A Sub-mW 18-MHz MEMS Oscillator Based on a 98-dB Ω Adjustable Bandwidth Transimpedance Amplifier and a Lamé-Mode Resonator

Abstract

This paper presents a microelectromechanical system (MEMS)-based oscillator based on a Lamé-mode capacitive micromachined resonator and a fully differential high-gain transimpedance amplifier (TIA). The proposed TIA is designed using TSMC 65 nm CMOS technology and consumes only 0.9 mA from a 1-V supply. The measured mid-band transimpedance gain is 98 dB Ω and the TIA features an adjustable bandwidth with a maximum bandwidth of 142 MHz for a parasitic capacitance C P of 4 pF. The measured input-referred current noise of the TIA at mid-band is below 15 pA/ Hz . The TIA is connected to a Lamé-mode resonator, and the oscillator performance in terms of phase noise and frequency stability is presented. The measured phase noise under vacuum is -120 dBc/Hz at a 1-kHz offset, while the phase noise floor reaches -127 dBc/Hz. The measured short-term stability of the MEMS-based oscillator is ±0.25 ppm.

Keywords: 1-dB compression point; Lamé-mode MEMS resonator; MEMS-based oscillator; electrostatic actuation; input-referred noise; phase noise; quality factor; transimpedance amplifier.

Figure 1

Simplified diagram of the (a) exploded and (b) assembled views of the Lamé-mode MEMS resonator with corner supports [1].

Figure 2

SEM micrograph of the Lamé-mode MEMS resonator with corner supports [1].

Figure 3

MEMS-based oscillator functional diagram.

Figure 4

Circuit schematic of: (a) the proposed fully differential TIA design, and (b) the AGC.

Figure 5

Simplified equivalent circuit of the RGC input stage used for noise analysis.

Figure 6

Test setup of the MEMS-based oscillator in open-loop (solid lines) and closed-loop (dashed lines) with micrographs of the TIA and resonator.

Figure 7

Normalized resonator transmission characteristic curves for various output input amplitude levels for (a) V${}_p$ = 100 V and (b) V${}_p$ = 200 V.

Figure 8

Measured TIA (a) gain and (b) bandwidth, for different values of $V_{CTRL\_A}$ and $V_{CTRL\_BW}$.

Figure 9

Measured TIA input-referred current noise.

Figure 10

Measured TIA gain for different input power levels, outlining the 1-dB compression point.

Figure 11

Measured open-loop gain and phase shift of the oscillator loop under vacuum at a polarization voltage of 100 V.

Figure 12

Measured phase noise in vacuum for polarization voltages of 100 V and 200 V.

Figure 13

MEMS oscillator signal short time stability at a 17.93 MHz central frequency (averaged over a five-minute timespan) with a polarization voltage of 100 V.