At the IEEE International Solid State Circuits Conference in San Francisco earlier this year, Texas Instruments revealed details of its on-chip bulk acoustic wave oscillator technology, that can replace external crystals in Bluetooth and other RF transceivers.

The firm is claiming better than ±30ppm frequency stability over -40°C to 85°C, package stress and 10 years of ageing.

The resonator is a ‘dual-Bragg bulk acoustic resonator’ (DBAR) fabricated on a silicon substrate.

Much like light in a laser, acoustic energy at a certain wavelength is trapped inside the resonating medium, a layer of aluminium nitride in this case (see diagram), by mirrors – acoustic Bragg reflectors made from alternating layers of silicon dioxide and titanium-tungsten alloy.

AlN is piezoelectric, allowing energy to be pumped in through a pair of electrodes like a standard crystal.

The 2.52GHz resonant frequency of the DBARs is temperature-dependent, and the first line of compensation for this is an additional mechanical layer in the structure which acts to eliminate much of its first-order temperature dependence across -40°C to 85°C, according to TI at ISSCC 2020.

When diced, the resonator is a 0.24mm2 solid block which can be moulded into a conventional non-hermetic no-void plastic IC package with no ill effects. Before moulding, TI first wire-bonds it on top of a 65nm CMOS chip which contains the rest of the circuitry needed to run the DBAR, which fits into 0.05mm2, alongside a microcontroller and a Bluetooth transceiver.

Driving the DBAR from the CMOS is a low-power oscillator that is fairly conventional except for an automatic level control that keeps the DBAR excitation at 300mV – a level which produces adequate phase noise at minimum power consumption.

To produce the 48MHz necessary to run a Bluetooth transceiver, the 2.52GHz is divided by 52.5 using a cascade of simple dividers plus a phase rotator to achieve the fractional divide (if you have not heard of a phase rotator, they are worth a web search).

The 48MHz is then fed into the phase-locked loop (PLL) that generates all the necessary ~2.4GHz Bluetooth carriers.

However, the 48MHz clock still has second-order and remnant first-order temperature dependencies.

ISSCC-2020-TI-BAW-oscillator-graph-hiresTo deal with these, the CMOS die includes a proportional-to-absolute-temperature (PTAT) sensor.

After the DBAR and CMOS are co-packaged, die thermometer and oscillator frequency readings at three temperatures are recorded on the CMOS.

In operation, these three pairs of readings are used to electronically compensate for remaining temperature effects (see graph left) by an on-board polynomial predictive algorithm that corrects the PLL multiplier value.

Power consumption is 1.1mW for the oscillator and 300μW for the divider.

To measure the effect of shock and vibration on the oscillator, TI has tested it to MIL-STD-883H and found that the BAW oscillator is one fourth as sensitive to shock as an equivalent crystal measured on the same board with the same CMOS radio chip.

ISSCC paper 3.1: ‘An integrated BAW oscillator with <±30ppm frequency stability over temperature, package stress, and aging suitable for high-volume production’