An Advanced Circuit Protection Approach for RF Interfaces
One aspect that sets circuit protection for radio frequency (RF) apart from other types of applications is the range of frequencies in which RF circuits operate. With abbreviated clock cycles, the transition time of the data leaves little room for error. Transitions must be clean and reliable to ensure the integrity of the data and interface. Circuit protection characteristics such as capacitance become important in terms of their effect on these high-frequency signals and it often complicates the design.
While capacitance can be designed into the circuit to provide the benefit of smoothing the transition of the data, it is often at the cost of the data becoming stable too late in the clock cycle, resulting in errors. A proven method to provide circuit protection includes the use of an electronic current limiter (ECL), which reacts when either the voltage or current is out of the safe range of operation. Particularly useful for RF applications that operate up to 3 GHz, this solution is available to complement other technologies, and provides protection from overcurrent, overvoltage, and open or short circuits.
Applications that follow Gigabit Ethernet (GbE), Serial Digital Interface (SDI), and Low Voltage Differential Signaling (LVDS) standards are well suited to an ECL solution. An ECL introduces negligible capacitance, which makes this method of circuit protection an optimal choice.
RF and LVDS Vulnerability
RF interfaces include a variety of sensitive electronic equipment such as cable drivers and equalizers. While signals may be transmitted wirelessly, an abundance of interface circuits is required in order to transmit, receive, and process the signals. Thus, while the signal itself does not require protection, the parts that make up the interface do. On the wired side of the RF application, there are numerous circuits that require circuit protection beyond the nominal or even enhanced electrical overstress protection provided with the devices. Depending on the end application, wired components in the RF signal chain may be susceptible to lightning and other faults that require extended protection from severe overstress. Many of these applications interface with equipment that uses highspeed LVDS signaling.
LVDS technology is useful for interface applications requiring high bandwidth. As use of communications networks increases in a wide range of applications and markets, LVDS signaling will be employed on transmission lines that may be exposed to electrical overstress. Any LVDS communications lines that interface with the outside environment will most likely include external lines exposed to lightning surge and power cross-threats, such as high-speed lightning transient levels at 1 MHz and 10 MHz. Hundreds or thousands of volts can be introduced to these lines due to a lightning strike, whether direct or indirect, and in orders of magnitude greater than the 1.25V signaling of LVDS signals. A highly effective protection method is required that protects the electronics while maintaining the wide bandwidth requirements of LVDS applications.
Typical RF Application Protection
A variety of technologies is employed for circuit protection in RF applications, including TVS diodes, coaxial arresters, and gas tubes at the connector interface. While TVS diodes are suitable for electrostatic discharge (ESD) protection where connection and disconnection of cables occurs, and in providing protection for overvoltage conditions, they typically will not be sufficient for lightning standards. Systems are available that predict when a lightning event will occur and will switch to standby or otherwise prepare for the event in advance, but this interruption may not be acceptable. A reliable level of continuous protection is preferable.
Five standards are typically followed to determine the level of protection a design requires and to which it is tested based on the application. Lightning protection solutions suitable to meet these standards for interface devices at speeds up to 3 GHz consist of pairing gas discharge tubes (GDT) with ECL protectors. Until recently, the overall performance of the signal interface was inherently degraded due to the load that the protection devices introduced to the circuit.
An Advanced Approach
Technology advancements in the area of circuit protection devices now make it possible to provide robust overstress protection without major impact to the performance of the interface. There are two combinations of interest that cover all five standards for lightning protection. The resistance of the solution is 10 ohms with 1 pF capacitance for GR-1089 Intra- B, IEC61000-4-5 Class 0-3 applications. To meet GR-1089 Intra-B Enhanced, IEC61000-4-5 Class 4-5, ITU-T K.21, a solution with 14 ohms and 1 pF capacitance is used. Each combination consists of a GDT and an ECL.
An ECL is a compact series device that introduces virtually no capacitance or inductance and that does not consume power in standby mode. It protects against overvoltage and overcurrent conditions, reacting in hundreds of nanoseconds. Benefits of this barrier for sensitive equipment include reset capabilities after a fault, high bandwidth, low installed cost, and a longer design lifetime since it does not bring energy into the printed circuit board on which it is installed. In stark contrast to the typical solution, which generally has an insertion loss that would limit the effective rate-reaching performance of the transmitted signal, the capacitance of the line is not an issue with this solution. And, the ECL style of protection has a 3-dB point of approximately 3 GHz, which makes it ideal for the high-speed data interface typical of LVDS.
An ECL is transient in nature, reacting within a small fraction of a microsecond to protect loads from exposure to very fast surges. The ECL provides protection by blocking the surge from affecting the circuit, thereby preventing any damage within the rated limits of the device. At low currents, the ECL has a resistive characteristic. Once above a set blocking current, it switches to high impedance and effectively creates an open circuit. For example, if a short-circuit condition occurs, current limiting comes into effect once the trigger current is reached. The voltage limiting circuit disconnects voltage and current, and the circuit is protected. In response to a 1.2 μs rise time waveform, the ECL protects within 100 ns. A total let-through of 100 nJ over approximately 800 ns can be expected. The result is well-protected equipment because the ECL and the equipment are not exposed to harmful damage levels.
This combination of GDTs and ECLs can be used in high-speed designs without introducing capacitance to the line, so it does not significantly affect performance. Engineers at Bourns have conducted tests on this combination solution to evaluate the signal integrity for protection against high-speed lightning situations. The solution was installed in series with devices typical in high-speed, point-to-point applications, including a cable driver and equalizer combination, and a high-speed LVDS buffer and repeater device. The test board featured high-bandwidth SMA connectors, controlled 50-ohm impedance microstrips, and inner layer cutouts under the protection devices for minimal parasitic capacitance. Signal path impedance was also controlled so that the filtered impedance profile of the test board remained within a 40- to 60-ohm range.
Per the electrical description of the solution, it was expected that the impact on the signal integrity of the link would be minimal when installed in line within the circuit. A timedomain reflectometer was used to verify that the impedance impact to the link was minimal. Frequency domain analysis showed minimal degradation or impact. Based on the insertion loss (shown in Figure 1), the -3 dB bandwidth of the GDT and ECL board is approximately 3 GHz. Measured from the GDT side (shown in Figure 2), the return loss remains below -15 dB at frequencies up to 1.5 GHz. From the ECL side (shown in Figure 3), this limit is maintained up to 2 GHz.
These characteristics qualify the performance of the solution as optimal for bit rates up to 2 Gbps and acceptable up to 3 Gbps. Two applications were tested to obtain eye patterns to show the time domain analysis and overall jitter with and without the solution. Figure 4 shows the interface without the solution, and Figure 5 shows the same interface with the solution added in line. Notice the minimal jitter the solution adds to the link. At 3.125 GHz, the introduced jitter was approximately 15-20 ps, and the applications operated within their specifications in both cases.
When longer cables are used, the series resistance of the solution may impact the performance. For instance, when an adaptive equalizer is used with a 75-100 meter cable, the resistance at the transmit side of the link can be reduced to ensure signal quality remains acceptable. Lower series resistance further reduces these minor impacts. Time domain test results confirm that these frequency domain measurements are valid. The solution was shown to have little impact and allow reliable performance at bit rates up to 3 Gbps.
Maintaining RF System Integrity
When enhanced and extended circuit protection is needed to protect applications from severe electrical overstress such as lightning and other serious surge threats, the combination of GDT and ECL technologies offers highly effective protection with minimal impact to system performance. This combined solution allows the rapid clock transitions of high-frequency LVDS signals up to 3 GHz to operate reliably within specification. It also protects against severe lightning threats and other open or short conditions that can cause overvoltage and overcurrent conditions.
When used in RF applications, the integrity of the system is maintained and the system’s lifetime is enhanced by not introducing additional energy onto the circuit board. Added benefits of this combined circuit protection method include packages that feature a small physical footprint and long product life.
This article was written by Ian Doyle, Product Line Manager, Semiconductor Products, at Bourns, Inc., Riverside, CA. For more information, Click Here
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