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Designing with Diodes: Protecting Sensitive Components   Jan. 22, 2019

receiver protect limiter blog.jpg (!blank)Sensitive low noise amplifiers (LNAs) in radar or radio receivers cannot tolerate large input signals without sustaining damage. What’s the solution? Receiver-protector limiter (RPL) circuits, the “heart” of which typically comprises PIN diodes, can be utilized to protect sensitive components from large input signals without adversely affecting small-signal operation.

RPL circuits do not require external control signals. These circuits comprise at least one PIN diode connected in shunt with the signal path, along with one or more passive components, such as RF choke inductors and DC-blocking capacitors. A simple (but possibly complete) RPL circuit is shown below.

diagram 1.png

When there is no RF input signal or when only a small RF input signal is present, the impedance of the limiter PIN diode is at its maximum value, the magnitude of which is typically in the few-hundreds of ohms or greater. Consequently, the diode produces a very small impedance mismatch and correspondingly low insertion loss.

When a large input signal is present, the RF voltage forces charge carriers into the PIN diode’s I layer, holes from its P layer and electrons from its N layer. The population of free charge carriers introduced into the I layer lowers its RF resistance, which produces an impedance mismatch as seen from the RPL circuit’s RF ports.

This mismatch causes energy from the input signal to be reflected to its source. The reflected signal, in concert with the incident signal, produces a standing wave with a voltage minimum at the PIN diode since it temporarily presents the lowest impedance along the transmission line. There is a current maximum collocated with every voltage minimum along the transmission line. This current flows through the PIN diode, enhancing the population of free charge carriers in the diode’s I layer, which results in lower series resistance, a greater impedance mismatch and a “deeper” voltage minimum. Eventually the diode’s resistance will reach its minimum value, which is determined by the design of the PIN diode and the magnitude of the RF signal. Increases in the RF signal amplitude force the diode into heavier conduction, thus further reducing the diode’s resistance until the diode is saturated and produces its lowest possible resistance. This results in an output power vs. input power curve as shown below.

graph 2.PNG

After the large RF signal is no longer present, the diode’s resistance remains low (and its insertion loss remains large) if the population of free charge carriers in the I layer is large. Upon cessation of the large RF signal, the population of free charge carriers will decrease by two mechanisms: conduction out of the I layer and recombination within the I layer. The magnitude of the conduction is determined primarily by the DC resistance in the current path external to the diode.

The rate of recombination is determined by several factors, including the free-charge-carrier density in the I layer, the concentration of dopant atoms and other charge-trapping sites in the I layer, etc. Due to the required parameters of the diodes, the greater the RF signal which a PIN diode can safely handle, the longer its recovery time to low insertion loss will be.

The properties of the I layer of the PIN diode determine how this circuit performs. The I layer’s thickness (sometimes referred to as its width) determines the input power at which the diode goes into limiting – the thicker the I layer, the higher the input-referred 1 dB compression level (also known as the threshold level). The thickness of the I layer, the area of the diode’s junction and the material of which the diode is made determine the resistance of the diode as well as its capacitance. These parameters also determine the diode’s thermal resistance.

The simplest implementation of a PIN RPL circuit comprises a PIN diode, an RF choke inductor and a pair of DC blocking capacitors. The RF choke inductor is critical to the performance of the RPL circuit, with the primary function to complete the DC current path for the PIN diode. When a large signal forces charge carriers into the diode’s I layer, a DC current is established in the diode. If a compete path for this DC current is not provided, the diode’s resistance cannot be reduced, and no limiting can occur. This current will flow in the same direction as a rectified current would flow, but it is not produced by rectification.

Implementation of the choke inductor in the RPL circuit can be challenging, since inductors are the least ideal of the components in the RPL circuit. Inductors all have series and parallel resonances due to their inductance and their parasitic inter-winding capacitance. Care must be taken to ensure that series resonances do not occur within the operating frequency band. Additionally, the choke’s DC resistance must be minimized in order to reduce the recovery time of the RPL circuit.

Note: the DC blocking capacitors are optional. They are only necessary if there are DC voltages or currents present on the input or output transmission lines which might bias the PIN diode.

A Practical Example

Assume the maximum input power which an LNA can tolerate is 15 dBm. This power level sets the requirement for the I layer thickness of the PIN diode in the RPL circuit, which in this case is approximately 2 microns. A designer can then determine the acceptable capacitance of the PIN diode from the frequency of the RF signal and the maximum acceptable small-signal insertion loss. If they assume the RPL operates in X Band and the maximum acceptable insertion loss is 0.5 dB, then the maximum capacitance of the diode can be calculated.

The insertion loss (IL) in decibels of a shunt capacitance is given by:

equation 1.PNG

We can solve that equation for C:

equation 2.PNG

For f = 12 GHz, IL = 0.5 dB and Z0 = 50 Ω, C = 0.185 pF.

Along with the I layer thickness, this value of capacitance will determine the area of the diode’s junction.

The combination of thin I layer and small junction area creates a diode which has relatively high thermal resistance, which cannot dissipate very much power without forcing the junction temperature to exceed its maximum rated value of 175 °C. Typically, a 2 micron diode with 0.185 pF capacitance can safely handle a large CW input signal of around 30 to 33 dBm. A larger signal can potentially damage or immediately destroy this diode due to the Joule heating produced by the current flowing through the diode’s electrical resistance.

PIN diode RPL circuits reliably protect sensitive components like low noise amplifiers in radar or radio receivers from large incident signals. For RPL applications which require very low flat leakage output power but high input power handling, additional diode stages and other circuit enhancements can be added at the input side of the RPL circuit.

Members of MACOM’s applications engineering team are ready to help you select the optimal diodes and circuit topologies for your RPL application. For more information on MACOM’s solutions, visit: https://www.macom.com/diodes


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What’s the Role of 200G Optical Connectivity on the Pathway to 400G?   Dec. 05, 2018

200g future blog.jpg (Datacenter)The ever-burgeoning bandwidth demands on Cloud Data Center infrastructure are intensifying the pressure on optical module providers to enable faster connectivity solutions at volume scales and cost structures. This is fueling tremendous uptake for 100G CWDM4 (4 x 25G) modules and accelerating the ramp to 100G single lambda (PAM-4) modules on the pathway to mainstream adoption of 400G (4 x 100G).

Technology vendors from across the optical networking industry are working hard to drive this progress, leveraging interoperability plugfests among other opportunities to ensure seamless compatibility among a growing ecosystem of components, modules, and switch systems. This activity reflects the urgent need for faster Data Center links, and also underscores the extreme effort and design precision required to achieve coherence among the heterogeneous products coming to market.

With 100G in widescale deployment today and the promise of mainstream 400G deployment seemingly ubiquitous, Cloud Data Centers are eager to take advantage of any and every opportunity to bridge the throughput gap and keep pace with the data deluge. 200G (4 x 50G) optical modules answer this immediate need head on.

ANALOG ADVANTAGES

200G modules provide several key benefits, chief among them the flexibility to leverage a fully analog architecture, the merits of which we assessed in an earlier blog post focused on optical modules for high performance computing (HPC) applications. Though somewhat more difficult to implement than mainstream digital signal processor (DSP) based solutions, fully analog optical interconnects can provide 1,000X lower latency than DSP-based solutions – a crucial attribute for enabling system and network performance at the fastest possible speeds. And while DSPs will remain essential for designing 100G single lambda and 400G modules, DSPs aren’t necessary for 200G module enablement today.

In the absence of DSPs, fully analog 200G optical modules consume much less power and dissipate considerably less heat. Leveraging existing optical components, it’s now possible to enable module-level total power consumption under 22 milliwatts per gigabit. This translates to a 200G optical module for 2km applications with power consumption as low as under 4 watts. A DSP-based module would likely clock in at 2 to 3 watts higher, which doesn’t sound like very much, until you aggregate the resulting power consumption penalty across a Data Center hosting thousands of optical modules. In this context, a 2 to 3 watt power savings per module is hugely advantageous for optimizing OPEX and cooling efficiency.

Low latency and power consumption are important attributes, but not the only performance metrics that matter. Signal integrity is another critical performance criterion given the cascading consequences of transmitting bit errors into the data stream. This poses a particularly daunting challenge as data throughput speeds increase from 100G to 200G and beyond.

The ability to maintain optimal signal integrity performance at 200G in the absence of a DSP is due, in large part, to continued advancements in clock data recovery (CDR) devices and the underlying signal conditioning technology. The newest generation of analog CDRs deployed in fully analog 200G modules have demonstrated the ability to enable a low bit error rate (BER) and better than 1E-8 pre-forward error correction (Pre-FEC), on par with DSP-based 200G modules.

HIGH VALUE, HIGH VOLUME

None of the aforementioned advantages of a fully analog 200G optical module would be worthwhile if the cost structures weren’t approaching comparable alignment with mainstream commercial solutions. But here again, the fully analog 200G module architecture wins against DSP-based 200G modules.

At the device level, the streamlined design of a fully analog 200G module reduces overall component count and sidesteps the costs of DSP development and implementation. At the broader market level, while 100G technology is already mature and component integration is well established, 200G end-to-end interoperable chipsets have just recently hit the market. Looking to the past as our guide, in the short term, 200G modules are expected to emulate cost structures akin to 100G modules when they entered the market a few years ago, and follow a similar downward cost curve as component integration is further standardized and volume shipments accelerate. In due course, 200G modules are expected to achieve a cost structure that’s comparable to today’s 100G modules.

As an intermediate step between 100G and 400G, 200G optical connectivity is a compelling solution for Cloud Data Centers challenged to implement faster optical links at scalable volumes and costs. DSPs will undoubtedly play a pivotal role on the path to 400G, and in the interim, the fully analog 200G module architecture lights the path to faster, cost effective connectivity beyond 100G.

MACOM is committed to leading the evolution of Cloud Data Center interconnects from 100G to 200G and 400G, and at ECOC 2018 we demonstrated a complete, fully analog 200G chipset and TOSA/ROSA subassembly solution that affords optical module providers seamless component interoperability to reduce design complexity and costs. To learn more about MACOM’s optical connectivity solutions for Cloud Data Center infrastructure, visit https://www.macom.com/data-center


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The Health and Economical Benefits of Solid-State Cooking   Oct. 23, 2018

Featured Image (Courtesy of the RFE Alliance):

RFE Diagram.PNGThe ability to generate and amplify RF signals is nothing new – but solid-state RF energy has enormous potential beyond data transmission applications. As companies like MACOM and collaborative organizations such as the RF Energy Alliance (RFEA) continue to pioneer and develop this technology, enabling greater efficiency and control than previously possible with conventional technologies, the full potential of this technology for mass-market applications is beginning to take form.

Microwave cooking is one application that is already being radically transformed with solid-state RF energy, enabling healthier eating and broad economical benefits. Solid-state RF energy transistors generate hyper-accurate, controlled energy fields that are extremely responsive to the controller, resulting in optimal and precise use and distribution of RF energy. This offers benefits unavailable via alternate solutions, including lower-voltage drive, high efficiency, semiconductor-type reliability, a smaller form factor and a solid-state electronics footprint. Perhaps the most compelling benefit is the power-agility and hyper-precision enabled by this technology, yielding even energy distribution, unprecedented process control range and fast adaption to changing load conditions, not to mention a lifespan of more than 10 years.

Enabling Healthier Eating

Precise temperature control is essential for maintaining proper nutrients of food during the heating/cooking process. Microwave ovens leveraging solid-state power amplifiers enable precision and control of directed energy, which helps preserve the nutritional integrity of food, and prevent cold spots that negatively impact the dining experience.

Since today’s magnetron-based microwave ovens aren’t equipped to adapt to energy being absorbed by or reflected from the food as it cooks, they rely on open-loop, average heating assisted by the rotating turntable at the base of the cavity. This imprecise delivery of energy often results in over-cooking and hot spots that can lower the food’s nutritional value.

By using multiple solid-state power amplifiers and antennas with closed-loop feedback to adjust for precise energy absorption, the energy can be directed with greater precision to exactly where it’s needed and in a controlled way that ensures optimal temperature control. Rather than relying on moisture sensors that measure humidity in the cooking cavity – an indirect mode of measurement that’s sometimes implemented in modern magnetron-based microwave ovens – solid-state microwave ovens measure the properties of the food itself while it cooks, and adapt accordingly. This promotes the retention of the nutrients, moisture and flavors of the food.

Economical Impact

The adoption of solid-state microwave heating is expected to commence in the industrial and commercial cooking market, where the value that these systems provide will be well worth the modest increase in cost. Customers stand to gain significant advantages centric to system reliability, food processing speed and throughput.

With regard to system reliability, solid-state RF transistors can provide 10X longer lifespans of typical magnetrons – this is a major benefit in 24/7 production environments where frequent magnetron failures can slow production and require numerous, expensive service calls. By eliminating the rotating platters common to magnetron-based microwave ovens, system reliability is further increased due to the reduction of mechanical moving parts, which are a common point of failure.

Food processing speed and throughput are increased due to solid-state microwave ovens’ ability to heat and cook food much faster than magnetron-based systems, owing to the rapid energy transfer enabled by solid state RF power adapting to the changing food dielectric. Solid-state RF technology is particularly valuable for food defrosting processes, enabling food to be defrosted much faster and more evenly than it can today, without drying or damaging the food.

With continued innovation in solid-state GaN-based RF technology and cost structure improvements, this technology is expected to eventually migrate to consumer kitchens, and in so doing has the potential to change perceptions of the modern microwave oven. Its value will evolve from that of a simple heating device, to a device that’s capable of cost-effectively cooking healthier, multi-course meals with unprecedented efficiency.

Proven Technology

This revolutionary cooking technology is already being successfully demonstrated. At IMS 2016, MACOM demonstrated this with our 300 W RF transistor in a solid-state oven baking muffins. The following year, at IMS 2017, MACOM announced their RF Energy Toolkit aimed at accelerating customers’ time to market by making it easier to fine tune RF energy output levels to maximize efficiency and performance.

Earlier this year, at IMS 2018, MACOM demonstrated the controllability of GaN-on-Si-based solid-state RF energy by successfully cooking the traditional Japanese Onsen Tamago. This dish is traditionally slow cooked using rope nets in the water of onsen hot springs in Japan at 70 °C for 30-40 minutes, enabling the egg yolk and egg white to solidify at different temperatures. The result is a dish of unique texture, with both a creamy outer layer and firm inner yolk. With the controllability enabled by solid-state RF energy, MACOM cooked this traditional dish in only 6-8 minutes, achieving the same desired consistency accomplished in the onsen hot springs.

Looking Forward

As with any emerging technology, the speed of RF energy technology’s commercial adoption hinges in part on collaborative industry efforts to establish common standards. Organizations like the RF Energy Alliance, composed of industry leaders spanning semiconductor vendors, commercial appliance OEMs and more, aim to help standardize RF energy system components, modules and application interfaces. In turn, this standardization will help to  reduce system costs, minimize design complexity, ease application integration and facilitate rapid market adoption (learn more about MACOM’s RF Energy Toolkit).

Thanks to continued advances such as these, the RF industry is closer than ever to enabling a more advanced, smarter kitchen for commercial restaurants and consumers around the world.


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Optical and Photonic Networks for 5G: Innovating at the Component Level   Sep. 04, 2018

MACOM in conversation with Verizon

vivek verizon.PNGAs technology continues to advance and new innovative solutions seem to appear daily, all eyes are looking to what’s next. For MACOM and Verizon, the next big step is the enablement and deployment of 5G communications networks. Verizon, being a telecommunications carrier, is eager to provide faster, more reliable connections to their customers, and MACOM is in a unique position to leverage its technology portfolio to develop the components that will make these new 5G networks possible.

Recently MACOM’s Vivek Rajgarhia, SVP and GM of the Lightwave Business Unit, had the opportunity to sit down to discuss recent product and technology innovation with Glenn Wellbrock, Director, Backbone Network Design, of Verizon. It was an interesting conversation throughout, spanning the growth and evolution of MACOM, to the direction in which the telecommunications industry is moving, and most importantly how these things intersect.

Rajgarhia began the conversation by explaining the history of MACOM, from the company’s founding nearly 70 years ago in the RF microwave industry, to present day where optics and photonics have become a significant focus for MACOM, highlighting how the portfolio of technologies has expanded to include Silicon Photonics (SiPh), Silicon Germanium (SiGe) and many more. More than ever before, MACOM is positioned as a preeminent supplier from RF to Light.

With 5G trending worldwide, companies and consumers alike are interested in the optimal development of 5G networks and the ability to deploy at a larger, more reliable scale to meet the ever-increasing demand for data. Growth in the telecommunications sector and the evolution of the components that will enable 5G is a main discussion point in the video, with both Wellbrock and Rajgarhia discussing how current technologies are growing toward a larger, more connected network.  The challenges being worked through while developing these technologies will optimize them for use in large-scale network architectures that will allow faster, more reliable connections between users. Wellbrock spoke about the evolution of laser technologies and how the jump from 10G lasers to 100G speeds occurred out of necessity to meet capacity requirements, while the jump from 100G to 400G will be an optimization move. Much of the foundation for the 5G network is already in place, but the challenge lies in optimizing this backbone in order for it to be built upon.

With the concepts of Smart Cities—cities that are interconnected with data collection sensors to supply information that can be used to better manage assets and resources, and the Internet of Things—connecting everyday objects with computing devices that enable them to send and receive data, comes the challenge of producing enough volume to satisfy demand, as well as the task of creating robust parts that will be able to handle the challenges that come along with connecting an entire city on one network. MACOM is committed to developing products that can be used in conjunction with each other to enable cohesive systems. For example, Silicon Photonics is positioned to be the most scalable solution for optical interconnects, delivering the lowest cost per bit for 100G and 400G, a key solution to the challenge that comes with deploying in remote areas of networks where replacing malfunctioning parts can be difficult and costly.

As technology continues to evolve, to satiate demanding higher speeds, an innovative, cost-effective and cohesive offering of components must be developed to enable next-generation connections. Wellbrock and Rajgarhia explore some of these exciting developments, and with some of the brightest minds in the industry working together, the full potential of 5G will soon be realized.

MACOM in conversation with Verizon


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