Electronic Design

Existing In-Home Coax Can Support Data Comm

Even though it exists in almost every home, the coax communication channel is underused. Coax ties the cable head or satellite antenna to one or more TV sets. Now, astute designers can use it to make broadband communication within the home as near as the closest F connector in the room.

Jack Terry, president of JT Laboratories in Cumming, Ga., discussed coax's broadband potential at January's 2001 Conference of Emerging Technologies in New Orleans, La. He believes the bandwidth and economic capabilities of the huge installed base of TV coax wiring in the home have been largely ignored. Terry added that data-transmission speeds well above 10 Mbits/s can be achieved economically using simple modulation and available high-volume technologies. These include an in-home data transport protocol for uninterrupted communication, like digital video and contention-based protocols for data like Ethernet.

Telephone wiring is prone to unwanted radiation, interference pickup, and reflections. But TV coax has relatively good shielding, exceptional bandwidth, and very low vulnerability to interference at frequencies above 900 MHz. It also isolates echo signals very well.

The frequency spectrum of 0.9 to 1.0 GHz is above the relatively high field-strength radiation of portable cell handsets. Yet it's low enough so the performance of presently installed splitters and couplers isn't compromised. Though rated for the TV bands, passive splitters and couplers perform adequately in terms of loss and port isolation when carrying modulated digital signals up to 1 GHz. Rather than being a problem, losses that occur in building coax wiring help attenuate echoes at these higher frequencies and permit much simpler equalization in the digital receivers.

To enable communications between high-speed data devices, a central means is needed to selectively echo the data signals but not the unwanted signals. The latter includes TV channel echo or ghost signals caused by poor matching-impedance TV tuners, set-top boxes, loose connections, and poorly constructed or damaged coaxial cable. Similarly, spurious interference like TV set/set-top box local oscillator conducted interference should not be echoed. This is difficult to achieve with passive coax distribution systems containing the elements needed for satisfactory TV distribution. But it can be realized by using a frequency-changing, regenerative device instead of frequency-selective echo devices. In addition to frequency changing, amplification can be added to reduce or offset losses introduced by splitters, creating a low-loss LAN.

A typical home LAN system might employ a gateway/regenerative frequency changer and three client-data interfaces (see the figure). The gateway/ frequency changer receives all upstream traffic from the client interface, providing Internet access via a cable modem. Meanwhile, the client-data interfaces handle both upstream and downstream traffic via the TV coax to the gateway/frequency changer.

For details, contact Terry via e-mail at at [email protected]

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