US3439303A - Rf isolation relay - Google Patents

Description

April 15, 1969 A. z. PURZYCKI I RF ISOLATION RELAY Filed May l, 1967 FIG. I

INVENTOR ALFRED Z. PURZYCKI ATTOR EY United States Patent O 3,439,303 RF ISOLATION RELAY Alfred Z. Purzycki, Mount Prospect, Ill., assignor to Teletype Corporation, Skokie, Ill., a corporation of Delaware Filed May 1, 1967, Ser. No. 635,136 Int. Cl. H01h 51/28 U.S. Cl. 335-154 16 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Modern day communications equipment is required to meet increasingly rigid standards with respect to the emission of radio frequency interference. It has been found that ordinary shielded cable does not provide sufficent protection against radio frequency interference leakage and that double-shielded cable must be employed if such standards are to be met. Of course, the use of doubleshielded cable would be a futle gesture if the relays, switches, connectors, etc., adaptedfor use with doubleshielded cable were not capable of providing the same degree of radio frequency isolation provided by doubleshielded cable.

SUMMARY OF THE INVENTION In the preferred embodiment of the nventon a radio frequency isolation relay adapted for use with doubleshielded cable is comprised of a switch, an Operating coil for the switch and a switch lead-out tube positioned around the switch within the coil so that current passing through the switch does not aflect the coil in any way. Shielding for the switch is provided 'by two nonmagnetic, conductive shields both positioned within the coil. One of the shields surrounds the switch and the leadout tube and is electrically connected to the inner shield of the double-shielded cable. The other shield surrounds the first shield and is electrically connected to the outer shield of the cable.

DESCRIPTION OF THE DRAWING A more complete Understanding of the invention may be had by referring to the following detailed description when taken in conjunction with the drawing wherein:

FIG. 1 is a sectional view of a radio frequency isolation relay employing the present invention;

FIG. 2 is a View of a length of double-shielded cable in which the various layers have been cut away at different points more clearly to illustrate the various components of the cable; and

FIG. 3 is a sectional View of the device shown in FIG. 1 taken along the line 3-3 in the direction of the arrows.

DETAILED DESCRIPTION Referring now to the drawing, wherein like reference numerals designate like parts throughout the several views, with particular reference being had to FIG. 2, there is shown a length of double-shielded cable 10. The cable is comprised of conductors 11 each of which is 3,439,303 Patented Apr. 15, 1969 surrounded by an individual layer of insulation 12. Although only two conductors 11 are shown in the draw ing it should be understood that any number of conductors may be employed in a given application. The conductors 11 and their individual layers of insulation 12 are surrounded by a layer of insulation 13 which is in turn surrounded by an inner shield 14. The inner shield 14 extends the entire length of the cable 10 and is constructed by weaving small diameter conductive wires into a tubular shell. The inner shield 14 is surrounded by a layer of insulation 15 which is in turn surrounded by `an outer shield 16. Like the inner shield, the outer shield is constructed from small diameter conductive wires which are woven into a tubular shell and extends the entire length of the cable 10. The outer shield is surrounded by an outer layer of insulation 17.

It has been found that ordinary shielded cable is unsatisfactory for providing radio frequency isolation in communications equipment. Consider first an ordinary twisted pair of wires used to carry current to and from an elcctrical device. The twisting of the wires and the fact that the current fiows in opposite direction through the wires causes the magnetic field which is generated around each of the wires by the current flow through it to tend to cancel the field around the other wire so that no net field is generated. Suppose a shield is placed around the twisted pair of wires in a further attempt to eliminate the magnetic field. If the shield is connected to signal ground at each of its ends a portion of the return current fiows through the shield and the cancelling efiect of the twisted wires is reduced. If, on the other hand, the shield is connected to signal ground at only one of its ends a potential gradicnt exists along the length of the shield and the shield acts as a quarter-wave antenna. Thus, the shield tends to radiate the very signals it is intended to isolate,

The problems which arse with the use of single shielded cable are virtually eliminated if double-shielded cable, such as the cable 10, is used. In use one end of each length of the inner shield 14 is connected to signal ground and the other end of each length is disconnected from signal ground to prevent return currents from flowng in the shield 14. The outer shield 16 is grounded and extends in an unbroken manner along the entire length of the cable 10 to eliminate the aerial effect of the shield 14. The combination of the two shields provides the desired radio frequency shielding without the danger of the shield acting as an aerial.

Referring now to FIG. 1 there is shown a relay useful in combination with cables such as the cable 10 for providing radio frequency interference shielding. The relay is comprised of a reed switch 20 including an evacuated glass envelope 21 and a pair of ferrous reeds 22. The reeds 22 support contact members 23 which are of the normally open variety. Since the reeds 22 are constructed from a ferrous material the switch 20 may be operated by subjecting it to a magnetic field oriented in a proper direction which moves the reeds 22 toward each other and closes the Contacts 23.

The reed switch 20 also includes a pair of terminals 24 which are electrically connected to the reeds 22. One of the termnals 24 is soldered directly to a conductor 11 of a double-shielded cable 10 of the type shown in FIG. 2. The other terminal 24 is soldered to a lead-out tube 30 which is in turn soldered to the other of the conductors 11 of the cable 10. The lead-out tube 30 is a hollow, circular tube formed from a non-magnetic, conductive material having a closed end to which one of the terminals 24 is connected. The tube 30 substantially surrounds the entire reed switch 20 and is oriented so that the conductors 11 of the cable 10 may be attached, one to the tube 3 30 and one to a terminal 24 at approximately the same position axially of the cable 10.

The tubular shape of the lead-out 30 is particularly important. If a wire lead-out extending parallel to the switch 20 were to be used, the wire and the reeds 22 would form a single turn coil which could couple signals passing through the switch into the magnetic field which operates the switch thereby -transferring the signals out of the relay. The tubular shape of the lead-out 30 causes the portion of the lead-out 30 positioned on any one side of the reeds 22 to affect the field in an equal and opposite manner from the portion of the lead-out 30 on the oppoiste side. Therefore, signals passing through the switch 20 are not coupled into the Operating field of the switch.

Positioned around the conductive tube 30 is a first shield member 31. Like the tube 30, the first shield member 31 is a hollow, circular tube formed from a nonmagnetic, conductive material having one closed end and one open end. The closed end of the first shield 31 is positioned adjacent the closed end of the lead-out tube 30 and, like the tube 30', the first shield 31 substantially surrounds the entire reed switch 20. The first shield 31 is electrically connected to the inner shield 14 of the cable by means of a conductive lead 32.

Positioned around the first shield 31 is a second shield 32. Like the lead-out tube 30 and the first shield 31, the second shield 32 is a hollow, circular tube formed from a non-magnetic, conductive material having a closed end and an open end. The closed end of the second shield 32 is positioned adjacent the closed end of the first shield 31. It should be noted that the conducto's 11 of the cable 10 extend from the cable 10 to the reed switch 20 through the open ends of the shields 31 and 32.

Positioned around the second shield 32 is an `Operating electromagnet 33 which is comprised of a bobbin 34 mounted on the second shield 32 and a coil of conductive material 35 formed on the bobbin 34. The coil of conductive material 35 is constructed so that `when it is connected to a source of current (not shown) it generates a magnetic field of sufficient strength to move the reeds 22 of the switch 20 toward each other thereby closing the contacts 23.

The lead-out tube 30 is attached to, is physically separated from and is electrically insulated from the first shield 31 by a layer of an epoxy resin 40. Similarly, the second shield 32 is attached to, is physically separated from and is electrically insulated from the first shield 31 by a layer of epoxy resin 41. It should be understood that it is not necessary to insulate and separate the members 30, 31 and 32 with respect to each other by the use of epoxy so long as the components are electrically insulated one from the other. It has been found that suitable insulation may be provided by coating the outer surfaces of the tube 30 and the shield 31 with varnish. Further, it has been found that suitable electrical insulation may be provided by surrounding the tube 30 and the first shield 31 with a layer of plastic tubing, preferably of the "heat-shrink" variety.

The relay assembly shown in FIG. l is mounted on, and the second shield 32 is electrically connected to, a conductive frame or wall segment 42 by means of a conductive nut 43 which is engaged with a threaded end portion 44 of the second shield 32. The second shield 32 is also electrically connected to the outer shield 16 of the cable 10 and is mechanically attached to the cable 10 by means of a jam nut connection 45.

The jam nut connection 45 is obtained by inserting a ring 46 of deformable metal between the layer of insulation and the outer shield 16 of the cable 10 and by surrounding 'an exposed portion of the outer shield 16 and a length of the outer layer of insulation 17 of the cable 10 with a second deformable ring 47. The ring 47 is inserted in a tapered collar 48 on the conductive nut 43 and a jam nut 49 is tightened around the collar 48 so that a connection is formed with the cable 10 in which the nut 43 is electrically connected to the outer shield 16 and is also mechanically connected to the entire cable 10. It should be noted that the jarn nut connection forms a continuous, uninterrupted connection between the shield 16 and the shield 32.

In some applications the interior of certain cabinetry may be entirely surrounded by a grounded shield. In such a case the use of cable having an outer shield 16 is not required since the grounded cabinetry of the apparatus forms the outer shield of the cable. The relay apparatus shown in the drawing is useful in such an application since the 'second shield 32 may be connected directly to the shielded cabinet of such an apparatus by means of the conductive nut 43. In other applications it may be desirable to support the relay shown in the drawings by means other than gripping a wall segment between the second shield 32 and the nut 43. In such an application, double-shielded cable such as the cable 10 is used and the relay structure may be supported in any suitable manner.

Although only one embodiment of the invention is shown in the drawings and described in the foregoing specification, it will be understood that invention is not limited to the specific embodiment described, but is capable of modification and rearrangement and substitution of parts and elements 'without departing from the spirit of the invention.

I claim:

1. A shielded switch including:

a magnetically operable switch having an input and an output; i

a first conductive, non-magnetic shield substantially surrounding the switch;

a second conductive, non-magnetic shield substantially surrounding the first shield and insulated therefrom;

a cable including a pair of leads, one electrically connected to the input and the other electrically connected to the output of the switch, and a conductive shield surrounding the leads and electrically connected to the first shield; and

magnetic means positioned outside of the second shield for operating the switch.

2. The switch according to claim 1 wherein:

the cable also includes an outer shield surrounding the conductive shield and electrically connected to the second shield; and

the magnetic means is an electromagnet positioned on the second shield.

3. The switch according to claim 1 further including a lead-out tube connected between the output of the switch and the other conductor of the cable and positioned around the switch so that current owing through the switch does not aifect the field of the magnetic means.

4. The switch according to claim 1 wherein the first shield is a tube having an open end and a closed end, wherein the second shield is a tube having an open end and a closed end -which is aligned with the closed end of the first shield and wherein the leads of the cable extend through the open ends of the shields.

5. A switch for use with double-shielded cable of the type comprised of at least two conductors, a first conductive shield around the conductors, a layer of insulation around the first shield and a second conductive shield around the insulation, including:

a switch having an input connected to one of the conductors of the cable and having an output connected to the other of the conductors of the cable;

a first conductive, non-magnetic shield substantially surrounding the switch and electrically connected to the first shield of the cable,

a second conductive, non-magnetic shield substantially surrounding the first shield and insulated therefrom and electrically connected to the second shield of the cable; and

means mounted on the second shield for Operating the switch.

6. The switch according to claim 5 wherein the switch is positoned in a predetermined orientation with respect to the Operating means and further including a lead-out tube connected between the output of the switch and the other conductor of the cable, and positoned around the switch within the Operating means.

7. The switch according to claim 5 wherein the shields are each tubes closed at one end with the closed end of the first shield positoned adjacent the closed end of the second shield and with the double-shielded cable entering the shields through their open ends.

8. The switch according to claim 7 further including a third closed end tubul-ar member positoned within the first shield with the closed end adjacent the closed end of the first shield and connected between the output of the switch and the other conductor of the cable for forming a lead-out for the switch.

9. A radio frequency isolation relay including:

a frame;

an outer shield mounted on the frame and electrically connected thereto;

switch Operating means mounted on the outer shield;

an inner shield positoned within the outer shield and electrically insulated therefrom;

a switch having an input and an output positoned within the inner shield; and

a cable including a conductor connected to the input of the switch, another conductor connected to the,

output of the switch and a conductive shield surrounding the conductors -and electrically connected to the inner shield. 10. The relay according to claim 9 wherein: the outer shield is a tube of conductive, non-magnetic material, the inner shield is a tube of conductive, non-magnetic material, the switch is a magnetically operated switch and is positioned within the inner shield, and the switch Operating means is an electromagnet. 11. The relay according to claim 10 further including a third tube positoned within the inner shield and connected between the output of the switch and the other conductor of the cable for forming a signal lead-out for the switch.

12. The relay according to claim 9 wherein: the outer shield is a substantially closed structure having a single opening,

the inner shield is a substantially closed structure having a single opening aligned with the opening in the outer shield, and

the conductors of the cable extend through the openings in the shield.

13. As an article of manufacture, a subassembly adapted for use with shielded cable including:

-a switch having an input terminal and an output terminal in juxtaposition with the input terminal;

a first conductive shield substantially surrounding the switch and having an opening adjacent the terminals of the switch;

a second conductive shield having an opening aligned with the opening in the first shield and fitted tightly around the first shield at least at the opening; and

means for electrically insulating the first shield from the second shield.

14. The subassembly according to claim 13 further including a lead-out shell positoned around the switch within the first conductive shield and electrically connected to the switch for forming the output terminal of the switch.

15. The subassembly according to claim 14 wherein:

the lead-out shell is a tube fitted tightly around the switch and having a closed end;

the first shield is a tube fitted tightly around the leadout tube and having a closed end adjacent the closed end of the lead-out; and

the second shield is a tube fitted tightly around the first shield tube -and having a closed end adjacent the closed end of the first shield.

16. The subassembly according to claim 15 wherein each of the shells is integrally formed from conductive, non-magnetic material and wherein each of the shells is separated from adjacent shells by a layer of electrically insul-ative material.

References Cited UNITED STATES PATENTS 8/1965 Santangel 335-154 X 5/1967 Morrison 335-151 U.S. Cl. X.R. 335-301

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