12 Feb SSTDR: Spread-Spectrum Time-Domain Reflectometry
Security and alarm systems become more complex every day as new services, features, technologies and capabilities are added. This complexity is powered by good old electricity, and without a reliable and consistent source of power, the systems are useless. All of the wiring configurations within each system that carry data, video, signal and voice, Cat-5, coax cable and Romex are the backbones that transmit the information about the overall health of each system.
Power over Ethernet, low-voltage sensor lines and AC and DC voltages all flow over these different types of conductors. If something goes wrong, you want to know about it as soon as possible and figure out what to do about it for the customer. But what happens when an intermittent condition pops up and trips alarms or sets off sensors? And then what happens when a technician responds and shuts off the power to check the circuits but finds nothing? How does a system get checked for electrical faults while it is running, without disturbing the service to the customer?
What if you could look into the electrical workings of any cables such as security and alarm, CCTV coax, data, electrical power, high-voltage utility cable and/ or aircraft cables while they are carrying voltage, video or even high-speed data, and see active faults such as intermittent shorts or arc faults? Well, now you can. A new technology is making its way to market, and the impact on all these things will be tremendous.
The new technology is an advanced form of spread-spectrum time-domain reflectometry (SSTDR), and it is being developed and commercialized by two American companies for worldwide applications of all types.
SST has been around for a long time. It was first thought of during World War II for use in secure communications from submarines. Qualcomm developed a version of this concept to help cellphones connect with each other over a noisy and uncertain wireless environment. The reason someone in Hong Kong can dial your cellphone and reach only you, not the other 7 billion people on earth, is mostly due to spreadspectrum implementation. What has happened over the wireless world is now coming to the wired world.
The advanced form of this technology promises to revolutionize the TDR cable-testing market and make it safer and easier to see exactly what is happening on live, energized circuits on all types of equipment and over all types of conditions.
A time-domain reflectometry (TDR) cable tester uses a generated pulse that is shot down a cable to seek out opens and shorts that exist in the cable. The reflected pulse shows a delay in time that can be translated into distance or location of the fault. Traditional TDRs can work only when the cable they are testing is off or unpowered. This is because the voltage noise on the line interferes with the pulse, and the reflection is lost in this noise. Advanced SSTDR fixes this by skipping or “floating” around the noise by generating multiple frequency pulses that cut through all the interference and capture the reflected pulse to show exactly where the fault occurs, when it occurred and what its characteristics are. In addition, unlike older TDR technology, SSTDR also can do dynamic testing, which can monitor an energized cable system while it is under load or stress and in use and wait for a fault to occur.
This is handy for those faults that are intermittent due to some outside influence, such as voltage surges or outside signal interference, that changes the character of the circuit. An example of this is a short in a power cord attached to a piece of machinery that shorts out only when a parts cart rolls over it, or an aircraft that has some abraded insulation in its wiring harnesses that is shorting for only nanoseconds during takeoff or landing.