Is the end of the quartz crystal near? Probably not. However, with the availability of new integrated bulk-acoustic-wave (BAW) devices, crystals are going to be replaced in many clock and timing circuits. This is something you should look into given that most engineers have to address or deal with clocks and timing circuits at some point.
- Tiny technology, global impact: TI BAW resonator breakthrough creates new electronic heartbeat
- SimpleLink™ crystal-less wireless MCU based on TI BAW technology— at the heart of IoT evolution
- TI BAW technology enables ultra-low jitter clocks for high-speed networks
Bulk Acoustic Wave Background
BAW devices have been used for years as high-frequency filters. Their small size and high Q selectivity have made them a competitor with surface-acoustic-wave (SAW) filters in cell-phone front-ends. SAW filters are good for frequencies up to about 1.5 GHz. Above that frequency, BAWs are superior up to 6 GHz or so, making them good candidates for the higher-frequency LTE and 5G cellular bands as well as Wi-Fi applications.
1. A BAW’s physical structure is similar to a quartz crystal, but much smaller. Its configuration is such that it can be made on a silicon chip, which isn’t possible with a quartz crystal.
A BAW is a small structure not unlike a quartz crystal. It’s made up of a super-thin piezoelectric film between two metal plates or films (Fig. 1). When excited by a voltage, the device oscillates at a specific frequency like a crystal. The frequency depends on the thickness of the piezoelectric film.
Figure 2 shows an equivalent circuit to the BAW. It’s similar to a crystal equivalent circuit. The two parallel plates form a capacitor CP that’s in parallel with a series RLC circuit of a specific resonance. The BAW is used as a stable frequency-determining element in an oscillator.
2. Shown here is the electrical equivalent circuit of a BAW.
BAWs have found a niche in the filter hierarchy and are seeing increased usage. Their prices have come down into a competitive range with SAWs. The big breakthrough, though, has been in conquering the problems of integrating them into a silicon chip. Now Texas Instruments has figured out how to make a BAW with standard processing practices and in fabs used to make silicon ICs. This gives rise to some interesting benefits and new products
The main benefit is that it eliminates the common need for an external crystal to set the clock speed of an MCU or other circuit. For many applications, the only external parts required for an SoC MCU is the crystal and a few bypass capacitors. The crystal is the larger device, so it dictates the PCB size and layout. Now, new products requiring a crystal can be replaced with equivalent devices that have an integrated BAW frequency-determining component. This tiny technology is going to have a global impact on MCU and devices that incorporate them (like everything) and any circuit that needs a clock generator.
Applications of BAW Frequency Sources
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. One of these radios with an MCU is the CC2652RB. These devices use the 2.4-GHz unlicensed band and employ a digital PLL with a digital VCO supplying the output. The PLL input reference is a BAW clock.
The BAW supplies the original signal to generate a 48-MHz PLL reference. The frequency error is less than 40 ppm over the full supply voltage range of 1.8 to 3.6 V and temperature range of −40 to +85°C range. It’s comparable to the performance of a 48-MHz crystal oscillator.
The BAW resonator does exhibit a slight increase in power consumption (<40 µA), but it doesn’t affect receiver sensitivity. The real bonuses are lower BOM cost and a smaller PCB footprint, leaving space for other parts and an improved layout.
Another application is to replace clocks in communications network equipment like routers and switchers. This equipment uses PLL synchronizers that detect valid clocks, filter the clock signals, and perform other functions. The reference source is usually a very stable but expensive temperature-compensated crystal oscillator (TCXO) or oven stabilized crystal oscillator (OCXO). These synchronizers can now be replaced with a new synchronizer IC from TI, the LMK05318. It integrates the BAW frequency references that can meet the jitter and noise standards of the ITU-T G.8262. This IC can comply with the strict <150 fs standard required by the forthcoming 400-Gb/s networks.