly, the maintenance of the associated circuitry becomes more difficult. A further arrangement involves riveting the substrates together with rivet heads soldered to each of the ground planes on opposite sides thereof and with an additional inside washer soldered to the transmission line and to the rivet. With this arrangement, the electrical location of the short is not accurately predictable. There is also a high leakage of microwave fields, and the performance is generally very poor at frequencies above 10 GHz.

In addition to the above, there are also substantial problems in producibility or repeatability in production rising out of the above techniques. These problems become quite severe when operating at frequencies above 10 GHz. For example, it will be appreciated by those skilled in the art that in the manufacture of microwave filters using tuning stubs at this frequency range, it is necessary that the shorts or grounds be precisely and predictably located since very small variations in effective tuning stub length will result in altering of the frequency characteristics of the filter.

The inventor has devised a stripline circuit and a method of grounding or shorting the same which avoids the above problems, resulting in a closed microwave stripline circuit with reliable shorts at predictable locations, good serviceability and excellent producibility. The specific circuit for which my invention has been devised is a microwave frequency diplexer; however, those skilled in the art will recognize that its teachings are just as applicable to multiplexers and other microwave circuits operating in the microwve frequency range, especially those using microwave filters or tuning stubs.

I form a stripline circuit which is chemically milled or otherwise formed to the desired dimensions and which is then placed on a dielectric substrate. The substrate may have printed circuit conductor tracks which are in contact with the stripline, and such tracks are a usual method of making contact at the connectors. These connections may be soldered or may be sufficiently secure when an upper dielectric substrate is clamped tightly against the stripline. Both substrates whose nonadjacent sides have conductive layers serving as ground planes and the stripline are placed in a housing designed to just accommodate them plus one or more shorting tabs formed with the stripline and which extends beyond the substrates into a space or chamber in the housing. One or more shorting bars are secured and clamped in position over the tabs with the shorting bars having a wall or edge of slightly greater height than that of the chamber. A cover for the housing has inserts of a conductive elastomer such as silver-impregnated synthetic rubber, which inserts are in registry with the edges of the shorting bar or bars so that when the cover is tightly secured to the housing the shorting bar wall or edge penetrates somewhat into the conductive elestomer, making electrical contact with it and, hence, with the entire housing. The location of the short is thus quite predictable as coinciding with the edge of the shorting bar, and where such bar is used to terminate a series of tuning stubs, the electrical characteristics are thereby precisely determined. No undesirble air gaps are caused by the requirement that the shorting bar be high enough to make good electrical contact with the chassis cover, nor because a tab must be soldered to a printed circuit track under the cover.

FIG. 1 is a plan view of a microwve diplexer incorporating my invention with the cover and the upper dielectric substrate removed.

FIG. 2 is a partial sectional view taken along lines 1--1 5 of FIG. 2, but with the cover and upper dielectric substrate in place.

FIG. 3 is a partial view of the structure of FIG. 3 enlarged to show detail .

Referring now to FIG. 1, a chassis in the form of a metal housing 10 is shown having an inlet port 12 and outlet ports 14 and 16. The unit shown is a diplexer whose function is to divide input signals ranging over a frequency from 2 GHz to 18 GHz into two separate bands. Thus, the housing has an interior chamber 18 which is dimensioned to receive a substrate 20 of dielectric material. The conductor 22, which is preferably chemically milled of 1.5 mil or smaller beryllium shim stock, is laid on the dielectric member 20 and is secured between this member and a similar member located on top of the conductor 22. It will be observed that conductor 22 divides into two sections, a first section 24 constituting a high frequency microwave filter section and a second 26 which is designed to pass frequencies of a lower frequency band such as 2 GHz to 10 GHz. This lower frequency band of signals is then conducted directly to the output port 16.

Referring now to the sectional view, FIG. 2, it will be observed that housing member 10 includes, in addition to the opening 18 for receiving the substrate 20, a pair of steps 28 and 30 of essentially the same height or thickness as the substrate 20, a part of which is located between said steps. Conductor 22, which for the most part lies directly over the substrate 20, also including shorting tabs 22a and 22b which overlie the steps 28 and 30, respectively. Overlying the shorting tabs 22a and 22b are a pair of shorting bars 32 and 34, respectively, and each of these tabs is firmly grounded on its corresponding step by means of one of screws 36 and 38. From consideration of FIG. 1, it will be apparent that there are actually three each of screws 36 and 38 shown anchoring the somewhat elongated tabs 22a and 22b. It is the function of the high frequency filter section shown at numeral 24 to pass frequencies of a known band to the detector section 40 which, after detection, supplies a video signal to outlet port 14.

The details of the invention may be more clearly understood from consideration of FIG. 3, which is an enlarged view of a portion of FIG. 2, Again, it will seen that housing 10 consists of a chamber 18 receiving a substrate 20 positioned next to the step 30 of essentially the same height. The conductor 22 overlies the substrate 20, and its tab 22b overlies the step 30. The shorting bar 34 which overlies the tab 22b is shown as having the wall or edge 34a of greater height than the part of opening 18 above step 30, which height is also essentially the same as that of the upper substrate member 42. The cover 44 of housing member 10 includes a groove 46 containing a seal 48 whose function is to prevent moisture, etc. from reaching the circuits inside of housing 10. Cover 44 also includes another groove 50 containing a member 52 of conductive rubber, which directly registers with the edge 34a of shorting bar 34. As shown, edge 34a is of such height that when cover 44 is installed, this edge penetrates the conductive rubber member 52, thereby providing a conductive path from the shorting tab 22b through edge 34a to conductive rubber member 52 and hence to the housing itself. It will also be observed that substrate members 20 and 42 each include a thin layer of conductive material such as copper on their nonadjacent sides and which are held in direct contact with housing 10 and cover 44, respectively, and serve as ground planes for the microwave circuit.

Since the depth of chamber 18 and the height of the step 30 are chosen to just accommodate the heights of substrate members 20 and 42 (and the stripline conductor 22), fastening of the cover 44 puts conductor 22 and the substrate members in compression, avoiding the creation of air gaps. The location of the short is clearly defined at the edge of the step 30, and thus the effective lengths of the tuning stubs in the high frequency microwave filter 24 are clearly defined.

While the invention herein has been described in terms of a diplexer, it will be apparent that invention is applicable to design of many microwave circuits. Applicant's system teaches means providing a complete short ground plane to ground plane, a circuit board assembly without air gaps at critical electrical locations, and an arrangement whereby assembly may be effected without critical mechanical tolerance controls. Thus, the arrangement shown herein makes possible good producibility of such circuits. The stripline circuit in many, if not most, cases can be formed of a self-supporting piece of stock for easy handling. In forming the circuit 22, for example, some thin supporting strips are formed between the outboard end of the low frequency section communicating with outlet port 16 and the shorting tabs 22a. These, of course, are removed in production. Also, as indicated above, it is frequently useful to form part of the circuit as a printed circuit on the surface of one of the dielectric substrate members with the chemically milled circuit making contact with the printed circuit conductor either through soldering or through the compressive force necessarily resulting when the cover 44 is pulled down onto chassis 10.