Time Code Distribution

Challenges in Time-Code Distribution

The challenges of time-code distribution typically fall into two categories: the number of outputs needed to support the number of IEDs, and environmental challenges such as Electromechanical Interference (EMI) and/or physical distance issues.

To begin, the clock itself may have a limited number of output ports, or the option slots available on the clock are in use for other purposes. Second, the environment may require distribution over long distances, and/or be susceptible to electromagnetic interference (EMI).

Arbiter Model 1073A is designed to distribute signals generated by a GPS Satellite Controlled Clock. The unit features three independent channels, each with one input and four output ports. Other clock models support a variety of input and output choices (refer to data sheet Timing Products Options and Accessories for specific model and option information) .

The problem of EMI can often be solved with the use of fiber optics because of it's immunity to electromagnetic interference. However, while substations may reasonably be considered high-EMI environments, the expense of fiber-optic cable and drivers is generally not justified for most connections, particularly between clock and IEDs in the same rack or control room.

This is because the galvanic isolation provided at the IED input also provides great immunity to damage from substation surge voltages. The occasional transient signal propagated to the optical isolator or transformer output is easily dealt with by the pulse-conditioning or demodulation circuits in a well-designed IED, and even if a transient is detected by the countertimers, it is easily identified and ignored. As a final protection, error bypass in the local clock
should guarantee continuous and accurate operation.

There are applications for fiber distribution of time codes, particularly between substations or control houses, where the length of the link makes copper connections undesirable. For these applications, where lengths can be many kilometers and losses require an ac-coupled signal, IRIG time code may be transmitted using modified Manchester encoding. This was first defined by PES-PSRC in IEEE Standard 1344-1995 (annex F) and later adopted by IRIG itself in IRIG Standard 200.

However, the cost of such systems must be weighed against the alternative of placing an additional GPS clock at the remote location. In almost all cases, the cost is lower, and reliability and flexibility greater, when a second GPS clock is used instead of a long fiber-optic link.

Arbiter produces multiple models that are fiber-optic capable, including distribution amplifier Model 1073A.