Coaxial Transmission Line Rf Switch

Abstract

A transmission line switch for general use at frequencies to 400 MHz and in certain applications to 1000 MHz, featuring strip-line circuitry. Specifications heretofore met only by more expensive switches are achieved with refined circuit design and improved lead support.

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to transmission line switches intended for use in the lower microwave region. More particularly, the invention relates to coaxial switches which will meet rigid military specifications but which are more economical to manufacture than switches heretofore available, by virtue of employing low-cost strip-line circuitry.

Switches of the type described herein must have the following characteristics:

1. Frequency: DC to 400 MHz.

2. RF Power rating: (non-switching) 100 watts CW minimum; (switching) 15 watts CW.

3. Impedance: 50 ohms nominal

4. Voltage standing wave ratio (VSWR): 1.3 to 1 (typical).

5. Insertion loss: 0.22 DB maximum (energized and deenergized).

6. Isolation: 33 DB minimum.

7. Operational Life: 10,000 cycles minimum

8. Coil current: 35 MA maximum

9. Operating voltage: 18-30 VDC.

10. Environmental requirements: Mil-S-32928B.

11. Vibration resistance: Mil-S-202C, method 204A, test condition B.

To meet such rigid specifications, it has heretofore been necessary to employ carfully hand-wired connections to relay terminals, with every connection being "critical" in terms of length, geometry and strength.

Typical of the prior art devices is the coaxial switch disclosed in U.S. Pat. No. 3,131,268. This switch provides for the input connector to be switched to either of two output connectors by the action of a push-rod assembly activated by relay. Leaf-spring contacts are attached to the output connectors, and the push-rod alternately connects one or the other to the input connector, the unconnected contact being grounded. Leaf spring contacts are, of course, subject to vibration.

The use of strip-line technology in switches of the general type described is not entirely unknown. In U.S. Pat. No. 2,842,637, a relay armature causes one line conductor to move selectively between a pair of parallel, spaced line conductors, following the general principle of a jack-type switch, but employing microstrip line conductors with associated ground conductors. Such a device would not have any degree of vibration resistance. In U.S. Pat. No. 3,114,887 a U-shaped loop is printed on a strip-line board which is used as a slider to connect an input line to either of two output lines. A device of this type would have low vibration resistance and, as with any type of sliding contact, is subject to wear.

As is well known, strip-line technology involves the essentially automatic production of circuitry on a high-volume, low-cost basis. The starting material is a circuit board having a layer of copper on each side. With photolithographic techniques, a positive resist is applied over the desired circuit pattern, and the remaining, exposed metal is removed by etching. The resist is then dissolved off, and the copper is plated, generally with nickel and hard gold. Such circuits can be produced in volume with precise accuracy and low cost.

Early efforts to adopt strip-line technology to coaxial RF switches which met the exacting requirements noted above were unsuccessful. Vibration tests were failed, resistance was too high, and other difficulties were encountered.

OBJECTS OF THE INVENTION

A general object of the present invention is to provide an improved coaxial transmission line RF switch.

A further object of the present invention is to provide a coaxial transmission line RF switch which meets or exceeds all of the above-noted requirements, and which is less expensive than switches heretofore qualified for the same service.

Another object of the present invention is to provide a coaxial transmission line RF switch which successfully employs strip-line circuitry.

Various other objects and advantages of the invention will become clear from the following detailed description of embodiments thereof, and the novel features will be particularly pointed out in connection with the appended claims.

THE DRAWINGS

Reference will hereinafter be made to the accompanying drawings, wherein:

FIG. 1 is a plan view, partly in section, showing a preferred embodiment of the invention;

FIG. 2 is a cross-sectional elevation taken along line II--II of FIG. 1;

FIG. 3 is a plan view of a circuit board adapted for installation in the embodiment of FIG. 1 and 2;

FIG. 4 is a schematic diagram of the circuitry of FIG. 3; and

FIG. 5 is a greatly enlarged cross-sectional elevation showing bonding of the center lead to the circuit board.

DESCRIPTION OF EMBODIMENTS

In essence, the switch of the present invention comprises a rectangular connector body 10 having a soldered-on cover or can 12. Four N-type coaxial connectors 14 are secured to the body, the center lead 16 of each extending interior of the body and soldered directly to the printed circuit board 18. The board 18 includes circuitry 20 (described hereinbelow in connection with FIG. 3) and has a relay 22 attached thereon with a mounting clamp 23. A pair of wires 24, 26 connect the relay 22 to a pair of insulated terminals 28, 30 mounted in the top of the can 12.

The rectangular box or body 10 is preferably fabricated of silver-plated brass. The bottom 32 thereof is much thicker than the sides, so that when the PC board 18 is attached thereto with screws 34, its upper surface will be in contact with the center leads 16 from the four coaxial connectors 14. Bottom 32 is tapped at appropriate locations to accomodate screws 34, and additional holes 36 are drilled to provide space for the relay terminals 38 and the ends of mounting clamp 23.

The sides of box 10 are provided with four identical round holes 40 to accomodate the connectors 14. As all four connectors are identical, only one need be described. The connector body 42 is also preferably fabricated of silver plated brass, and comprises a threaded cylinder completely open at one end but having only a small axial opening 44 at the other end 45. The diameter of body 42 at the end 45 is such that it can be fitted into opening 40 in box 10 and soldered into place. An annular shoulder 46 on body 42 keeps the end of body 42 flush with the interior surface of box 10. A cylindrical insulator 48 fits within connector body 42, and is preferably made of a polytetrafluoroethylene material such as Teflon (trademark). A projection 50 on the end thereof fills the opening 44. It is noted that projection 50 is deemed essential for vibration resistance; it may be a separate insulating member but is preferably an integral part of insulator 48. Insulator 48 is only about one-half as long as connector body 42, and extends up to an outwardly extending annular shoulder 52 in body 42. To insure tight friction engagement between insulator 48 and body 42, the outer surface of the former may be grooved or roughened. A cylindrical bushing 54 fits within body 42 and rests against shoulder 52, thus locking insulator 48 in place, since the axial opening 56 in bushing 54 has a smaller inside diameter than shoulder 52. Bushing 54 is locked in place by spinning over the end of connector body 42. The inside surface of bushing 54 is slightly tapered so as to snugly engage the male coaxial connector (not shown) when the same is inserted in the connector body. The connector 14 is completed with center lead contact 58, which is inserted in and retained by an axial opening 60 in insulator 48. Contact 58 has a contact portion 62, a body portion 64 and a lead 16. The material of choice for contact 58 is gold-plated beryllium copper. The contact portion 62 is the end which extends into the cavity defined by bushing 54, and comprises an axial opening and four equi-spaced slots. Portion 62 is adapted to resiliently receive the center lead of the male connector. The body portion 64 is a solid cylinder with the outer surface grooved or notched so as to be frictionally engaged by insulator 48 when inserted into opening 60. The lead 16 is connected to or integral with body portion 64 at the end opposite contact portion 62, and extends through an axial opening in projection 50 into the interior of box 10.

The connection and support of lead 16 are important aspects of the invention, and greatly enlarged detail thereof is shown in FIG. 5. With reference to that drawing, it is first to be noted that circuit board 18 with circuitry 20 is just below and in contact with lead 16. Dimensions are controlled to obtain an interference fit between leads 16 and the bonding pads on circuitry 20. Next, the extension of lead 16 beyond projection 50 is as small as possible, i.e., the minimum necessary for soldering. In practice this is about one-sixteenth inch. The solder 21 which connects lead 16 to the circuit 20 extends to substantially the entire exposed length of leqd 16. Thus, between projection 50 and solder 21, lead 16 is firmly supported along substantially its entire length. This is important for vibration resistance. Also, if lead 16 is somewhat larger than other requirements might dictate, vibration resistance is also improved (0.05 in OD is typical).

The printed circuit board 18 and circuitry 20 will now be described, and attention is directed to FIG. 3. The board 18 is preferably epoxy-bonded fibreglass with copper on each side. The copper on the top side is formed into the circuitry 20 as shown using well known photo-etching techniques which need not be described. However, it is important to note the the circuitry should be as broad as possible consistent with allowable separation. This reduces circuit resistance, improves the impedance match and lowers insertion loss. In general, the circuitry should be at least about one-eighth inch wide wherever possible, which means everywhere except directly beneath the points 68 a, b, c (shown in dotted lines) where the relay armature contacts the board. At these points, the circuitry should match the armature closely to insure proper operation. Sharp corners in the circuitry should be avoided, and after the circuit is etched it should be plated with nickel and hard gold (.0002 Ni and .00004 Au are typical). Additional holes 70 are provided at locations adapted to receive matching projections on the bottom of relay 22 (other holes are for screws 34).

FIG. 4 is a schematic diagram of relay 22 and the circuitry of FIG. 3, with contacts 1-4 of FIG. 4 corresponding to bonding pads 1-4 of FIG. 3. Thus, when the switch is not energized the armature of relay 22 covers areas 68a and 68b and current flows along the path 1-4 and 2-3. When relay 22 is energized, the armature pivots and covers area 68c while opening areas 68a and 68b. Contacts 3 and 4 are open and current flows in the path 1-2. It will be appreciated that this circuitry is exemplary only and other arrangements may be worked out by those skilled in the art.

The preferred relay for use with the invention is the "Printact" (trademark) relay No. 24 BW2GX12 manufactured by Executone, Inc. and described in U.S. re-issue Pat. No. 24,209. However, other relays adapted for PC board mounting and direct armature contact may be employed.

The terminals 28, 30 mounted in the top of can 12 are preferably of the feed-through capacitor type, such as Allen Bradley FASC 102W. This has a capacitance of 1000.mu. f and a maximum DC working voltage of 500 volts. An inductor 72 is attached to one or both terminals within the can (Delevan Part No. 1025-20 is satisfactory). The capacitors and inductor act to reduce cross-talk, a poorly understood phenomena attributable to the wires passing into the cavity. Possible short circuits can be avoided by installing thermo-shrink PVC tubing 74 over inductor 72 and its connections.

Assembly of the switch is simple and straight forward. The four connectors are assembled and soldered into openings 40. It will be noted that board 18 has four notches, adjacent to and on one side of bonding pads 1-4. Board 18 is placed in body 10 with these notches accomodating the leads 16. Board 18 is then slid into its proper position, with an interference fit between the leads 16 and the bonding pads. The leads 16 are soldered to the adjacent pads on circuitry 20, care being taken to avoid solder flow except on the pads. Terminals 28, 30 are soldered to the can 12. Wires 24, 26 are attached to the terminals 38 on relay 22. One wire is connected to inductor 72, which is connected to terminal 28, and the other wire is connected to terminal 30. The projections on the bottom of relay 22 are inserted in mounting holes 70, and metal clamp 23, which is generally U-shaped with inwardly-extending hooks on the ends, is installed by pressing down over the relay until the hooks engage the underside of the circuit board 18, through appropriate notches. Assembly is completed by soldering can 12 to body 10 at their mating edges.

It will be noted that one side of clamp 23 is close to bonding pad 2, and it has been determined that cross-talk and VSWR properties are improved if the clamp is notched or made narrower in this area. Further, the possibility of a short circuit with adjacent circuitry will be eliminated if a piece of insulating tubing is placed over this end of the clamp.

Soldering of can 12 onto body 10 can create problems of contamination, and it is preferred to leave a small hole in can 12. This helps dissipate the heat caused by the soldering and allows contaminating gases to get out of the can prior to sealing of the hole.

Various changes in the details, steps, materials and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as defined in the appended claims.

Claims

What Is Claimed Is:

1. A coaxial high frequency switching device comprising:

a housing;

a plurality of female coaxial connectors adapted to receive male coaxial connectors mounted exteriorly of said housing in a single plane and each connector having a center lead supported by an insulator, the lead extending into said housing a short distance;

a strip-line circuit board having a circuit pattern thereon including bonding pads at positions coinciding with the positions of said leads, said circuit board being mounted within said housing in the plane of said connectors at a level to effect an interference fit between said leads and said bonding pads, said leads being bonded to said pads;

a relay secured to said board and having an armature directly contactable with selected portions of said circuit pattern to effect desired switching; and insulated terminals in said housing connected to said relay.

2. The switching device as claimed in claim 1, wherein said insulator surrounds and supports each said lead through the wall of said housing, and said leads are soldered to said pads along substantially the remainder of their length, whereby said leads are rigidly supported along substantially their entire length.

3. The switching device as claimed in claim 1, wherein said terminals include feed-through capacitors.

4. The switching device as claimed in claim 1, wherein said circuit board includes notches adjacent to and to one side of said bonding pads, whereby said board may be inserted over said leads in said housing and slid into interference fit with said leads.

5. The switching device as claimed in claim 1, wherein said circuit pattern comprises copper plated with nickel and hard gold, and each conductor therein is at least about one-eighth inch wide wherever possible.

6. The switching device as claimed in claim 1, wherein said housing comprises a rectangular body portion having said connectors mounted in walls thereof and said circuit board mounted on a raised bottom thereof, and a can portion bonded to the walls of said body portion around the periphery thereof, said can portion having said terminals passing therethrough.

7. The switching device as claimed in claim 3, wherein an inductor is connected between one of said capacitors and said relay.

8. The switching device as claimed in claim 4, wherein said relay is secured to said board with clamping means engageable in a pair of said notches.

9. In a coaxial high frequency switching device including a plurality of N-type coaxial connectors secured to a housing, insulated center leads extending thereinto, and relay means within said housing cooperating with circuitry for effecting desired switching between said leads, the improvements comprising:

a strip-line circuit board having said circuitry on a surface thereof including bonding pads in interference fit with and soldered to said leads;

an insulator securing each said lead within each connector and extending through the wall of said housing, each said lead being supported by said insulator or said bonding pad and solder along substantially its entire length;

said relay being secured to said board and having an armature directly contactable with selected portions of said circuitry to effect desired switching upon energizing or deenergizing said relay.