MEMS technology helps wireless devices get rid of crystal oscillators, piezoelectric vibrators or become new favorites

Tokyo Institute of Technology and Japan's Information and Communication Research Institute (NICT) have developed a circuit technology that allows wireless communication circuits to eliminate the use of crystal oscillators, using MEMS (Micro-Nano Electromechanical Systems) technology integrated into the chip. Vibrator.

Existing wireless communication circuits use a crystal oscillator in an LO (Local Oscillation Circuit) that provides a reference frequency of an RF (Radio Frequency) signal. For example, an LO transmitting hundreds of MH to several GHz RF signals is constituted by a PLL circuit capable of multiplying the output of a crystal oscillator such as 40 MHz. This technology eliminates the need for a high-precision and expensive 40MHz crystal oscillator. However, a clock composed of a 32 kHz crystal oscillator used in an embedded device or the like is required.

According to Ito Hiroyuki, an associate professor at the Institute of Future Industrial Technology at the Tokyo Institute of Technology, it is not to remove the 32 kHz crystal oscillator for RF signals because "the crystal oscillator for RF signals is still a subject in the design of wireless communication circuits." The 32kHz crystal oscillator is used in many devices such as MCUs and is inexpensive, so that it can be integrated into the IC or built into the IC package in the future.

Using a piezoelectric vibrator

This time, in order to remove the 40MHz crystal oscillator, a vibrator composed of MEMS technology that can be integrated into the IC is used. The vibrator prototype uses an external piezoelectric element having a resonance frequency of several GHz.

The newly developed circuit consists of a piezoelectric vibrator and a two-stage clock circuit using a 32 kHz crystal oscillator. The former stage is a clock generating circuit composed of a piezoelectric vibrator, and has a function of detecting a frequency variation and a phase shift of the piezoelectric element with reference to a 32 kHz clock source. The latter stage is a clock adjustment circuit that compensates for frequency variations and phase shifts. The frequency variation and phase shift are controlled within the requirements of the wireless communication circuit.

Another advantage of using a piezoelectric vibrator is that it is easy to obtain the low phase shift (phase noise) required by the wireless communication circuit. Since the frequency of the crystal output signal (the resonance frequency of the vibrator) depends on the shape, it is not easy to increase. When used for RF signals such as GHz and tens of GHz, the multiplication ratio is increased, so the accuracy (phase noise) may be deteriorated. MEMS products such as piezoelectric vibrators are easy to achieve a resonance frequency of GHz, which can suppress signal degradation caused by multiplication.

The piezoelectric vibrator of the prototype is commercially available. The two-stage clock circuit uses TSMC's 65nm process product.

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