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Printed circuit board layout

Crystal-less Radio Provides Wireless Connectivity for IoT

May 13, 2019
Using radios that don’t need crystals saves space.

It is estimated the total spending on Internet of Things (IoT) devices and infrastructure will reach $1.2 trillion by 2022, up from an estimated $151 billion last year. Such rapid growth indicates that the IoT is penetrating deeply into many markets, from first-adopters a decade ago to today, with 90% of business executives in technology, media, and telecom considering IoT technology to be central to their business strategies. Considering that a major bottleneck to widespread IoT adoption has been lack of integration, the benefits of crystal-less radio for IoT will be tremendous.

A wireless SoC direct conversion architecture (device 1) uses an external crystal (device 2). Direct conversion transceivers make for a compact design with a single synthesizer.  depicts a digital PLL (phase look loop) which enables direct conversion of baseband to radio signals and vice-versa. The front end is comprised of a power amplifier (PA) and low-noise amplifier (LNA). The LNA is followed a by a mixer which feeds baseband signals to a DSP modem through analog-to-digital converters (ADC).

The heart of any wireless system-on-a-chip (SoC) is its clock or timing circuit. A critical block in any wireless transceiver circuit is a phase-locked loop (PLL) synthesizer. It is a versatile circuit used in signal synthesis, synchronization, modulation, demodulations, and signal tracking. PLL synthesizers generate a radio frequency signal (FRF) from a fixed reference signal (FREF). FRF can be adjusted by changing the divisor (N) in the programmable feedback divider. When FRF/N equals FREF, the loop is termed “locked,” then:

Generally, external crystal oscillators generate FREF, which have accuracies ranging from 5 to 50 ppm. When the PLL is locked, FRF’s accuracy will be the same as FREF. Quartz crystals have been considered the best option for generating FREF because they provide higher-frequency accuracy. There are other lower cost and/or accuracy oscillators such as ceramic oscillators, but for most IoT applications, quartz crystals are the way of generating FREF.

A generic SoC reference design uses wireless MCU, crystals, matching networks, and filter capacitors. Sometimes the crystal that generates the reference clock for an SoC’s radio is nearly as large as the wireless MCU itself. So, if area is a limiting constraint, the preferred choice is to operate without an external crystal to generate FREF and use an in-package piezo-electric resonator known as a bulk acoustic-wave (BAW) resonator instead.

This crystal-less direct conversion radio transceiver has the external crystal replaced by a BAW resonator. The Digital PLL FREF is generated from its own system-in-package (SiP) BAW resonator.

On the left, (a) shows a schematic of a BAW resonator. It consists of a piezoelectric material sandwiched between two electrodes which convert electrical energy to mechanical-acoustical energy and vice-versa. The mechanical resonance of the piezoelectric material generates the clock. In (b), a cross-sectional view of the crystal-less radio shows where the BAW mounts on the silicon die which rests on the package’s lead frame.

To achieve better frequency stability over temperature and battery (i.e., voltage) conditions, the BAW frequency is actively compensated. This is made possible by the digital PLL architecture used in crystal-less wireless SimpleLink MCUs.

This schematic shows the BAW resonator’s active-compensation mechanism. The device’s temperature sensor and battery monitor are read and then fed into the RF core processor. The RF core uses these readings, along with stored three-point frequency vs. temperature values to each device, to calculate the frequency of the BAW at any given temperature and voltage.

Once the BAW frequency is calculated, an offset is computed and fed to the device’s digital frequency synthesizer, which offsets the division ratio of the PLL to get an output frequency within a few parts per million (ppm) of the desired channel frequency. The following equation shows the second-order approximation used to calculate the digital PLL’s offset:

fBAW = P0 + T - 27 * P1 + T - 272 * P2

Where T is the voltage-compensated temperature-sensor reading, and P0, P1, and P2 are coefficients determined through a least-mean-square error calculation that relates BAW frequency to temperature, as determined by the stored three-point frequency vs. temperature curve.

Texas Instruments’ micro-electro-mechanical machine technology lets a broad portfolio of wireless Arm Cortex-M based SimpleLink MCU platforms go crystal-less. The newest product of this crystal-less SimpleLink MCU platform is CC2652RB (CC26). It features a built-in, high-frequency, faster-start-up time BAW reference clock. This innovative crystal-less design enables faster customer production and time-to-market without compromising on performance.

The Rx sensitivity chart compares the BLE 1Mbps mode receiver sensitivity of a CC2652RB crystal-less radio to that of CC2652R using an external crystal. Receiver sensitivity is an important metric of wireless radios and requires a high accuracy reference to not degrade over operating conditions. It can be seen that the sensitivity is comparable between the two. This proves the frequency of the BAW is accurate enough to give good performance. The RV Current chart compares the BLE 1Mbps active-mode receiver-mode current consumption of a crystal-less wireless MCU to that of one using an external crystal across temperatures.

The CC2652RB with a BAW consumes is only slightly more current during active receive or transmit operations. This approximate 500 µA current increase during active radio operation with CC2652RB is offset by TI’s real-time operating system software that enables BAW only when needed for radio operations. The overall increase in connection-profile current is small because most applications have high duty cycles. For example, a BLE connection has the following approximate current consumption:

Considering the active current is a weighted average of Tx and Rx current:

Where TPDU  is the active transmit time for PDU (packet data units) and βRX  is the ratio of receive time-to-transmit time (e.g., to account for guard-bands for packet reception). These equations don’t account for non-radio processing time because it is application-dependent. For approximation, considering the BAW increases the Tx and Rx current (ITX  and IRX) by 500 µA, and that with an external crystal, the ITX  is 9.7 mA and IRX  is 6.9 mA. Isleep  is approximated as 1 µA. Assuming βRX  is 1, and TPDU  is 328 µs, for connection of 7.5 ms or 4 s (fastest or slowest connection intervals for the core Bluetooth spec), and adding 1% increase as approximation error, the results are in the tables below.

This is the printed circuit board layout of CC2652R wireless Arm Cortex-M based SimpleLink MCU reference design. Areas of the 7 × 7 mm MCU package and a 48-MHz external quartz crystal are highlighted.

In applications such as asset trackers, the space savings offered by crystal-less radio can be very important. There are asset trackers on the market that are approximately the size of a typical car key fob (about 50 mm on a side). These can be made smaller by eliminating the high-frequency crystal needed for radio operation.

This table shows the approximate amount of space saved using a BA with (no crystal device) SimpleLink by removing the high frequency crystal. This assumes a routing efficiency of 80%, and a BOM that includes 26 0402 components, one 0603 component, one 0805 component, a 32 kHz crystal, a 48 MHz crystal, and a CC2642R chip. Removing the low frequency crystal, which is also possible with SimpleLink, results in even more space savings.

The space saving possible with crystal-less radios are crucial in many emerging applications, such as Medical IoT (MIoT). Medical device makers are already delivering implantable pacemakers and heart monitors smaller than AAA batteries (the smallest pacemakers now measure about 26 mm long by 6.7 mm wide). The above tables show BAW technology make many IoT devices nearly a millimeter shorter in the critical dimension (which would give a 12% reduction in the cross-sectional area of an example pacemaker that is 26 mm long by 6.7 mm wide).

It is already possible to run SimpleLink CC26x2 wireless MCUs without 32-kHz crystals at the expense of increase power consumption. When this approach is used with a CC2642RB device, it requires no crystals. Texas Instruments’ CC2642RB Arm Cortex-M based SimpleLink wireless MCU is a crystal-less radio with a fast-start-BAW stable reference clock, for saving space and a faster time to market.

James N. Murdock is a validation engineer working for Texas Instruments. Habeeb Ur Rahman Mohammed is a validation manager at Texas Instruments’ Connected Microcontrollers organization. To lear more about SImpleLink BAW MCU, click here.

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