Bench Talk

Play Acoustic: A Look at Bulk Acoustic Wave Resonators

(Source: imred/Shutterstock.com)

When I’m not pounding my fingers against the keyboard of my computer, I am often found pounding my fingers on the neck of my electric guitar. Even though all my electric rock guitar heroes are all getting up there in age, I continue to carry their torch into the future. Alas, sometimes even a rock ‘n’ roll animal like me has to lay down his electric guitar and play an acoustic.

Why, you might ask? Well, it is certainly not because I am losing my taste for rock ‘n’ roll and replacing it with interest in James Taylor. No! It’s because playing an acoustic guitar allows one to jump right into the music. All one has to do is pick it up, tune it, and play. There is no hassle with cables, cords, amplifiers, pedalboards, and the host of other associated pains in playing electric. One can feel the instrument move and breathe in your hands and against your body (Figure 1).

Figure 1: The author playing an acoustic guitar. (Source: Jessie Kilbane)

Today, clocking is often achieved through external quartz (Si0₂) crystals. One of the pains of using crystals is that they are external to the semiconductor they go with. For many applications, the only external parts required for a System-on-Chip (SoC) microcontroller (MCU) is the crystal and a few bypass capacitors. The crystal is the larger device, so it dictates the PCB size and layout. Together, they’re like the electric guitar. It is not an integrated, ready-to-go unit.

Here, we will discuss how bulk acoustic wave (BAW) technology can alleviate some of the pains design engineers experience by eliminating the need for an external crystal to set the clock speed of an MCU or other circuit. In addition, we will explore the benefits of using devices with an integrated BAW frequency-determining component.

Playing my acoustic guitar reminds me of how acoustic technology works for more things than mere musicians. Bulk acoustic wave (BAW) filters have been useful in higher-frequency LTE and 5G cellular bands as well as Wi-Fi applications. BAWs do so by working as a suitable high-frequency radio frequency (RF) filtering device. Their physical structure consists of a piezoelectric-film resonator placed between two thin metal plates or films. When excited by a voltage, the mechanism oscillates at a specific frequency like a crystal. The rate depends on the thickness of the piezoelectric film. These filters find applications in everyday RF devices such as smartphones.

Now, BAW technology is being transformed to provide an integrated clocking function to MCUs and related semiconductors. This tiny technology is going to have a global impact on MCUs and devices that incorporate them and any circuit that needs a clock generator. The BAW oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating micro-acoustic resonator (BAW) to generate a stable electrical signal through the piezoelectric effect. This precise signal, at a very high frequency, provides the clocking and timing reference for electronic systems.

BAW is a micro-resonator technology that enables integrating high-precision and ultra-low jitter clocks directly into packages containing other circuits. New products previously requiring a crystal can be replaced with similar devices with an integrated BAW frequency-determining component. BAW technology is less bulky than external quartz crystals—delivering cleaner wired and wireless signals over networks. It enables higher quality communications and more efficiency to a range of areas, from wireless consumer electronics to high-end industrial systems.

At the forefront of this innovation is none other than Texas Instruments (TI), a leader in semiconductor solutions for analog and digital embedded and applications processing. TI is now employing BAWs in SimpleLink^™ MCUs.

One of the first of several TI products to incorporate a BAW frequency source is its popular SimpleLink radio transceiver chips targeting IoT and other short-range wireless solutions for Bluetooth^®, Zigbee^®, Wi-Fi^®, and other standards. Texas Instruments SimpleLink™ MCUs offer the broadest portfolio of differentiated wired and wireless Arm^® MCUs with a single development environment (Figure 2). This environment delivers flexible hardware, software, and tool options for designers developing Internet of Things (IoT) applications. With a single software architecture, modular development kits, and free software tools, the SimpleLink MCU platform enables 100 percent code reuse across the portfolio. SimpleLink microcontrollers help manufacturers quickly develop and seamlessly reuse resources to expand their portfolio of connected products.

Figure 2: The Texas Instruments SimpleLink^™ MCU platform supports Wi-Fi, Zigbee, Bluetooth, Thread, sub-1GHZ, and Wired standards for IoT and short-range wired applications. (Source: Texas Instruments)

Texas Instruments CC2652RB (Figure 3) supports Thread, Zigbee^®, Bluetooth^® 5.1 Low Energy, IEEE 802.15.4, IPv6-enabled smart objects (6LoWPAN), and proprietary systems. This new product is the industry’s first crystal-less wireless MCU, which integrates a TI BAW resonator within the package.

Figure 3: Texas Instruments CC2652RB SimpleLink™ Wireless MCU is a multiprotocol 2.4GHz wireless crystal-less MCU with integrated TI BAW resonator technology. (Source: Texas Instruments)

The integrated BAW resonator technology eliminates the need for external crystals without compromising latency or frequency stability (Figure 4). This MCU allows design engineers to create simpler, smaller designs while increasing performance and lowering costs. It will also speed time to market since designers will be able to eliminate the process of selecting, calibrating, and installing external quartz crystals.

Figure 4: The BAW technology fabricated on a silicon die. (Source: Texas Instruments)

The device is optimized for low-power wireless communication and advanced sensing in building security systems, HVAC, medical, power tool, and wired networking. It is also optimized for portable electronics, home theater and entertainment, and connected peripherals markets.

The benefits an MCU with BAW Technology include:

• Small package size
• Simplified development
• Robust and durable design
• Physical security

Let’s look at these benefits in more detail.

Small Package

Space savings can be essential in applications such as wearable medical devices. Because they’re integrated into chip packages, circuit designers will no longer need to use separate clocking devices mounted on circuit boards. The CC2652RB is designed for space-constrained wireless applications. Removing the external 48MHz crystal reduces the footprint of the traditional 7mm x 7mm QFN wireless MCUs by 12 percent. This also provides you with increased flexibility for routing your sensors and peripherals to the GPIO pins.

Simplify Development

Quartz crystals are one of the most common pain points in the RF layout of wireless radios. They are often the source of issues, such as noise, cross-talk, or crystal frequency tuning. These issues often require new PCB spins to fix during development, increasing your development costs. All of these risks can be avoided by utilizing CC2652RB.

Robust and Durable

Many of today’s innovative products reside in harsh environments. From the constant vibration of an industrial robot to the mechanical shock of a dropped device, wireless devices need to operate in many situations reliably. BAW resonator technology is a viable long-term design option for rugged environments. It is more resilient to mechanical shock and vibration environments, with three times lower ppm variation than an external quartz crystal when tested against the industry standard, MIL-SD-883H. The device is also rated up to a 10-year lifetime, versus the typical five to six years of a quartz crystal. This means a more accurate radio, along with fewer timing and transmission errors, over nearly double the lifetime. Quartz crystals are also more susceptible to breaking under mechanical shock, terminating the wireless functionality of the device. If the external crystal shatters, it would need full replacement to restore wireless functionality. This fragile part can be removed from the application with the BAW resonator.

Physical Security

Using BAW resonator technology diminishes a potential point of weakness in MCU security. Removing the external crystal mitigates the opportunity for a timing-related side-channel attack on the system.

Well, as I conclude my keystrokes on my computer, I can turn my attention to getting home and playing my guitar. I hope you have learned how playing with acoustics will do nothing but help you rock ‘n’ roll your jam away on your new designs. Now, let me turn up the AC/DC as I rush to beat the rush hour.