Frequency tunable magnetostatic wave filters with zero static power magnetic biasing circuitry

Abstract

A single tunable filter simplifies complexity, reduces insertion loss, and minimizes size compared to frequency switchable filter banks commonly used for radio frequency (RF) band selection. Magnetostatic wave (MSW) filters stand out for their wide, continuous frequency tuning and high-quality factor. However, MSW filters employing electromagnets for tuning consume excessive power and space, unsuitable for consumer wireless applications. Here, we demonstrate miniature and high selectivity MSW tunable filters with zero static power consumption, occupying less than 2 cc. The center frequency is continuously tunable from 3.4 GHz to 11.1 GHz via current pulses of sub-millisecond duration applied to a small and nonvolatile magnetic bias assembly. This assembly is limited in the area over which it can achieve a large and uniform magnetic field, necessitating filters realized from small resonant cavities micromachined in thin films of Yttrium Iron Garnet. Filter insertion loss of 3.2 dB to 5.1 dB and out-of-band third order input intercept point greater than 41 dBm are achieved. The filter's broad frequency range, compact size, low insertion loss, high out-of-band linearity, and zero static power consumption are essential for protecting RF transceivers from interference, thus facilitating their use in mobile applications like IoT and 6 G networks.

Fig. 1. Tunable bandpass filter with magnetic biasing circuit.

The magnetostatic wave filters (MSWF) were placed in the center of the magnetic biasing component. The aluminum transducers were placed on top of the yttrium iron garnet (YIG) cavity. The magnetic biasing component consists of two permanent magnets, two shunt magnets wrapped with coils, and two magnetically permeable yokes which concentrate the magnetic flux in the MSWF. a Reconfigurable MSWF concept: The primary function of a radio-frequency (RF) filter is to selectively allow certain frequencies to pass while blocking others. With the implementation of a suitable RF filter, the amplitude of the out-of-band interfering signal is significantly reduced compared to the desired signal. This feature is particularly important in scenarios where the interference is much larger than the intended signal at the receive antenna. Moreover, the high out-of-band linearity of our filter plays a vital role in ensuring that intermodulation products generated by interfering signals do not adversely impact the desired signal. b Optical microscope image of the fabricated device assembly. c Scanning Electron Microscope image showing the aluminum transducers on top of the YIG cavity. This device has a width (W) of 200 μm and length (L) of 70 μm. d Summary of device schematic diagrams and equivalent single-mode circuit models of MSWF and magnetostatic wave resonator (MSWR).

Fig. 2. Circuit modeling for magnetostatic wave resonator (MSWR).

a Multi-mode circuit model for MSWR. b Comparison of the impedance magnitude of the measured MSWR, single mode circuit model, and multi-mode circuit model. c Comparison of impedance phase of the measured MSWR, single mode circuit model, and multi-mode circuit model. The series resonance frequency, f_s and parallel resonance frequency, f_p, of the single-mode circuit model have been labeled. This MSWR is with W = 200 μm and L = 70 μm with Al transducer width of 4 μm. The device is measured at an applied bias field of 500 Gauss.

Fig. 3. Comparison of magnetostatic wave resonator (MSWR) for various YIG cavity widths.

The effect of yttrium iron garnet (YIG) cavity width on (a), the device coupling coefficient, (b), the device Quality factor (Q-factor), (c), device figure of merit (FoM), (d), the magnetostatic resistance, R_m.

Fig. 4. S₁₂ Frequency response of the magnetostatic wave filters (MSWF) at different magnetic flux density (B).

a The impact of yttrium iron garnet (YIG) cavity width on the frequency response with a constant length of 70 μm. b The influence of YIG cavity length on the frequency response with a constant width of 150 μm.

Fig. 5. Capacitor voltage and coil current during capacitor discharge.

During the capacitor discharge, a current pulse was applied to the coil to magnetize/demagnetize the AlNiCo magnets.

Fig. 6. Integrated device.

a Measured magnetic flux density under different capacitor charging voltages. b Insertion loss vs. frequency comparison for the magnetostatic wave filters (MSWF) measured under an external magnetic field generated by an electromagnet and the magnetic biasing circuit. c MSWF response measured with the magnetic field supplied by an electromagnet on a magnetic probe station. d MSWF response measured with the magnetic field supplied by the zero DC power tunable magnetic biasing circuit. e Zoomed in ${\mathrm{S}}_{12}$ frequency response of MSWF measured with magnetic field supplied by the tunable magnetic biasing circuit.