MEMS Switch Enabled Spatiotemporally Modulated Isolators

TL;DR

This work tackles the lack of ferrite circulators in ultrasonic underwater acoustic channels by introducing a magnet-free, spatiotemporally modulated (STM) isolator built from MEMS switches. The authors design and implement a 3rd-order lumped-element STM bandpass filter where three LC resonators are periodically loaded with a modulated capacitance $C_m$ via MEMS switches, driven at frequency $\omega_m$ with phase progression $\phi_m$, yielding nonreciprocity and enabling self-interference cancellation (SIC) in in-band full-duplex UWAC. The center frequency follows $\omega_0 = \frac{1}{\sqrt{L_r (C_r + D C_m)}}$ and the modulation strength is $\xi = \frac{C_m}{C_r}$, with optimization guided by spectral admittance matrix (SAM) analysis and harmonic balance methods. A PCB prototype using MM5230 MEMS switches achieves a maximum isolation of $15.99$ dB at $62\text{kHz}$ modulation, in good agreement with SAM predictions, and demonstrates a path toward higher center frequencies (up to $0.6$ MHz) with improvements in modulation strength and loss reduction. These results suggest magnet-free STM isolators can enable in-band full-duplex UWAC when integrated with digital SIC, and they outline design strategies (higher order, harmonic beamforming) to reduce modulation requirements and expand the applicable frequency range.

Abstract

This work reports the simulation, design, and implementation of a compact MEMS switch based spatiotemporally modulated (STM) bandpass filtering isolator to improve self-interference cancellation (SIC) in underwater acoustic communication networks. Conventional ferrite circulators are unavailable in ultrasonic frequency ranges limiting SIC to techniques such as spatial cancellation and adaptive digital cancellation. This study details a sub-megahertz electronic non-magnetic filtering isolator. High power-handling, compact, and reliable MEMS switches enable the periodically time varying filter circuit to be non-reciprocal. A printed circuit board (PCB) implementation shows strong agreement with spectral admittance matrix simulations with a maximum measured isolation of 15.99 dB. In conjunction with digital SIC methods, this isolator can enable in-band full duplex underwater communication, environmental sensing, and imaging.

Figures (14)

• Figure 1: (a) PCB implementation of a $3^{rd}$-order lumped element STM bandpass isolator using packaged (b) Menlo Microsystems, Inc. MEMS switches. The active isolator area highlighted in red measured $32.9mm\times 11.5mm$. The single-pole, four-throw MEMS switch is configured with two gates shorted together, routing the signal from RF 1 to RF 2 as indicated in (b). The remaining two throws are unused.
• Figure 2: Circuit diagram for a lumped-element $3^{rd}$-order STM isolator. $L_r$ and $C_r$ form identical resonators and $C_{ki}$ are coupling capacitors chosen to realize a $0.05dB$ Chebyshev filter response. $C_m$ represents the modulation capacitors which are periodically connected to the LC-resonator using Menlo Microsystems switches.
• Figure 3: Simulated isolation for a $3^{rd}$-order lossless STM isolator as the (a) modulation frequency ($\omega_m$) and (b) modulation strength ($\xi$) are swept with $\phi_m=60^\circ$. In (a)$\xi$ is fixed at $0.3$ and in (b)$\omega_m/\omega_0$ is fixed at $0.1167$. A normalized modulation frequency of $\omega_m/\omega_0=0.1167$ is chosen to maximize isolation and isolation bandwidth while minimizing forward transmission loss.
• Figure 4: Accounting for component losses in the isolator simulations from the inductors' quality factor ($Q=70$) and resistance ($R_{dc}=0.824Ω$) as well as the switches' on-resistance ($R_{switch}=1.5Ω$) shows that the required modulation strength for optimal isolation increases by $67\%$ as compared to the lossless case from Fig. \ref{['3rdOrder_Ideal_Isolator_ModStrSweep']}.
• Figure 5: Experimental setup to measure the forward and reverse transmission response of the $3^{rd}$-order STM isolator. A Siglent SDG2042X and a Keysight 33220A arbitrary waveform generators with a shared $10MHz$ clock generate the low voltage modulation square waves. Two TEGAM 2350 high-voltage amplifiers are used to increase the modulation voltage above the switches' pull-in voltage to $90V$. A HF2LI Zurich lock-in amplifier measures the STM isolator's transmission response.