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Designing with PIN Diodes: Discrete, HMIC or AlGaAs?   Jul. 10, 2018

discrete small blog.jpg (903548438)At first glance, implementing a solid-state RF or microwave switch does not seem to be a daunting task: identify and evaluate an appropriate integrated circuit (IC) switch and if none exists, select PIN diodes to include in a discrete, custom design. Millimeterwave RF switches seem to be a bit more challenging, due to the high frequency of operation.

As one investigates the products that are available for this task, their diversity can be overwhelming. Silicon discrete PIN diodes? MACOM’s silicon Heterolithic Microwave Integrated Circuit (HMICTM) PIN diode integrated circuits? Gallium arsenide (GaAs) discrete PIN diodes? Aluminum gallium arsenide (AlGaAs) ICs? To compound matters, there are several viable circuit topologies, such as series-diodes-only, series-shunt diodes, shunt-diodes-only and more. How does a designer determine how to proceed?

 

 

Shunt PIN Diode SPDT

High Isolation Generic PIN SPDT

 
 
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macom product.pngThe Discrete Design vs. Integrated Circuits

It is generally easier to implement an integrated-circuit switch into a system than to design a discrete switch circuit. The IC switch designer is a specialist who chooses the optimal diode characteristics and switch topology to meet the switch’s required performance specifications. The IC switch user need only select the best IC and then design the appropriate control-signal bias decoupling networks. Some products, including MACOM’s IC switches, even include the bias decoupling networks.

Discrete diode switch designs can be optimized for specialized performance requirements for which an IC switch may not exist, such as insertion loss, isolation, power handling and switching time.

 

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The HMICTM Switch Process

MACOM’s HMIC technology joins glass and silicon into a single monolithic structure. The HMIC process uses automated batch process fabrication and testing technologies in order to produce diode structures, resistive and reactive lumped elements, all of which can be produced with small size and low loss for high performance microwave integrative circuits. Applied at the wafer level, this aims to substantially reduce the size, cost and performance limitations of the device, while also significantly improve the repeatability of conventional diode, active, passive, hybrid and chip-and-wire microwave circuits.

Due to characteristics of the HMIC wafer process, all of the PIN diodes in a HMIC switch design must have the same I layer thickness. HMIC switches can only include silicon PIN diodes, for which the upper frequency bound of operation is approximately 30 GHz.

 

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The AlGaAs Switch Process

MACOM’s proprietary AlGaAs technology offers notable benefits, including ultra-broad bandwidth capability in the millimeter wave bands, low insertion loss, low bias current consumption, compact die sizes, BCB scratch protection and the availability of integrated bias networks.

In MACOM’s proprietary AlGaAs PIN diode technology, the anode layer of the diode is doped with a carefully-controlled concentration of aluminum. This can produce a larger band gap at the interface of the P and the I layers, which ultimately can produce a PIN diode with lower series resistance than an otherwise-identical GaAs PIN diode structure (see: Designing with Diodes: Why Choose AlGaAs?). The expected result is improved performance at millimeter wave frequencies up to and higher than 100 GHz.

 

Making the Best Decision

The approach a designer chooses to follow to implement a PIN diode switch function is influenced by many factors: the frequency and bandwidth of operation, power handling requirements, switching time requirements, maximum acceptable insertion loss, minimum required isolation and, not least, the circuit designer’s level of knowledge and experience with switch design. MACOM offers HMIC integrated circuit switches from single pole two throw to single pole four throw, AlGaAs integrated switches from single pole single throw to single pole eight throw and among the industry’s widest variety of discrete silicon, GaAs and AlGaAs PIN diodes.

RF Matters here at MACOM, and our industry leading applications engineering team is ready to help you succeed!


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Trending at IMS 2018: 5G and the Importance of RF   Jun. 11, 2018

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Why Choose RF Energy for Automotive Ignition?   May. 09, 2018

automotive ignition.jpgPerhaps one of the most lucrative commercial applications available to solid-state radiofrequency (RF) energy is in the automotive industry, known for its dedication advancing vehicle technology with every model. Traditionally enabled by spark plugs, vehicles can now benefit from RF solid-state plasma technology, which offers improved fuel efficiency and reduced carbon footprint.

What is RF Energy?

The concept for RF energy powered ignitions dates back to 1992, following the 27th Intersociety Energy Conversion Engineering Conference in San Diego, California, where the idea of lean, precise automotive ignition without the use of the traditional spark plug was first introduced. Automakers grappling with increasingly stringent vehicle emissions protocol, allowing for only a small carbon footprint and facing potential fines for every car which omits over protocol, were in need of a seamless solution to improve efficiency without completely redesigning the engine. Solid-state RF energy presented an opportunity to offer an unprecedented level of control and efficiency to auto ignitions, thereby reducing waste and cost, with equivalent footprint. At the time, further analysis was required in order to control the physical volume of plasma created by the RF discharge.

Today, due to recent advancements enabled by organizations like the RF Energy Alliance, innovative new technologies such as MACOM’s GaN-on-Si displacing incumbent LDMOS and collaborations like STMicroelectronics and MACOM, RF energy is now poised to achieve the optimal performance, scale and cost structure to enable this formerly “futuristic” application. Fuel efficiency improvement has become a choice not only for car manufacturers, but for consumers to demand as standard.

Spark Plugs vs. RF Plasma Ignition

The concept of spark plugs are simple: deliver an electric current to a vehicle’s combustion chamber, emit a one-time electric spark—similar to that of a lightning bolt—to ignite the compressed fuel and air mixture, which then moves the pistons of the engine and voila. Due to the erratic, natural behavior of the initial spark, the technology is inconsistent in terms of duration per start and amount of gas expended per start. Generating a high voltage spark in combination with the timing it takes to start the “cranking” of an engine, any gasoline which lies in the ignition chamber is burned, even if gas levels are low or diluted. Though fractional in seconds, the use of unnecessary gas combinations accumulates. Despite these inefficiencies, many car manufacturers have stuck with spark plugs due to the familiar technology and low cost structure (previously unavailable with an RF plasma solution).

RF plasma, on the other hand, offers a constant array of energy. The constant energy source is beneficial to a vehicle, bringing higher combustion ratios, the ability to control and manipulate the plasma, and operate flawlessly under extreme pressure time and time again—whereas traditional spark plugs can only work to a certain pressure constraint, resulting in fuel mixtures which are sub-optimal.

By leveraging RF energy in lieu of a spark plug, fuel in the combustion chamber can be ignited much more evenly than with conventional ignition systems. The precision control afforded by RF plasma-powered ignitions can largely reduce exhaust emissions and save on fuel by using a plasma to accurately optimize the fuel combustion process. Aggregated globally, a 10% improvement in fuel efficiency for combustion-powered vehicles would represent a significant leap forward in carbon emissions control. 

Looking Forward

RF plasma ignitions not only have the potential to better serve the automotive industry, but can optimize performance in motor bikes, diesel trucks, large industrial engines and more. MACOM has recently started to develop an integrated circuit, the Analog Lock Loop, which is expected to provide a cost effective control loop for the solid state amplifier to deliver energy efficiently to the RF plasma ignition under the controlled environment. Following MACOM’s recently announced collaboration with STMicroelectronics to commercialize GaN-on-Si for RF markets, a plasma ignition could be in a low-end car market sooner than you think!


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5G Wireless Fronthaul: Achieving the Benefits of C-RAN and CPRI with 100G Optical Connectivity   Apr. 24, 2018

5g wireless fronthaul.jpg (544581870)In a previous blog post we assessed the evolution of 5G wireless standards and industry expectations for the incremental deployment of 5G infrastructure in the years ahead. Continued forward progress on these fronts brings us ever closer toward a future of breakthrough bandwidth capacity and data delivery speeds. There will of course be daunting technology challenges on the pathway to 5G, but these challenges present tremendous opportunities for wireless operators to evolve our wireless infrastructure in a scalable and sustainable manner that enables continuous improvements in bandwidth, power, management and cost efficiencies.

Wireless fronthaul is a prime example of this dynamic, inviting a fresh look at the underlying network architecture extending from remote radio units (RRUs) to baseband units (BBUs). Taking a comprehensive view of wireless fronthaul trends and standards, we see a clear opportunity to combine parallel advancements in RF and optical technologies to realize a more elegant and cost effective fronthaul architecture that’s optimized to accommodate the huge increase in 5G data throughput.

CENTRALIZED NETWORK ARCHITECTURE

At the RF antenna layer, advanced phased array technologies are enabling massive MIMO configurations that multiply the capacity of antenna links leveraging anywhere from 16 to hundreds of densely clustered antennas. Meanwhile, each individual antenna is targeted to serve more and more bandwidth, from 60 MHz today to as high as 200 MHz for the 3.5 GHz frequency band, and far higher – as high as 800 MHz – for mmWave frequencies.

The data throughput enabled by this advanced RF architecture is immense, but introduces significant dilemmas about how best to partition the attendant signal processing burden between the RRU and BBU to maximize overall data throughput. If the baseband processing is integrated into the RRU via an “All-in-One BTS” approach, then RRU power consumption will rise dramatically, as will maintenance and upgrade costs. This approach runs counter to the inherent benefits of the Centralized/Cloud Radio Access Network (C-RAN) architecture, whereby the RRU design is greatly simplified and the processing intelligence instead resides at a centrally managed, maintained, and cooled BBU servicing a multitude of RRUs over long distances.

C-RAN leverages the natural efficiencies of virtualized, general purpose server infrastructure, with highly flexible, software defined programmability. With C-RAN, it’s also much easier to administer and implement algorithms to mitigate inter-cell interference and improve spectral efficiency.

CPRI vs eCPRI

Though C-RAN promises many benefits for 5G wireless fronthaul, it’s not without its challenges. Chief among them, the long distance functional split between BBU processing and RU requires an ultra high speed optical interconnect to minimize latency. The Common Public Radio Interface (CPRI) protocol, established in 2003, is commonly employed for this purpose today over distances of tens of kilometers, but has been bandwidth constrained in the absence of low cost, high speed optical connectivity solutions.

Published in 2017, the CPRI Over Ethernet (eCPRI) specification aims to address this issue in part by moving a portion of the processing workload from the BBU over to the RU, where the dataflow can be pre-processed in a manner tailored for the requirements of specific 5G uses cases including fixed wireless, massive broadband, and ultra low latency applications. But here again, this approach runs counter to the core value proposition of C-RAN and the centralization of processing workloads and network administration at the BBU.

5G network split options under consideration (image courtesy of eCPRI Overview/IEEE)

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As an alternative approach to eCPRI, data compression techniques are now being developed for implementation at the RRU, targeted to accommodate the extreme data throughput introduced with massive MIMO while ultimately reducing fronthaul bandwidth requirements to a level commensurate with today’s LTE MIMO architectures – a CPRI data rate that’s serviceable with 100G optical interconnects. By compressing the dataflow from the RRU to the BBU, the combined benefits of CPRI and C-RAN can be readily achieved, enabling BBU-level consolidation of baseband processing and management operations.

MACOM’s innovative 100G single-wavelength PAM4 DSP and advanced silicon photonics technology deliver a high-performance, cost-effective 100G optical solution for C-RAN networks. Where previously the CPRI protocol enabled streamlined C-RAN architectures but fell short in data delivery speeds, the mainstreaming of 100G optical connectivity has bridged this gap, thanks also to the volume scale deployment of 100G connectivity in cloud datacenters, and the resulting reduction in 100G cost structures. Wireless operators are now well positioned to exploit the speed and cost efficiencies of 100G interconnects for wireless fronthaul in preparation for the 5G data deluge, while enabling a flexible C-RAN architecture designed for seamless scalability into the future.

MACOM’s leadership across the RF and optical networking domains gives us unique visibility into the technologies, trends and standards driving the development and deployment of 5G connectivity. To learn more about MACOM’s 5G solution portfolio, visit https://www.macom.com/applications/wireless-networking-and-communic/5g


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