Constant Impedance Transmission Line Routing Network

This invention relates to distribution networks and the use of relays to provide a plurality of transmission line paths in which each possible path routing does not differ in impedance from any other path routing, and in which there are no impedance discontinuities.

With the use of digital signals, squarewave signals and a multitude of other variety of high frequency signals, it is often necessary that a transmission line maintain a uniform impedance characteristic in order that digital signals or high frequency signals transmitted along the line are not distorted, reflected or altered in phase when the transmission line is carrying or conveying the signals. Certain digital data processing systems, data communications systems, testing systems and other switching systems using high frequency signals require that the input end of a transmission line be routed over to a variety of other output terminals after being switched through different paths.

In order to keep the disturbance to the line as small as possible, this requires a switching system with a controlled impedance matching the impedance of the transmission line and maintaining the constant impedance no matter how many switches are in the line. Generally, these types of transmission line systems are designed to have a characteristic impedance somewhere between 50-100 ohms, which theoretically remains constant at the designed impedance; however, any changes in length or routing may lead to problems in high frequency switching systems if the impedance-constancy of the transmission line is not maintained.

One place in the digital processing field where the problem of line variance arises is in the testing of high speed integrated circuits. In this situation, pulses with fast edges rates (high-speed rise-time and fall-time) are used. These types of pulses contain a wide range of frequencies which complicates the problem of switching the signal through various other transmission lines without introducing distortion to the signal pulses.

The problem of maintaining a constant impedance transmission line and still permitting switching of the line into different output paths has heretofore been handled by the use of so called "coaxial relays" which are precisionally built and extremely expensive in cost per unit. These coaxial relays can switch an input signal to one of two (sometimes more) outputs. These generally are well designed and introduce very little distortion to the input signal travelling through.

Generally, when switching systems have to be applied to computer applications, such as the testing of high speed integrated circuits, these may involve the connecting of hundreds of terminals to be tested. Since a multitude of coaxial relays are required, the cost of building a test system using these coaxial relays becomes prohibitive. For example, a test system (which may routinely need to switch an input signal to one of 50 available output terminal lines) would require such a multitutde of coaxial relays, to do the job properly, that it would make this application unreasonably expensive. Thus, commercial systems which would require the use of this type of switching with coaxial relays find that it is prohibitive.

A number of manufacturers have attempted to handle this problem in the fashion shown in FIG. 1 by connecting many small single throw relays 11.sub.o to one point and closing only one relay at a given time. Generally, these relays are the small inexpensive reed relay types. However, a persistent problem that arises with this solution, as seen in FIG. 1, is that each relay connected to a common node 9 will form a stub and add a capacitative load or discontinuity. For example, in FIG. 1, while one relay is closed, there are three other open relays hanging on or connected to the common node 9 which introduced a set of stubs making a capacitative load. In FIG. 1, for example, if the transmission system is designed as a 50 ohm system, which is customary in IC test systems, each relay is made as a "50 ohm" relay. This means that such a closed relay will behave electrically as if it were a piece of 50 ohm transmission line. However, each "open" relay connected to the node 9 behaves as a short stub. Thus, the only way to reduce the amount of load capacitance at that node is either to reduce the size of the relay or to reduce the number of relays, which again tends to defeat the problem of switching a transmission line. This cluster-of-relays approach in FIG. 1 can never be a truly controlled-impedance system.

The practical answer to the problem is provided by the use of small inexpensive single-pole double-throw reed-type relays which can be placed on or embedded in a printed circuit board. The preferred apparatus described herein uses a single-pole double-throw reed relay which has one input and two output lines and does not have an off position. The input line is always connected either to one or the other of the two output lines. This should be contrasted with the "single throw" type relays which have one "off" position and one "on" position only. One typical type of the preferred reed relay is designated as a Form C Reed Relay as manufactured by Hamlin, Inc., Lake and Grove Streets, Lake Mills, Wis. 53551. Another similar type of such relays are the miniature mercury-wetted relays known as Log-cells such as manufactured by Fifth Dimension, Inc., Post Office Box 483, Princeton, N.J. 08540.

The problems of providing a constant-impedance transmission line, which can be switched and routed to different destinations without altering the constant-impedance, do not have to be denied of practical use because of impossibly-expensive components such as coaxial switches or other devices.

A use of simple inexpensive double-throw reed-relays may be combined with high frequency transmission lines placed on a printed circuit board having a ground plane providing a "low side" or a common ground. Each switch means is electromagnetically shielded by an encompassing cover which is also grounded to the ground plane at each end of the shield. Further, each switch means may be encompassed by an actuating coil or by adjacent magnetic poles for actuation of the switch means. The switches have one input signal line and two output signal lines. Each one of the outputs of the initial relay switch then is connected by transmission strip lines of equal length and character to the inputs of two subsequent relay switches which then provide four output signal lines. Thus, no matter which routing is switched on by the relay switches, there is no change in the characteristic impedance at the transmission line and hence no distortion of high frequency signals. Whatever the desired impedance value of the transmission line, each relay switch is designed to match this impedance and all relay switches in a system are duplicates of each other and in impedance value of the transmission line.

FIG. 1 shows one type of switching system used in the art for high frequency transmission lines.

FIG. 2 is a schematic drawing showing an improved system for switching and routing constant-impedance transmission lines.

FIG. 3A and FIG. 3B show a specific practical embodiment of a switching system for a constant-impedance transmission line.

FIG. 4 is a schematic drawing showing one configuration of a high frequency transmission which is routable to any one of a large number of output signal lines.

FIG. 5 is a drawing illustrating one simplified embodiment of the switching network.

As seen in FIGS. 2 and 3A, a switchable transmission line providing continual constant impedance without the need for expensive coaxial relays may be seen. For example, reed relay switches in single-pole, double-throw versions may be chosen for the proper size of inner conductor and the proper size of the outer glass tube to provide a true controlled-impedance, single-pole, double-throw coaxial relay switch which are negligibly expensive in comparison to other devices such as the precision "coaxial type" relays. These inexpensive reed relay switches 11 may be connected in a tree structure as shown in FIG. 2 such that one signal can be routed to any number of destinations with no alteration or interruption of the characteristic impedance (or other desired impedance) of the transmission line system. There are no open stubs caused by open relays hanging on the line, as there are in FIG. 1.

The relay switches, FIG. 3A, may consist of an inexpensive readily available reed relay which is surrounded by conductive shield 23 having ground connections 23.sub.gi and 23.sub.go directly to ground plane 22. The actuating coil 30 can be wound directly on the shield or on a separate pole piece placed in close proximity to the reed relay.

In FIG. 1, which illustrates one version of the prior art, it will be seen that the relay switches 11 are single-pole single throw in which there is normally no connection until the relay is actuated at which time a closed connection is made. The relays 11 of FIG. 2 are to be contrasted in that they are single-pole double-throw relays in which, whether energized or not, there is always a connection between the input and one of the output lines.

As seen in FIG. 3A these inexpensive sealed relays may be mounted on a printed circuit board 20 and connected to strip lines or transmission lines 21 embedded in the printed circuit boards.

Referring to FIG. 3A and FIG. 3B there is seen a printed circuit board 20 having a ground plane 22 connected to each shield 23. Within the surface of the printed circuit board there are recessed indentations or grooves 24 into which may be placed a series of relay switches 11.

Each relay switch 11 is enclosed within a shield 23 which may be a copper mesh shield connected onto the ground plane 22.

Referring to FIG. 3A the electromagnetic shield 23 which encompasses each of the reed relay (single-pole, double-throw) switches is connected to the ground plane 22 at two separate and distinct locations. The input side of the shield has a ground connection 23.sub.gi directly to the ground plane 22 while the output side of the shield 23 has a similar direct to ground connection shown at 23.sub.go. As can be seen in FIG. 3B these connections from the shield to ground are provided at either extremity of the shield by means of the direct-line connections 23.sub.gi and 23.sub.go which connect directly to the ground plane 22. It may be noted that the ground plane 22 is the "low" side of the transmission line and it is not necessary that this side be connected to an external earth ground.

Each relay switch 11 may be encompassed by an actuating winding 30 which is used to activate and change the internal contacts of the relay switch 11.

As seen in FIG. 3B, the inner mechanism of relay switch 11 may consist of a pair of stationary arms 11.sub.b, 11.sub.c between which there may move a movable arm 11.sub.a. Thus, depending on non-energization of the winding 30, the arm 11.sub.a will maintain contact with its normally closed arm 11.sub.c or, upon energization of the coil 30, the movable arm 11.sub.a will make contact with the arm 11.sub.b.

The arms 11.sub.b, 11.sub.c are connected to transmission line strips 21 which are arranged on the surface of the printed circuit board 20. These transmission line strips, FIG. 3A, are symmetrically balanced in physical and electrical characteristics in their connective pattern between any two hierarchies of relay switches. Thus, a "zero level" relay switch 11.sub.A has two output transmission line strips 21.sub.b and 21.sub.c which are symmetrically balanced to connect to a "first level" of relay switches 11.sub.B and 11.sub.C.

The embodiment of FIG. 3A may be schematically illustrated in FIG. 2 wherein a single input transmission line may be switched and routed onto a multiple number of selected paths depending on the relay switching at each node such that a continuous transmission line will occur from input terminal to output terminal and will maintain a constant impedance without any capacitive stubs or other distorting factors hanging on to the transmission line which might alter the impedance and change the character of high frequency signals which are transmitted.

Referring to FIG. 4, the input transmission line 8 to the apparatus may be a coaxial line having a center lead which is connected to the input line of the first relay switch 11A. The other side of the input coaxial line which is designated as the "low" or ground side is connected to the ground plane 22 (FIG. 3A).

Referring to FIG. 4, there is seen another preferred embodiment wherein an input signal line 8 is connected to a single-pole double-throw relay switch 11. Each output arm of the first relay switch 11.sub.A connects to a second stage where two relay switches 11.sub.B, 11.sub.C can connect the signal transmission line in one direction or the other. For example, a printed circuit board, such as 20 of FIG. 3A, would support and mount the configuration shown in FIG. 4. Each relay is shielded by shield 23 which is connected to ground plane 22. Likewise each relay can be actuated by a winding 30.

Thus, the input signal 8 may be connected to any one of 32 output lines as shown by Q.sub.1, Q.sub.2, Q.sub.3 . . . Q.sub.30, Q.sub.31, Q.sub.32. In each case the transmission line from input 8 to any given output such as Q.sub.30 will establish a constant impedance transmission line which has no capacitive stubs or other distortion making characters, thus permitting a true and accurate transmission of the input signal at 8. Each relay switch 11 is, of course, made to match the impedances of the transmission line and each relay switch in the system will have the same impedance characteristic. Likewise, each strip transmission line 21 will be balanced (at the output side of each relay switch 11) to maintain the proper characteristic impedance.

Referring to FIG. 5 another simplified embodiment of a high frequency transmission line system may be provided which is supported completely by coaxial cable and electromagnetic shields. The switching devices 11 may be effectively integrated onto a coaxial cable by continuously connecting the coaxial cable 8 to the shield 23 which encompasses the switching means 11. Thus, the input line coaxial cable 8.sub.a has its outer "low" side or "ground" side connected to the input side of the electromagnetic shield 23. The output side of shield 23 is connected to the low side of cables 8.sub.b, 8.sub.c. The input conductor 8 connects to the input single pole 11.sub.a of the switch 11. The output contacts 11.sub.b, 11.sub.c are connected to a first and second coaxial cable 8.sub.b, 8.sub.c wherein both of these coaxial cables are connected together on the low side to the shield 23 and also to each other.

Likewise, each cable 8.sub.b and 8.sub.c may have a reed relay switch integrated within it to branch out again into two more coaxial cables. In this case, the coaxial lines and switch-shields form a continuously integrated transmission line which branches out to form other integrated transmission lines of like nature whereby activation of selected coils 30 can be used to form a desired routing for high frequency signals without distortion.

Since the use of the reed relay switch in this combination permits the elimination of expensive precision switches, then the cost per relay-switch is reduced by approximately one-hundred fold and this one-hundred fold cost reduction is multiplied by the total number of relays used in the system, thus making it practically feasible to use and provide a distortion free transmission line which is switchable to a multitude of output terminals.

In order to provide a particular or selected path from input to output, it may be noted that each level of the switching tree could be considered to be a "binary bit" where a "one" indicates that the corresponding level is "actuated" (away from its normal position). This makes the structure most readily adaptable to computer control, that is to say, whereby binary signals can be used to actuate any given pattern of relays in the system of FIG. 4 in order to provide a distortion free constant impedance transmission path between the input 8 and any one of the selected outputs such as Q.sub.1, Q.sub.2, etc. Switching of relay trees is known in the art and is described in the AIEE Transactions, on page 582, Vol. 68, 1949 in an article by S. H. Washburn.

The major application of the system described herein is to high frequency signals or digital signals having fast-rise and fast-fall times and wherein it is important that the integrity of these signals be maintained when they are transmitted over different switching paths and through differently routed transmission lines.

The above described systems which, in one case, provides a transmission line with integrated switches, and, in another case, permits simple and inexpensive relay switches embedded adjacently parallel to the ground plane of a printed circuit board and wherein each of the individual relay switches have a dual grounded shield at each end to provide a tree structure such that, no matter what configuration of switches is used, each transmission line path that is routed will provide the proper constant-impedance distortion-free transmission line which will transmit high-frequency signals without distortion over paths which may vary in length or in configuration.