Distribution Device for Distributing Very High Data Rate Digital Signals

The invention relates to very high data rate digital signal transmission, and in particular it relates to distributing such signals via lines branching from a distribution line.

At present, the data rates of digital signal transmissions may reach values such that spectrum components thereof reach the microwave range.

In order to conserve the waveform of the transmitted signals, it is essential to use perfectly matched transmission media since the slightest non-matched connection can be catastrophic from the transmission point of view.

At very high data rates, e.g. about 100 Mbit/s and above, the maximum length that can be tolerated for a branch line connected to a bus is very short, and a function of the data rate; the higher the data rate, the shorter the maximum length. For example, at a data rate of 100 Mbit/s, each branch line on a bus cannot be more than a few centimeters long since even when terminated with its characteristic impedance, it nevertheless gives rise to a degree of mismatch on the bus. If longer than 5 centimeters, a branch line gives rise to a non-acceptable degree of disturbance to the digital signals it transmits.

In the very common situation of distributing very high data rate digital signals to electronic devices mounted on a common printed circuit card, this limited line length imposes severe constraints on the locations that the electronic devices may occupy on the card, and these constraints may limit the number of electronic devices that can be placed on the card to one or two, even though there is room on the card for more devices.

The object of the invention is to make it possible to distribute very high data rate digital signals without disturbance via lines branching from a bus.

Another object of the invention is to avoid constraints on the location of electronic devices on a given printed circuit card, which constraints are due to the very short lengths of branch lines.

The present invention provides a distribution device for distributing very high data rate digital signals, the device comprising a distribution line connected at one end to a transmitter, and branch lines having the same characteristic impedance each connected to a respective electronic circuit, the distribution line being constituted by at least one bus of varying impedance, the bus being constituted by n lengths of different impedances, each length being constant in width, the width of successive lengths reducing from a widest end of the bus connected to the transmitter, each change in width between two successive lengths constituting a junction, and each branch line being connected to a junction, wherein the branch lines are of arbitrary lengths and the lengths of the bus are also arbitrary.

Embodiments of the invention are described by way of example with reference to the accompanying drawings, in which:

FIG. 1 shows a distribution device of the invention for asymmetrical distribution;

FIG. 2A shows a variant of the FIG. 1 distribution device, and FIG. 2B shows a variant of the FIG. 2A bus;

FIG. 3 shows a distribution device of the invention for symmetrical distribution;

FIG. 4 shows a variant of the FIG. 3 distribution device;

FIG. 5 shows a variant of the electronic circuit of FIGS. 1 and 2;

FIG. 6 shows a variant of the electronic circuit of FIGS. 3 and 4;

FIG. 7 shows another variant of the electronic circuit of FIGS. 1 and 2;

FIGS. 8A and 8B show two other variants of the electronic circuit of FIGS. 3 and 4;

FIG. 9 shows a variant of the FIG. 2 bus;

FIG. 10 is a perspective view of a variant of two FIG. 4 buses;

FIG. 11 shows an asymmetrical distribution device of the invention used as a mother board; and

FIG. 12 shows a symmetrical distribution device of the invention used as a mother board.

FIG. 1 shows a distribution device of the invention for asymmetrical distribution. In this figure, the distribution line is a bus B of varying impedance connected to lines L2, L3, . . . , Ln of arbitrary lengths, each of said lines having the same characteristic impedance Z0.

The bus B is made up of lengths B1, B2, . . . , Bn, with the lengths Bn being connected to a transmitter E which is in turn connected to an input LE (differential or otherwise), with the length B1 being connected to an electronic circuit D1. Each length is constant in width and of arbitrary length. The widths of the lengths increase from one length to the next, with the length B1 being the narrowest. Each junction between two lengths is obtained by a symmetrical change in width of the length about its long direction, said variation taking place on both sides of each length.

The transmitter E is of conventional type and serves to shape the digital signals which it receives via the input line LE. The bus B is a conductor, e.g. a printed circuit track, and the lengths B1 to Bn have respective impedances Z1 to Zn. At each junction between two lengths, one of the lines L2, . . . , Ln connects the bus to an electronic circuit D2, D3, . . . , Dn. At the input to each of the electronic circuits D2 to Dn, the lines are terminated on their characteristic impedance Z0. The end of the last length B1 is applied to the input of electronic circuit D1 which is terminated by an impedance Z1 which is preferably equal to Z0.

In order to obtain good digital signal transmission, the lengths are terminated on their own impedances; and the impedances of the lengths are then as follows:

where Z1=Z0 if the length B1 has impedance Z0.

Putting Z0=100 ohms, then Z1=100 ohms, Z2=50 ohms, Z3=33.3 ohms, and Z4=25 ohms. Using electronic circuits D1, D2, . . . , Dn implemented in ECL logic, it is possible to distribute to four electronic circuits since the value 25 ohms is the lower limit specified by circuit manufacturers as an output load on an electronic circuit.

FIG. 2A shows a variant of the FIG. 1 distribution device. In this figure, one of the longitudinal sides of the bus C is rectilinear and the change in width between successive lengths takes place solely on the other longitudinal side of the bus. As in FIG. 1, the lines L2, L3, . . . , Ln are connected to the bus where the lengths join one another and they are terminated on their characteristic impedance Z0. The end length C1 at the end of the bus furthest from the transmitter E which is connected to length Cn is itself connected to an electronic circuit D1, with said length C1 being terminated on an impedance Z0 as in FIG. 1, assuming that the impedance Z1 of the length C1 is equal to Z0. The values for the impedances Z2 to Zn of lengths C2 to Cn are given by the above-specified equations.

FIG. 2B shows a variant way of connecting the lines L2 to Ln to the bus C of FIG. 2A. In FIG. 2B, the lines are connected to the rectilinear side of the distribution bus level with the steps which constitute the junctions between the various lengths of the distribution bus.

FIGS. 3 and 4 show a distribution device of the invention for symmetrical distribution. In these figures, the distribution line is a differential line constituted by two identical buses B and B', each transmitter having one end connected to the output of a differential E' whose input is connected to an input line LE, which may be differential or otherwise.

In FIG. 3, the two buses B and B' are as the same type as distribution bus of FIG. 1, i.e. there is a step in each side of the distribution bus at each junction between two successive lengths. In FIG. 4, both distribution buses C and C' are of the same type as the FIG. 2 distribution bus, i.e. each bus includes a rectilinear side and each junction between two successive lengths of the bus is constituted by a step in the other longitudinal side of the bus. In FIG. 4, the rectilinear longitudinal sides of the distribution buses C and C' face each other and are at a constant distance apart s equal to a few tenths of a millimeter.

Each of the electronic circuits D'2, D'3, . . . , D'n, is connected to a corresponding pair of junctions in the two distribution buses B and B' or C and C' via respective differential lines Ld2, Ld3, . . . , Ldn. The differential lines Ld2 to Ldn are of arbitrary lengths, and of the same impedance. Each of them is therefore terminated, at its end adjacent to the inlet to an electronic circuit, on an impedance Zd equal to its characteristic impedance.

In these two figures, the pair of end lengths B1 or C1 in the two distribution buses is connected to an electronic circuit D'1, and is terminated on an impedance equal to the impedance of the pair of end lengths, which impedance is preferably equal to the characteristic impedance Zd of the differential lines.

Using the notation Z1d, Z2d, . . . , Znd, for the impedances of respective pairs of corresponding lengths B1, B2, . . . , Bn in FIG. 3, or C1, C2, . . . , Cn, in FIG. 4, then:

where Z1d=Zd if the terminal pair of lengths B1 or C1 has an impedance equal to Zd.

In FIGS. 1 to 4, digital signals are distributed to electronic circuits D1 to Dn or D'1 to D'n. These electronic circuits are either circuits for making use of the received signals, or else they are transmission circuits each including a corresponding transmitter E or E' having its output connected to a distribution bus or to a coaxial line or to a shielded pair, for example, and naturally in FIGS. 1 to 4, some of the devices could be end use circuits and others could be transmission circuits. When all the electronic circuits are transmission circuits, then the card may be a mother board for distributing the digital signals it receives to said transmission circuits, as described below with reference to FIGS. 11 and 12.

In FIG. 5, references Li and Di designate a line and an electronic circuit of FIGS. 1 and 2. In this case, the electronic circuit is a transmitter having its output connected to a distribution bus BD having m lengths connected to electronic circuits Di1 to Dim, with the length BD1 being terminated on an impedance Zi0 equal to the characteristic impedance of said length. The electronic circuits such as Dim are connected by lines to the bus BD, said lines being of arbitrary length and having the same characteristic impedance, e.g. of value Zi0. All of them are terminated on their characteristic impedance.

In FIG. 6, Ldi and D'i designate a differential line and an electronic circuit as shown in FIGS. 3 and 4. This electronic circuit is a differential transmitter having its output connected to two distribution buses F and F' each having p lengths, referenced F1 to Fp and F'1 to F'p, which lengths are connected to electronic circuits D'i1 to D'ip, with the electronic circuit D'i1 being connected to the ends of the lengths F1 and F'1 of the two buses F and F', and with the other electronic circuits such as D'ip being connected via respective differential lines to corresponding lengths of the two buses F and F'. The differential lines are of arbitrary length and have the same impedance and they are terminated on impedances Zd0 equal to their characteristic impedance. The ends of the lengths F1 and F'1 are connected via an impedance Zd0, assuming that the line constituted by the pair of lengths F1 and F'1 has an impedance of Zd0.

In FIG. 7, the references Li and Di designate a line and an electronic circuit as shown in FIG. 1. In this case, the electronic circuit is a transmitter connected to a coaxial cable LC.

In FIGS. 8A and 8B, Ldi and D'i designate a differential line and an electronic circuit as shown in FIGS. 3 and 4. In this case, the electronic circuit is a differential transmitter having its output connected to two coaxial cables LC1 and LC2, (FIG. 8A) or to a screened twisted pair LBi (FIG. 8B).

In FIGS. 5 and 7 the transmitter could be a differential transmitter, in which case it should be connected in FIG. 5 to two identical buses, and in FIG. 7 to two coaxial cables or to a screened twisted pair. Similarly, in FIGS. 6 and 8A, 8B, the transmitter could be a transmitter connected to a single bus or to a single coaxial cable, in order to provide asymmetrical distribution. As is well known, a single bus for asymmetrical distribution is typically associated with a ground plane separated from the bus by a dielectric (not shown).

The bus for asymmetrical distribution or the two buses for differential distribution need not be rectilinear and could be bent through an arbitrary angle in order to make good use of the printed circuit card area when locating the electronic circuits. FIG. 9 shows a bus BD for asymmetrical distribution having a bend in its second length BD2.

FIG. 10 shows two buses BD and BD' for differential distribution, with both of the uses being bent in its second length BD2 or BD'2 respectively. For differential distribution, both buses are preferably superposed, although naturally they must be insulated from each other, e.g. by means of a substrate S, thereby enabling the two wires of each differential line to have the same length and also enabling corresponding lengths of the bus to run parallel so that the impedances of each pair of corresponding lengths are well defined.

FIGS. 11 and 12 relate to mother board distribution, with FIG. 11 relating to asymmetrical distribution and FIG. 12 relating to symmetrical distribution.

In FIG. 11, a mother board includes a transmitter EP and a distribution bus BP connected to the output of the transmitter, which in turn has its input connected to a line LEP, which may be differential or otherwise. In this figure, a bus is shown having a rectilinear side like the bus C in FIG. 2, however it should naturally be understood that the bus BP could have junctions obtained by symmetrical changes in the widths of the lengths as shown in FIG. 1. Each junction is connected by means of a connector K to transmission circuits CD1, CD2, CD4, and CD5. Transmission circuit CD1 is of the type shown in FIGS. 1 and 2. Transmission circuit CD2 is of the type shown in FIGS. 3 and 4. Transmission circuit CD4 is constituted by a differential transmitter E' having its output connected to two coaxial cables LC1 and LC2 which could be replaced by a screened twisted pair line. Transmission circuit CD5 is of the type shown in FIG. 5 and is connected by a connector K to the end length of the bus BP. In transmission circuits CD1, CD2, CD4, and CD5, the transmitters are connected to the distribution bus BP by branch lines of arbitrary length and common characteristic impedance Z01. The circuit CD3 is a plug circuit likewise connected by a connector K to a junction in the bus BP, and constituted solely by a line terminated on its characteristic impedance Z01, which is the same as the impedance of the other lines of the transmission circuits. This plug circuit serves to conserve the impedance of the bus BP and the corresponding junction, either because the transmission circuit that would normally be connected to said junction is out of order, or else because there is no intention of connecting said junction to a transmission circuit.

In FIG. 12, the mother board comprises a differential transmitter EDP and two distribution buses BP1 and BP2 each having a rectilinear side as shown in FIG. 4, but naturally, they could be of the same type as the buses B and B' as shown in FIG. 3. Each pair of junctions in the buses BD1 and BD2 is connected via a connector K1 to transmission circuits. By way of example, various different transmission circuits CT1, CT2, CT4, and CT5 are shown. Transmission circuit CT1 is of the same type as shown in FIGS. 1 and 2 with an input line LE which is a differential line. Transmission circuit CT2 is of the type shown in FIGS. 3 and 4 and its input circuit LE is a differential line. Transmission circuit CT4 is of the type shown in FIG. 8A, and the two coaxial cables could be replaced by a screened twisted pair line as shown in FIG. 8B. Transmission circuit CT5 is of the type shown in FIG. 7 and is connected by a connector K1 to the two end lengths of the buses BP1 and BP2. In transmission circuits CT1, CT2, CT4, and CT5, the transmitters are connected to two distribution buses BP1 and BP2 via differential lines of arbitrary length and common characteristic impedance Zd1. The circuit CT3 is a plug circuit, likewise connected by a connector K1 to the two buses BP1 and BP2, and including a differential line terminated on its characteristic impedance Zd1 which is equal to the impedance of the differential lines of the transmission circuits. This plug circuit performs the same function as the plug circuit CD3 in FIG. 11.

In FIGS. 11 and 12, the transmission circuits and the plug circuits are carried by printed circuit cards. The various circuits shown are given purely by way of example and naturally all of the transmission circuits could be identical and of any desired type. Similarly, the transmission circuits may be of one or several of the types shown, and the plug circuits could be omitted, so long as none of the transmission circuits is out of order.