BUC and SSPA SWaP Considerations for Communications on the Move

by Daniel Lopez, Product Manager, Norsat International

Size, Weight, and Power (SWaP) are the three most critical physical attributes which Block Upconverter (BUC) and Solid State Power Amplifier (SSPA) designs must address in Satellite Communications on the Move (COTM) systems. Minimizing size and weight while maximizing power are competing aims, and optimizing this balance is at the heart of BUC/SSPA design.

But why is SWaP so critical in the first place? How does a successful design minimize the form factor, reduce weight, and achieve high power output? And what elements most affect how a BUC/SSPA achieves a good versus poor SWaP balance? And in actual COTM applications, how do these principles work out in real life? These are the questions that designers, manufacturers, suppliers, and users of COTM-oriented BUCs and SSPAs have to contend with.

Benefits of SWaP

The benefits of low SWaP in COTM come from smaller and lighter BUCs that are therefore easier to position, access, operate, and if needed, service. In airborne COTM applications, for example, the constraints of limited space, and critical fuel consumption margins result in the criticality of small system size and low weight, respectively.   By contrast, in maritime COTM situations, weight is somewhat less important than space — where the constraints of radome mounting impose extremely compact form factor requirements, in addition to thermal and other considerations. In the case of land-based vehicular COTM, once again, space is severely limited, and weight also imposes penalties on performance that must be addressed. Power efficiency (output power versus power consumption) is a factor for all COTM applications, since there’s always a finite level of useable power in a moving vehicle, whether land-based, airborne, or maritime. So, all other factors being equal, the smallest and lightest BUC or SSPA outputting the highest power with the lowest possible power draw, is what SATCOM customers are looking for.

Tradeoffs of SWaP

In the context of BUC and SSPA design, SWaP is challenging to achieve as there is a mutual relationship between the three aspects. In order to optimize one aspect, there needs to be a compromise in another area. For example, the natural tradeoff for a lightweight component is the lack of durability and ruggedness. When a component is required to work in a harsh environment which may include extreme temperatures and vibration, opting for a lightweight product may not be the best option. As well, adding more power to a unit also results in more heat, which creates the need for more components in order to cool it down and not overheat. Cost also plays an important factor. For example, die-level designs (as opposed to packaged device designs) achieve both weight and size reduction — as well as increasing power efficiency — but at an increase in development complexity and cost.

So then, on what design factors does an effectively engineered SWaP COTM upconversion solution hinge?   The answers are in two domains — the mechanical architecture of the BUC or SSPA, and its RF architecture.

SWaP Design Considerations: Mechanical Design

In the mechanical realm, SWaP begins with a compact and lightweight design. Beginning with how the critical components such as the power module, amplifier and, in the case of a BUC, the synthesizer are laid out. If fans are used in the cooling solution, where they are located, and where the various power, RF, and M&C connectors are placed. But what, in turn, informs a designer as to where and how to architect these components? The key lies in the intended specific COTM application for the BUC or SSPA. Specifically, around the physical and environmental standards to which the on-the-move BUC or SSPA must perform. So, for example, environmental EMC standards for land use are spelled out in MIL-STD 461. For airborne application, MIL-STD 704B spells out shock, vibration, and EMC parameters. And for maritime environments, it’s MIL-S-901 standards that spell out shock performance criteria. In order to achieve these standards, designers of BUCs and SSPAs must take into account not only the physical architecture to allow for all those performance criteria — but also factor in the additional circuitry that may be involved to address those standards — the inclusion of which also affects size, weight, power consumption — and cost.

In addition to addressing application and performance requirements, a good BUC and SSPA design must also be user-friendly with respect to its users throughout its operational life. It begins with a design where all the components are integrated together in a logical manner, and continues with an architecture that can also be manufactured and tested in large quantities.   Finally, the BUC or SSPA needs to work efficiently for the customer at delivery, but ideally, keep in mind the very likely possibility that future customizations may be required.

For example, Norsat’s ATOM 25W Ku-band BUC is manufactured for SWaP and is only 6.5” x 3.2” x 6.1” and weighs an impressive 5 lbs.

The design concept for this 25W Ku-band BUC was to make it compact, lightweight, and power efficient, in other words, low SWaP, as shown in the mechanical. However, a challenge sometimes arises when customizations are requested, as any additions or changes to features will most likely result in a bigger product. Though this is not always the case, it is important to keep in mind that a SWaP design sometimes does not lend itself well to further customizations after it has been built.

Conversely, another approach used by some BUC and SSPA manufacturers is pay weight and power penalty for achieving greater customization flexibility. This is done by employing a common physical form factor platform across various power levels. The result is higher parts commonality, which ultimately translates to lower unit costs. Additionally, customizations are easier and faster to turn around. This tradeoff is harder to justify when it comes to on-the-move SATCOM, where space and power constraints are more likely to trump cost savings. Nevertheless, the diversity of BUCs and SSPAs in the marketplace, even at common power sizes, shows the divergence of preference over whether that tradeoff is acceptable or not.

Another mechanical design consideration for COTM upconverter SWaP is the thermal solution to handle how the heat is dissipated. Due to their comparatively higher power consumption requirements, BUCs and SSPAs create considerable thermal energy. At various power levels —whether saturated power (Psat), 1 dB compression point ( P1dB), or 3dB off P1dB — a BUC or SSPA draws much more power than output power, on account of heat related and other power wastage.  This ratio — power efficiency — can range from as high as around 20% down to 5%. For example, the Norsat ATOM 25W Ku BUC draws 147W of power at P1dB, yielding a 17% efficiency at that particular level. At saturated power, that efficiency drops to 15%. While the efficiency will vary from model to model, what remains is an inherent power wastage that both reduces the BUC’s efficiency and also produces a heat load that needs to be dealt with.

How the heat is physically dissipated varies by output power, design, and application. At lower powers, such as 4W BUCs, cooling fans and heat sinks usually suffice. As the power outputs and sizes increase, the resulting higher thermal loads higher and begins to require cooling fans, spreaders, and heat pipes as the preferred methods of creating the necessary heat transfer. Even fans have limitations — especially low speed fans, which yield very concentrated heat densities, and are disproportionately large — limiting their cooling capability and placing large size penalties on the BUC or SSPA design.

In the case of the ATOM BUCs, high speed fans are used, since in the end, the tradeoff of higher power consumption is more than offset by a much greater cooling efficacy — and due to their small size — achieves a better size vs cooling tradeoff. Pictured in Figure 4 is Norsat’s ATOM 40W Ku-band BUC which uses high speed fans to cool down internal components. In the end, lower heat leads to less weight and power drain. There is a huge engineering design challenge when it comes to thermal and it can set apart competitors when evaluating low SWaP products — all the more so in the domain of on-the-move SATCOM.

Hard choices around thermal solutions become more critical when, in addition to these general design principles, the specific COTM requirements are layered on.  For example, land and maritime OTM applications will likely need IP 66 or better environmental rating and may need MIL-STD-810 salt/fog testing to ensure protection from long term salt exposure to moving fan components. By contrast, airborne applications, particularly those at higher altitudes where lack of sufficient air pressure renders fans ineffective, instead have thermal solutions centered upon cooling plates.

Not only are the choices in components such as board layout and cooling solution themselves the only factors in design, but also decisions around how much modularity versus how much standardization are incorporated into the design. Designing for the smallest design would mean integrating as much circuitry on as few boards as possible. However, this leads to extra complexity in meeting stringent environmental requirements (e.g. EMI) and reduces the ability to customize without huge NRE and effort. As an example, Norsat keeps its size down by waveguide combining at the die level, but uses a modular architecture for the internal circuitry to give the ATOM BUCs and SSPAs flexibility for such things as adding internally shock mounted synthesizers to meet airborne vibration requirements. Similarly, for EMI standards, Norsat separates that componentry on its own filter board in order to not burden customers with the cost if they don’t need it.

So the role played by the physical architecture of an on-the-move BUC or SSPA is primarily focused on the orientation of the intended mobile applications — whether on land, airborne, or on water. Cost plays a factor, but only insofar as the physical constraints of the larger terminal allow.  But if the physical design speaks to successfully overcoming the challenges of the particular on-the-move environment, is that where the quest for good SWaP ends? No, because the components in the RF chain are themselves part of the solution to the COTM SWaP challenge.

SWaP Design Considerations:
RF Architecture

In reality, while the design specialties of the mechanical design and the RF design are two very different domains, they necessarily converge in the overall design of on-the-move upconverter products.

In terms of a BUC’s RF architecture and available semiconductors, SWaP can be achieved through a variety of different approaches. The traditional way to architect the amplifier inside of a BUC is to use a set of matched field-effect transistors (FETs).  Since FETS are low gain, a lot of FETs are needed in order for them to work properly. They are then cascaded within the component and require  either a microscript or a waveguide combiner between each FET, resulting in a larger SSPA assembly.  There are pros and cons when choosing either a microscript or waveguide as the semiconductor. A microscript such as the Wilkinson combiner has high signal loss, is large, and not very efficient. The waveguide is more efficient, but is larger still in size.

Another way to build a SSPA is to use high gain microwave and millimeter-wave integrated circuits (MMICs). For example, Norsat’s ATOM Ku- and Ka-band product lines use MMICs as they are high gain and are more space efficient than FETs. Norsat chooses to save space by using high gain MMICs knowing that they may be less powerful than FETs, but compromises by designing for more power combining. Manufacturers may choose different component parts to put together their SSPAs, but the challenge is always to find an efficient way to combine these choices.

Another option is to use a bare die. Matched FETs and high gain MMICs are packaged, while dies are smaller. The advantages of dies are that they are more efficient than traditional methods and smaller in size, but the disadvantage is that they are harder to assemble and create heat in a more concentrated area. Dies require soldering and wire bonding, which is a more complicated process than assembling and bolting down packaged matched FETs and high gain MMICs to a baseplate. The second major issue with dies is the amount of heat that is built up within a small space. The power and heat energy remain the same regardless of size, so in the case of bare die, it just becomes more concentrated which makes it harder to dissipate. If, on the other hand, in the case of a packaged MMIC, the heat is spread out over a larger area, it is easier to dissipate. Comparing bare dies that are 4mm x 4mm versus packaged MMICs of equal performance that are in a package of roughly 15mm x 15mm, means that the reduced surface area of the bare die yields less heat transfer capability. Additionally, this is an expensive process to develop from an R&D perspective.

GaAs vs. GaN

Another decision faced by RF designers developing on-the-move BUCs and SSPAs centers on the substrate choice between the tried-and-true gallium arsenide (GaAs) versus the newer gallium nitride (GaN) technology. The advantages of GaN center around its higher efficiency and higher voltage capability compared to GaAs. By contrast, GaAs is limited in output power of the final device and manufacturers have increasingly looked to GaN to solve these issues.

In the context of on-the-move upconverter products, the GaN products’ higher gain and high power products within a smaller package signals a clear SWaP advantage. And with GaN’s higher efficiency reducing the need for complex and bulky power combining, the GaN option makes even more sense for mobile applications. Indeed, Norsat offers GaN technology in its Ka-band products and there is further product development focused on GaN technology underway. However, is the case for GaN that simply clear cut?

To begin with, it is hard to compare GaAs and GaN products as they behave differently and the usable power ranges occur at different points. To begin with, the critical power thresholds are different between GaAs and GAN products.  Whereas GaAs products focus on saturated (PSat) and P1dB power levels, GaN products focus on PSat and Linear (Plin) power. Some measurements such as saturated power may seem to be the same for both, but it is difficult to compare GaAs at P1dB versus GaN PLin, since those two powers are not equivalent, and the compounds in question each behave in their own way. Trying to establish specification equivalence is extremely difficult, and the relative newness of GaN technology means that the broader market needs more information and product information.

Typically, maximum-linear power is used as the standard measurement and specification for GaN products. However, there are multiple definitions of maximum-linear power in the industry; for this article, maximum-linear power is defined as per MIL-STD-188-164B under the single carrier maximum-linear power and two carrier maximum-linear power definitions. As per MIL-STD-188-164B, for a single carrier, the maximum-linear power is defined as the carrier power where the first spectral regrowth sidelobe (measured at 1.0 symbol rate [expressed in Hz from the carrier center frequency]) of the modulated carrier is -30 dBc. MIL-STD-188-164B defines two carrier maximum-linear power as the maximum combined transmit power of two equal amplitude continuous wave (CW) carriers, when any individual intermodulation product power is -25 dB relative to the combined power of the two CW carriers.

Using these definitions, GaAs and GaN can be compared more evenly in terms of PSAT and maximum-linear power. The MIL-STD-188-164B shows that the PSATs are the same for both GaAs and GaN, but the maximum-linear power for GaAs is actually better. However, in many cases it is not necessary to operate at the maximum-linear power and in other cases, specifically in a system where there is a single carrier, it is possible to operate higher than the maximum-linear power. Overall, saturated power for both GaAs and GaN may be the same, but generally the GaAs device will have the higher maximum-linear power when compared to the GaN device. Though, this observation does not mean that the operating points are necessarily the same. It is often a perception that GaN is equivalent to low SWaP, but in terms of maximum-linear power, it may not be the case, and most especially given that good power linearity is GaAS’s chief RF benefit. Careful consideration of the pros and cons of each gives customers guidance to select the product that best meets all stated requirements.

Balance of Tradeoffs

So while there is no silver bullet for a perfect on-the-move SATCOM upcoversion design solution, the competing aims of size, weight, power, and cost mean there are general themes that define well designed BUCs and SSPAs. A well integrated form factor that speaks to the BUC’s or SSPA’s ease of integration into the COTM terminal of choice will achieve the particular mobile operational capability. At the board and component level, a technology architecture must also reflect the choices spelled out by the overall terminal’s mission-critical requirements.

But all these design decisions must be grounded within the constraints of the broader picture — chiefly that COTM BUCs and SSPAs are not ends in themselves, but components of a greater whole system of antennas, radomes, and other RF equipment.  And depending on whether the COTM terminal is on the move on the ground, up in the air, or on the water, the BUC or SSPA, along with all other components, must consider the respective EMI, Shock, or Vibe MIL standards of that mission.

Consequently, a better way to look at SWaP is not as an abstract design objective, but rather the holistic outcome of a carefully considered solution that has optimized size, weight, and power — all within the context of the operational standards that yield the customer mission-critical upconversion capability that is on budget, and on specification.

Author Bio: Daniel Lopez is the Product Manager, Satellite for Norsat International Inc. in Richmond, BC, Canada.  He served in the Canadian Forces Telecommunications Branch for nine years, both as an operator and an officer.  Subsequently, he has worked in technical product management and consulting roles for over 17 years.

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