Common-Mode Interference from Cable System
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| Sep 13th 2011, 15:13 | |
W1MGJoined: Apr 4th 1998, 00:00Total Topics: 0 Total Posts: 0 |
If a TV or other Cable Company termination in a residence puts a common mode RF signal on the cable, and that signal causes harmful interference to another (nearby) spectrum user, is the Cable Company required to put a choke on the cable? It’s not leakage, and it’s not a source being operated by the cable company. Where is the burden of responsibility in a case like this? |
| Sep 13th 2011, 16:08 | |
N0IVNJoined: Apr 4th 1998, 00:00Total Topics: 0 Total Posts: 0 |
Cable TV networks operate on the principle of frequency reuse, where a cable company uses frequencies inside of its cables for purposes that may be completely different than what the same frequencies are used for in the over-the-air environment. As long as the shielding integrity of the cable TV network is intact, signals inside of the cables will not leak out and interfere with over-the-air services, and over-the-air signals will not leak in to the cable network and interfere with cable services. The FCC Rules in Part 76 require a cable company to maintain leakage of the signals inside of its cable network -- that is, its own signals -- below certain field strength limits, as well as to not cause harmful interference (even if the leakage is below the defined maximum allowable field strengths). Cable companies are also responsible for signal leakage from cable company-owned terminal equipment (set-tops, cable modems, etc.). Interestingly, they also are responsible for signal leakage from poorly-shielded subscriber-owned devices such as cable-ready TVs connected directly to the cable TV network. The FCC does not expect a cable company to repair a defective or poorly shielded subscriber-owned TV, of course, so responsibility for the latter is limited by the Rules to disconnecting the cable TV service to eliminate the signal leakage (most cable companies consider disconnecting cable service a last resort, and typically would try other things such as install a set-top box or reduce the amplitude of signals in the cable). Non-cable interference that might be coupled to the cable TV coax shield via any of several means is not the responsibility of the cable company. One example is interference that might be coupled via code-required neutral bonds. The cable TV company is required by code (typically the National Electrical Safety Code for outside plant and National Electrical Code for premises) to bond the shield of its coaxial cable to the power company neutral conductor and the household grounding electrode system. Telephone companies also are required to bond their cable(s) to the neutral conductor, as are satellite service providers. The purpose of these common bonds is to minimize a difference of potential among the power neutral, telephone, cable TV, and satellite cabling grounds, for safety of people and equipment. One downside to the aforementioned bonding is that interference may be coupled among various services. Interference from a device connected to any of the services (power, telephone, cable, satellite) may be coupled to the other services via the code-required bond, and then radiated by the wiring or cabling of any or all of those services. The culprit here is not the bonded services, but the device causing the interference. Plasma TVs can be notorious sources of RFI, both radiated and conducted. Obviously there is little that can be done about interference radiated directly by a plasma TV. Conducted interference might be able to be reduced by the usual methods (common mode chokes, brute force AC filters, etc.). See a related thread on plasma TV interference in this forum. Assuming the interference in question is typical plasma TV "noise" rather than cable TV signals leaking from a poorly shielded set, the cable company not responsible for installing common mode chokes. 73, Ron Hranac, N0IVN ARRL EMC Committee Member ARRL Colorado Section Technical Specialist |
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