An edge-coupled magnetostatic bandpass filter

Abstract

The further development of 5G and 6G communication systems introduced new frequency allocations beyond 6 GHz, necessitating the development of compact bandpass filters that can operate over wide gigahertz frequency ranges. Herein, we report on the design, fabrication, and characterization of an edge-coupled magnetostatic forward volume wave bandpass filter (MSFVW). Using micromachining techniques, we fabricate both 2-pole and 4-pole filters from a yttrium iron garnet (YIG) film grown on a gadolinium gallium garnet (GGG) substrate with inductive transducers. By adjusting an out-of-plane magnetic field, we demonstrate linear center frequency tuning for a 4^th-order filter from 4.5 GHz to 10.1 GHz while retaining a fractional bandwidth of 0.3%, an insertion loss of 6.94 dB, and a - 35 dB rejection level. We characterize the filter nonlinearity in the passband and stopband with IIP3 measurements of - 4.85 dBm and 25.84 dBm, respectively. In this work, we demonstrate a compact octave tunable narrowband channel-select filter with a significant degree of design flexibility and performance comparable to the state-of-the-art.

Fig. 1. Tunable edge-coupled MSW bandpass filters.

a Chip microphotograph of multiple MSW bandpass filters and 1-port resonators fabricated on a YIG on GGG chip using YIG micromachining technology^,. A 4-pole filter (on the left) and a 2-pole filter (on the right) are highlighted in red. A 1-port resonator is highlighted in orange. b Rendering of a 4-pole bandpass filter featuring four YIG resonators with gold electrodes conformally deposited over the etched YIG. Magnetic bias is oriented out-of-plane along the z-axis.

Fig. 2

Lumped circuit model of an edge-coupled 4-pole MSW bandpass filter with electrically short transducers.

Fig. 3. Measured resonator impedance.

Frequency response of the 1-port resonator highlighted in Fig. 1a at 3962 Oe showing a Q = 2206 and $k_{eff}^2=1.53\%$.

Fig. 4. Filter tuning response and experimental setup.

a Measured 4-pole MSW bandpass filter frequency response at different out-of-plane magnetic biases from 3205 Oe to 5303 Oe. b Experimental setup showing the fabricated filter chip resting on the pole of an electromagnet, two GS probes connected to one device under test, and an optical microscope used for probe landing and device alignment.

Fig. 5. Measured filter passbands.

Frequency responses for the a 2-pole and b 4-pole filters highlighted in Fig. 1 near the passband at 3864 Oe. b also shows a comparison of the fitted lumped element model in Fig. 2 with measured S21. The fitted circuit assumes all four resonators are identical and excludes the spurious passbands caused by higher order MSFVW modes.

Fig. 6. Measured 3 dB bandwidth and center frequency of the 4-pole MSFVW filter.

The center frequency tunes rate of 2.7 MHz/Oe.

Fig. 7. Filter comparison with different metal thicknesses.

a Measured S21 and b total loss for 2-pole filters with 300 nm and 3 μm thick gold transducers biased at 3652 Oe and 3660 Oe, respectively. Frequency is plotted relative to the center frequency to account for the slight difference in bias strength.

Fig. 8. Two-tone IIP3 measurement in the passband of a 4-pole filter at 3652 Oe bias.

The two input tones are separated by 15 MHz with the high tone at 5.799 GHz.