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Embedded systems have been almost entirely digital throughout their long history, while RF and microwave technologies were separate subsystems with no effective interface between the two. For many reasons, this “RF/digital divide” should finally be connected.

Mercury Systems, which makes embedded systems (i.e., board-level digital and RF subsystems), proposed that manufacturers of embedded and microwave subassemblies participate in its initiative called OpenRFM. The company's goal is to make it possible to integrate RF and microwave technology into current “digital-only” embedded form factors for the first time. If you're not in the embedded systems business, you're probably wondering, “Why is this just happening now?” If you're into the designing of RF and microwave subsystems, you might ask, “Why bother?”

The answer to the first question is that digital and microwave designers have always operated in separate domains and ignored each other. The answer to the second is that integration of these two disparate technologies simply makes sense regardless of what “camp” you reside in, and for the following reasons:

  • It would allow these two technologies (digital and microwave) to be integrated in a standard form factor followed by the embedded systems industry.

  • It would be a major step toward realizing two major goals of the U.S. Department of Defense (DoD): to dramatically increase function integration, and to allow radar, electronic warfare, and other systems to be used in multiple platforms without major redesign.

  • All embedded systems manufacturers and prime contractors could follow a single design roadmap while retaining the flexibility to differentiate their products from others by using their own OpenRFM-compatible products.

  • Systems could be constructed that are smaller, lighter, consume less power, are less expensive, and shorten the time to market for both embedded systems and end products.

Although Mercury targeted OpenRFM to sectors of the defense industry in which it participates, there is no reason why it could not be adopted in other embedded markets such as rugged industrial, scientific, and medical systems, and telecommunications, for example. As some of these applications make use of wireless communications in some form, OpenRFM should be useful for manufacturers of commercial systems that incorporate it.

To affect this integration on a broad scale, Mercury will need to have it appended to the current family of standards used in defense embedded systems — OpenVPX, whose champion is VITA (formerly the VMEbus International Trade Association). There is at least a reasonably good chance this will occur as it fits neatly into the association's roadmap, which has already expanded to include software-defined radio and space qualification.

Figure 1. This is an OpenRFM module riding atop an OpenVPX board. Traditionally, the functions performed in the OpenRFM module would have been implemented in a larger separate housing.

OpenRFM is a modular, open architecture built around OpenVPX that combines hardware, firmware, and software, and allows high channel density, advanced interconnect technology, and employs a “building block” approach. The company has already produced OpenRFM products in 3U and 6U OpenVPX form factors, as shown in Figures 1 and 2. The real estate available for OpenRFM in the OpenVPX environment is sufficient to allow a broad array of integrated microwave subsystems (today referred to as Integrated Microwave Assemblies or IMAs) to be accommodated.

One of the most common is the downconverter, which effectively forms the front end of every system in which a signal must be captured over the air and lowered in frequency to one that can be handled by an analog-to-digital converter. The downconverter's opposite is the upconverter, which does just the reverse: taking a signal at a lower frequency and increasing it to one required by a specific application. Both of these IMAs are required elements of every type of military and commercial system, from satellite communications terminals to microwave point-to-point links, and almost every type of communications system.

Figure 2. The 6U VME carrier (bottom) and three OpenRFM modules (top): a quad downconverter (left), wideband tuner (center), and direct-digital synthesizer (right).

These are complex products that include many active and passive microwave components, and range from relatively small to quite large, depending on their requirements. As these subsystems typically don't reside on a board-level product but rather are separate functional blocks, OpenRFM has the potential to make the whole system smaller. Other typical IMAs include complete digital receivers that, when combined with traditional signal processing, general-purpose processing, signal distribution, and other functional blocks, form a larger subsystem.

Why Is This Just Happening Now?

As we are all painfully aware, the defense industry has been driven primarily by technology rather than cost. This is because new radar, electronic warfare, and other systems required by the DoD are extremely complex and push the envelope of achievable performance. It's also because the Pentagon has to combat both current and perceived next-generation capabilities of its adversaries and because these systems remain in the field for decades with upgrades over the years. Both of these requirements don't come cheap.

However, the days of this “just do it” mentality are rapidly coming to an end as the DoD scrambles to squeeze as much as possible from its annual budgets. It's obviously not the first time Pentagon spending has been under the microscope, but it may be the first time that, technologically speaking, there are ways purveyors of digital and microwave technology can broadly help make it happen. OpenRFM is well suited to become one of these enablers.

By standardizing the electromechanical, software, control plane, and thermal interfaces used by IMAs, OpenRFM could almost completely change the way defense systems are built, providing a standards-based roadmap that would allow systems to be used on more than one platform, which is the opposite of why they are designed and built today. Even though a radar built by manufacturer A for one aircraft may perform very similar functions as one built by manufacturer B on another aircraft, they are invariably almost totally different, incorporating proprietary designs that can't easily be “ported over” from one platform to the next.

There Are Precedents…Sort Of

There are market sectors in which digital and microwave technologies comfortably coexist in a quasi-standard form factor — smartphones and other wireless devices being a classic example. In all these cases, integration was driven by need: There's simply no room in these small, typically battery-powered devices for disparate technologies singing different tunes.

Bluetooth products and Zigbee owe their very existence to wireless communications, as do Wi-Fi access points, some medical devices, and other products in which size or some other constraining factor dictates that digital/microwave integration is essential. Their small size and specific digital and microwave requirements allow this integration to be accomplished at the wafer level using a System on a Chip (SoC) approach that can integrate a truly astonishing amount of functionality in a single device. A good example is Silicon Labs’ Si468x CMOS digital receiver ICs that are essentially low-power, multi-band, digital broadcast receivers that support AM, longwave, shortwave, FM, AM and FM HD radio, digital audio broadcast, and digital multimedia broadcast.

Among other things, they include an OFDM channel demodulator, audio DSP processing, on-chip source decoding, I2S digital audio output, a stereo audio DAC, VCO, PLL, frequency synthesizer, AGC SPI, and I2C control interfaces. Typical applications include multimedia handsets, media players, GPS devices, tablets, all types of fixed and portable radios, boom boxes, and entertainment systems.

One other SoC that packs broad functionality in a single device is Broadcom's BCM20736 WICED SMART Bluetooth SoC that allows OEMs to develop applications on its ARM Cortex-M3 processor. It includes an RF and embedded Bluetooth stack, support for wireless charging, software for profiles, the stack, APIs, and application software development kits, and has two serial peripheral interfaces (SPIs). The device, designed to be powered by a single coin cell at 1.2 VDC, is pin-compatible with Broadcom's Bluetooth Smart SoCs, and allows secure updates to be implemented over the air.

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