Telecommunications equipment must be able to survive surges and power faults as defined in the relevant standards. Survivability is achievable by providing protection remotely, at the terminals of the equipment, or both. In addition, or alternatively, we can provide reliable protection by making the equipment more robust.
When designing a circuit-protection strategy, it is important to consider the complete system. To reduce cost, we can diminish the capabilities of the protection scheme somewhat, though the other components must be more robust to compensate. In such a case, the cost of enhancing the reliability of the downstream components may exceed the cost savings of a less robust protector. A good design will optimize the tradeoffs.
VDSL CIRCUIT PROTECTION CONSIDERATIONS
Very high-speed digital subscriber line (VDSL) technology facilitates the delivery of information at speeds of up to 52 Mbits/s. Standard VDSL deployment uses a frequency spectrum up to 12 MHz, whereas VDSL2 allows up to 30 MHz as an option.
The capabilities of VDSL depend on the distance between the operator and end-customer equipment as well as the condition of the existing copper plant and copper infrastructure outside the plant. Depending on loop conditions, VDSL can support varying bit rates and high-bandwidth services such as a channel of HDTV programming over telephone copper pairs.
Since VDSL equipment connects to the copper infrastructure of the public switched telephone network (PSTN), the equipment runs the risk of exposure to overcurrent and overvoltage hazards from ac-power-cross, power-induction, and lightning surges. One possible solution deploys resettable polymeric positive temperature coefficient (PPTC) overcurrent protection devices in a coordinated protection scheme with overvoltage devices such as gas discharge tubes (GDTs) and thyristor surge suppression devices.
REDUCING INSERTION AND RETURN LOSS
Because signal spectrum is increasing from 10 MHz to 30 MHz, VDSL system designers face a number of new challenges. The most important issue is reducing insertion and return loss and the effect on reach and rates in high-speed applications.
The capacitance of overvoltage protection devices becomes a concern in the upper range of the VDSL frequency spectrum, as the devices used to protect the system may increase system insertion loss. Tests demonstrate that low-capacitance thyristors and GDTs are suitable for use in high-data-rate circuits including VDSL applications.
The test results in Figure 1 illustrate the effects of capacitance on insertion loss in several overvoltage protection configurations. The results show that low-capacitance GDTs, in the realm of 1 pF, have the lowest insertion loss with standard 50-A thyristors (15 pF at 50-Vdc bias) and 100-A micro-capacitance thyristor devices (20 pF at 50-Vdc bias) having slightly greater insertion loss.
Regarding test procedures, the inset modules in the test diagram consist of either a three-pole, 230-V GDT or two 270-V in-series thyristors interfacing with two 0.3-m pieces of CAT5e cable. An Agilent 8753ES Vector Network Analyzer with two North Hills 0301BB 50:100 Ω wideband transformers performs the insertion-loss measurements.
The transformers are for measuring the insertion loss of the modules under 100-Ω impedance conditions, which is equal to the line impedance over the VDSL frequency spectrum. An HP 4195 low-frequency impedance analyzer performs capacitance measurements at 1 MHz with no bias.
IMPLEMENT A LOW-CAPACITANCE VDSL SOLUTION
In Figure 2, the circuit diagram shows a VDSL solution that effectively reduces capacitance and energy let-through while optimizing the circuit-protection scheme. Visible in the circuit diagram, GDT1 provides primary protection from 350 V to 1 kV, and the GDT2 and GDT3 components work in series with the thyristors. In this scenario, the thyristor helps lower the breakdown voltage of the GDT and reduces the let-through energy in the case of a surge. Additionally, the PPTC devices help coordinate the primary and secondary protection.
Test results for this protection method, found in Figures 3, 4, and 5, demonstrate that the GDT and thyristor combination does not break down under ringing voltage and does not clip the ringing voltage. In the oscilloscope screen shot seen in Figure 3, the input voltage rate is at 100 V/s and the dc-breakdown voltage is at 287 V, which is higher than the ringing voltage of 200 V.