CATV operators are rapidly expanding their networks and capabilities as the government encourages triple play services. At the same time, users continue to place more and more demands on service providers—high-speed Internet, high-definition video, clear telephone voice service.
Hybrid Fiber Coax (HFC) based triple-play services require reliable components, both optical and Electronic. The reliability of fiber optic networks, excellent quality of service, has received widespread attention and praise for significant improvements in performance, and as a result we certainly believe that the reliability of electronic components is equally important.
Achieve high service quality and reliability
Many electronic components in use today were designed and developed to simulate CATV systems. These early electronic components were optimized to provide a superior analog TV signal and are still the foundation of modern HFC systems. In fact, many of these components are still in use today. Originally used only for CATV systems, these same HFC networks are now being expanded to provide enhanced triple play services with significantly improved transmission distances. This dramatic expansion in service and coverage has increased the complexity of these systems, placing ever more stringent quality and reliability requirements on electronic components.
One of the main electronic components in a triple play network is the system amplifier. These amplifiers are used to boost the signal going downstream throughout the network (Figure 1). Effectively boosting the signal requires a delicate balance – increasing power levels for long range coverage, reducing signal distortion to maintain picture quality, and reducing power consumption to save energy and reduce operating costs. Without the cooperation of these electronic components, the triple play service cannot be successful.
Figure 1: CATV system schematic illustrating the application of the system amplifier
A system amplifier consists of many electronic components, one of the most important of which is the line amplifier, which provides the final amplification stage and the vast majority of signal boosting. The line amplifier consumes a lot of power at the same time, and if it cannot be solved properly, it will have an impact on the reliability of the triple play network. The line amplifier largely determines the quality and reliability of the network due to its important role in providing the final amplification.
Design Considerations for Line Amplifiers
A line amplifier with high design quality and reliable performance needs to meet certain criteria. Component technology and manufacturing processes for developing line amplifiers are as important as design skills in ensuring quality and reliability. In the beginning, many semiconductor companies used discrete designs for their line amplifiers. These designs can provide suitable performance, but are inconsistent and difficult to mass produce. Later, as technology, design, and manufacturing techniques continued to improve, line amplifiers were designed as monolithic microwave integrated circuits (MMICs). The new MMIC design (Figure 2) can improve consistency, manufacturing process, and reliability, while improving performance through technological advancements. These fundamental features of MMIC designs can improve line amplifiers to meet the ever-increasing performance requirements and quality standards of systems without the need for fine-tuning of discrete designs. Since 1996, ANADIGICS has pioneered RF line amplifier monolithic microwave integrated circuit (MMIC) designs utilizing GaAs MESFET technology.
These line amplifiers have been the industry standard for the last 15 years. Because line amplifiers can produce the high output power needed to transmit signals over long distances, they lose 8 to 10 watts of power. This requires large heat sinks to ensure performance and reliability, which necessitates tuning of the modules. In addition, line amplifier MMICs are not field repairable, and in the event of a failure, the entire system amplifier must be replaced.
Figure 2: Line Amplifier MMIC in a System Amplifier
The Hybrid Line Amplifier module was developed to provide conventional mechanical dimensions and to address thermal management issues that could cause the amplifier to overheat. Another major advantage of hybrid line amplifier modules is that they are field replaceable, so these system amplifiers can be easily repaired in the field. Figure 3 compares the size and packaging of hybrid modules and MMIC line amplifiers.
Figure 3: RF Line Amplifier Hybrid Module and MMIC
For these reasons, hybrid line amplifier modules are now very common in HFC network system amplifiers, and there are several manufacturers around the world producing such amplifier modules using both discrete components and MMICs. Hybrid designs utilize discrete components to customize performance characteristics such as Composite Triple Beat (CTB), Composite Quadratic Beat (CSO), Cross Modulation (XOMD), and Noise Figure Distortion, while providing design flexibility. However, using discrete components can lead to performance differences due to production differences. With discrete line amplifiers, network reliability is more difficult to guarantee because the reliability of individual components is not tested before system amplifier production. Line amplifier ESD protection and robustness are extremely important to network reliability, and if the system amplifier is designed with MMIC line amplifiers, components can be tested in advance. Table 1 compares the design advantages of MMIC line amplifiers versus discrete line amplifiers.
Table 1: Discrete Line Amplifiers vs. MMIC Line Amplifiers
Various techniques can improve the ESD protection and overall durability of discrete component line amplifiers. These techniques may sacrifice system performance, resulting in lower gain, poorer input and output return loss, and gain roll-off at lower frequencies. Figure 4 shows the elements of a hybrid module that affect system performance and reliability. Input and output ESD circuitry protects the amplifier without degrading performance, and the line amplifier can be either discrete or MMIC. Figure 5 shows that the lack of proper ESD circuitry in a hybrid-line amplifier can result in degraded electrical performance, reduced bandwidth, or reduced system speed.
However, hybrid line amplifier modules using MMICs can improve ESD protection without sacrificing performance, as long as the design incorporates optimized input ESD protection. See Figure 5 for details. The MMIC has built-in ESD protection on both the input and output, and is fully tested before assembly. Additional ESD protection comes from within the hybrid. The key is that the MMIC-based hybrid module optimizes design and performance, utilizing a known-good MMIC for dual ESD protection for improved reliability. Another major advantage of MMIC-based hybrid line amplifiers is better uniformity than discrete components. MMICs also simplify and improve the module manufacturing process, increasing yield and reducing component count, thereby reducing complexity.
Figure 5: Bandwidth of Hybrid Line Amplifier Module with and without ESD Protection
Line Amplifier Technology
The semiconductor component technology used in the design of line amplifiers evolves together with the design methodology. Silicon components can provide reliable performance for early short-range analog systems operating at speeds up to 770 MHz. The network operating speed has initially been increased to 870MHz and is now up to 1 GHz to provide additional bandwidth for more TV channels and triple play services. This requires the use of gallium arsenide semiconductor components to increase power and improve signal quality over a wider frequency range. The quality of the signal depends on the amplifier distortion characteristics, and the performance of gallium arsenide is much higher than that of silicon (Si). Therefore, GaAs MMIC line amplifiers are widely used in HFC system amplifiers and have become one of the most important component technologies for many years.
GaAs MMIC line amplifiers have historically been known for their superior performance and reliability, and now we can deliver similar performance with line amplifier hybrid modules for successful triple-play networks. In addition, proven technologies coupled with new hybrid-line amplifier design and manufacturing processes contribute to improved performance and reliability.
MMIC’s proven GaAs technology ensures that line amplifiers can deliver output power, gain and linearity without sacrificing reliability. While RF line amplifiers also use a variety of other technologies, only GaAs MESFETs have been tested and proven as a robust and reliable process technology over the past 25 years. One of the key indicators of the reliability of this technology is the mean time between failures (MTTF). Even at an extreme temperature of 125 degrees Celsius (Figure 6), the mean time between failure of GaAs MESFET technology is still around 106 hours. This standard is actually better than 100 years of continuous operation, ensuring that performance does not degrade or fail prematurely under normal operating conditions. Predicted MTTF for GaAs MESFET Hybrid Line Amplifiers
Figure 6: GaAs MESFET technology provides exceptional mean time to failure (MTTF)
This shows that GaAs MESFET line amplifiers are extremely stable, reliable, and reduce maintenance and operating costs even when operating at 85 degrees Celsius. GaAs MESFETs are the best choice for performance, cost and reliability compared to other IC technologies. The performance of silicon technology is not high enough, and other GaAs technologies such as HFET or PHEMT cannot provide the same level of linearity and cost or reliability of MESFET technology. These MTTF specifications can be verified with field data showing a direct link to durability and reliability.
Line Amplifier Test
Selecting the right IC technology and adopting the best design approach, while necessary, is not a sufficient criterion to ensure overall system performance and reliability. The testing methods used to develop and manufacture these components are key to ensuring that the technology and design actually benefit the triple play system. First, the key parameters of the line amplifier that affect system performance must be investigated. The performance of the line amplifier is related to the full load spectrum, including the aforementioned CTB, CSO, XMOD, and noise figure distortion. These standard performance measurement units are defined in Table 2, and the required level depends on the system design. The triple play system can meet the extreme electrical performance demands placed on the line amplifier by the system due to the number of TV channels, video on demand and other data services. The additional data transmission requires better linearity and distortion in the system and line amplifiers to ensure that the analog TV channels maintain excellent quality.
Main performance specifications
Table 2: Key performance specifications including CTB, CSO, XMOD and noise figure
Second, reliability testing will help ensure uninterrupted operation. There are several key tests to ensure a product achieves proper reliability, including ESD, durability, ring wave and stress testing. These tests introduce electrostatic discharges, voltage spikes, and sustained high currents to simulate harsh environmental and mechanical conditions such as power surges, lightning strikes, and load switching. These tests are difficult to perform and often permanently damage amplifiers, whether MMICs or hybrid modules, so special techniques have been developed to ensure the durability of these components without damaging them. Most of these key tests and techniques can be performed on the MMIC before it is assembled into the hybrid module. This ensures that hybrid modules can achieve higher reliability, which is not possible with discrete components. It is for this critical factor that we recommend choosing a hybrid line amplifier with proven MMIC design and technology.
Proven track record
While testing for durability and performance is important, many parameters related to reliability are difficult to relate to specific test results. Long-term, sustained results must be ensured through additional manufacturing processes and design techniques. These processes and techniques have been continuously developed and refined over many years and have been used to achieve the results demonstrated by existing HFC networks.
One of the measures of future line amplifier performance is to demonstrate the ability to develop and deliver stable products. For example, ANADIGICS has shipped more than 26 million RF line amplifiers over the past 15 years with a field failure rate of less than 2 ppm (Figure 7). To achieve this excellent business record, ANADIGICS utilizes a variety of technologies, designs, manufacturing processes, and testing best practices described in the sections above.
Cumulative Line Amplifier Shipments
Figure 7: Cumulative Line Amplifier Shipments vs. RMA, Demonstrating the Reliability and Durability of ANADICS Line Amplifiers
We believe that one of the keys to ensuring the performance and reliability of a triple-play network is selecting the right suppliers that can develop their own line amplifier MMICs using GaAs MESFET process technology. We recommend that you ask potential suppliers for details on the components of a hybrid line amplifier to see if they are fully tested prior to assembly and if both the hybrid section and the MMIC have built-in ESD protection without sacrificing performance.
what does that mean?
As system operators continue to refine their triple-play services and expand the reach of their networks, it is important to focus on the components that will enable them to succeed. RF line amplifiers are undoubtedly an important element in a triple play infrastructure, so it is imperative to select and specify line amplifiers that provide reliable performance. Therefore, we recommend considering the technology and design of the line amplifier, determine if you prefer MMIC or hybrid, choose a hybrid line amplifier based on proven MMIC technology, and insist on checking shipment records as well as RMA data to determine if it has a Good business record. We believe these best practices below help ensure reliable performance in triple play networks.
For more information on ANADIGICS line amplifier products, please contact: [email protected]
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