1. Field of the Invention
The invention relates to a differential transmission cable module used for transmitting a differential signal between electronic devices.
2. Description of the Related Art
Differential signaling is conventionally used for high-speed electrical signal transmission. In known differential signal transmission cables, three or more conductors are used to transmit one type of differential signal for the following reasons.
Firstly, to provide an outgoing path and a return path for electric current and also to define characteristic impedance of wiring, at least two conductors to be current paths are generally required in electrical wiring used for high-speed transmission. This is the minimum number of conductors required for electrical wiring used for high-speed transmission.
However, conductor wiring pattern on a printed circuit board constituting an electronic device is different in each device and this causes a problem that it is hard to accurately predict the amount of high-frequency noise to be generated. For this reason, electrical wiring for high-speed transmission on a printed circuit board is generally configured as differential transmission wiring.
In the differential transmission wiring, an outgoing conductor is arranged close to a return conductor, and the shape of the outgoing conductor and the shape of the return conductor are symmetric to each other. This allows high-frequency noise generated by the outgoing conductor and that generated by the return conductor to cancel each other out in the differential transmission wiring. Thus, the noise electromagnetic field level can be reduced in the differential transmission wiring. In this case, in order to even out the characteristic impedance of the differential transmission wiring in a longitudinal direction, it is necessary to arrange one or more wide ground conductors near the outgoing conductor and the return conductor. Therefore, in differential transmission wiring on a printed circuit board, at least three or more conductors consisting of one or more outgoing conductors, one or more return conductors and one or more ground conductors are required to transmit one type of differential signal.
Differential signal transmission cable is used to connect differential transmission wirings on different printed wiring boards to each other. Known differential signal transmission cable connects an outgoing conductor, a return conductor and a ground conductor constituting differential transmission wiring on a printed wiring board respectively to those of another printed wiring board. Thus, known differential signal transmission cable is configured to use at least three or more conductors consisting of one or more outgoing conductors, one or more return conductors and one or more ground conductors to transmit one type of differential signal.
FIGS. 7 and 8 show examples of structure of known differential signal transmission cable and connection structure between known differential signal transmission cable and printed circuit board.
A differential signal transmission cable 102 shown in FIG. 7 is configured to have two signal line conductors 121, a single layer of a shield conductor 122 serving as a ground conductor, and one drain wire 123. The two signal line conductors 121 are connected to two signal conductors 133 formed on a printed circuit board 103, and the drain wire 123 is connected to one ground conductor 134. In the differential signal transmission cable 102, the number of ground conductors is one when the shield conductor 122 is electrically connected to the drain wire 123, and the number of ground conductors is two when the shield conductor 122 is not appropriately electrically connected to the drain wire 123. Thus, in the example shown in FIG. 7, the number of conductors constituting the cable is three or four.
A differential signal transmission cable 202 shown in FIG. 8 is configured to have two signal line conductors 221 and a single layer of a shield conductor 222 serving as a ground conductor. The two signal line conductors 221 are connected to two signal conductors 233 formed on a printed circuit board 203, and the shield conductor 222 is connected to a ground conductor 234. In the example shown in FIG. 8, the number of conductors constituting the cable is three.
Meanwhile, a differential signal transmission cable described in Japanese patent No. 5214056 is configured to have two signal line conductors and two layers of shield conductors serving as ground conductors. In the differential signal transmission cable described in Japanese patent No. 5214056, one of the two layers of shield conductors is not electrically connected to anywhere and intra-pair skew is thereby reduced without increase in transmission loss of differential signal. Thus, in the example described in Japanese patent No. 5214056, the number of conductors constituting the cable is four.
As such, the known differential signal transmission cables use three or more conductors to handle one type of differential signal.