1. Field of the Invention
This invention is directed to a transmission line cable which allows ribbon cable segments to be formed into a round shape while maintaining a controlled relationship of signal and ground conductors so that the resulting electrical characteristics are consistent and uniform.
2. Description of the Prior Art
Flat transmission line cable, commonly referred to as ribbon cable, is widely used to facilitate mass terminations. Unlike multiconductor round cable, the conductors in ribbon cable remain in the same position with respect to a reference and can be connected to a mass termination connector, without the need to trace each individual conductor to its source. In conventional round cable, the multiple conductors twist and undulate within the cable so that it is impossible to determine by inspection without the use of color codes whether the relationship of the conductors at the end of the cable is the same as at the beginning of the cable, and mass termination techniques cannot be utilized.
While round cable is disadvantageous from the point of view of mass termination, it possesses other properties which provide advantages over ribbon cable. In particular, round cable is easier to shield, bends in any direction for easier handling and installation, and has a more compact profile for some applications.
In view of the distinct advantages of round cable in terms of shielding and handling, it has been proposed to configure ribbon cable into a round shape by folding the ribbon cable against itself and extruding a plastic jacket in a round configuration thereover. The ribbon cable is thereby configured into a round shape but can be re-configured as ribbon cable by stripping away the round jacket and unfolding the ribbon cable. The unfolded ribbon cable can then be mass terminated in the normal manner.
Prior attempts to configure ribbon cable into a round shape by folding and jacketing did not address the variations that occur with electrical characteristics. In such configurations, impedance and capacitance variations can be relatively high due to the position of the conductors when the ribbon cable is folded and cannot be considered controlled from a transmission line cable perspective, as the tolerances usually associated with such cable are relatively tight. The position of the conductors caused by folding the ribbon cable can also lead to increases in crosstalk.
Accordingly, there remains a need in the art for a transmission line cable with controlled and predictable electrical characteristics, which combines the handling and shielding advantages of round cable and the advantages of mass termination of ribbon cable.