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arxiv: 2505.18388 · v3 · submitted 2025-05-23 · 📡 eess.SP

Practical Demonstrations of FR3-Band Thin-Film Lithium Niobate Acoustic Filter Design

Taran Anusorn , Omar Barrera , Jack Kramer , Ian Anderson , Ziqian Yao , Vakhtang Chulukhadze , Ruochen Lu This is my paper

Pith reviewed 2026-05-19 12:33 UTC · model grok-4.3

classification 📡 eess.SP
keywords thin-film lithium niobateacoustic filtersXBAR resonatorsFR3 bandladder filtersinsertion lossfractional bandwidth6G wireless
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The pith

Thin-film lithium niobate acoustic filters achieve 1.79 dB insertion loss at 20.5 GHz with 8.58 percent bandwidth and over 14.9 dB rejection in the FR3 band.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper shows how to set both the center frequency and the bandwidth of small acoustic filters built in thin-film lithium niobate by choosing the right film thickness and crystal orientation. The method uses first-order antisymmetric bulk-wave resonators whose frequency shifts with thickness and whose response varies with in-plane direction in 128-degree Y-cut material. A working three-element ladder prototype reaches low loss and controlled bandwidth while staying smaller than one square millimeter and rejecting signals outside the target band. An eight-element version demonstrates that adding more stages improves rejection further. The result matters because future wireless systems will need many such filters packed close together at frequencies above 20 GHz.

Core claim

The implemented three-element ladder filter prototype achieves an insertion loss of only 1.79 dB and a controlled 3-dB FBW of 8.58 percent at 20.5 GHz, with an out-of-band rejection greater than 14.9 dB across the entire FR3 band, while featuring a compact footprint of 0.90 by 0.74 square millimeters. An eight-element filter prototype shows an insertion loss of 3.80 dB, a fractional bandwidth of 6.12 percent at 22.0 GHz, and a high out-of-band rejection of 22.97 dB.

What carries the argument

The technique that uses the thickness-dependent resonant frequency of first-order antisymmetric lateral-field-excited bulk acoustic wave resonators together with the in-plane anisotropic properties of 128-degree Y-cut thin-film lithium niobate to set filter center frequency and fractional bandwidth.

If this is right

  • Adding more resonator stages in the ladder network increases out-of-band rejection while keeping the same center frequency and thickness-tuning method, as shown by the eight-element prototype.
  • The same thickness and anisotropy controls can be used to build multiple filters on one chip, each tuned to a different FR3 sub-band.
  • The compact footprint allows many such filters to be placed side by side without increasing the overall size of a radio front-end.
  • The approach extends to higher-order filters without requiring new materials or fabrication steps.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Monolithic filter banks built this way could reduce the number of separate chips needed in a 6G handset, lowering both size and power consumption.
  • Temperature or power-induced drifts in resonant frequency might require on-chip trimming circuits that were not tested in the prototypes.
  • The same resonators could be combined with thin-film lithium niobate electro-optic modulators on the same substrate to create integrated transmit-receive modules.

Load-bearing premise

Small changes in film thickness and crystal cut direction will produce predictable and repeatable shifts in resonator frequency and coupling strength that match the values needed for the target filter response.

What would settle it

Fabricate resonators with a range of film thicknesses around the design value, measure their actual resonant frequencies, and check whether the observed frequency shifts match the predicted thickness dependence within the tolerance required for the stated 8.58 percent bandwidth.

Figures

Figures reproduced from arXiv: 2505.18388 by Ian Anderson, Jack Kramer, Omar Barrera, Ruochen Lu, Taran Anusorn, Vakhtang Chulukhadze, Ziqian Yao.

Figure 1
Figure 1. Figure 1: (a) shows the transmission response of a three-element ladder filter, with one series and two shunt resonators, alongside their admittance profiles. At frequency f1, the shunt resonators act as grounds due to high admittance, creating a transmission zero (TZ). At frequency f2, the series resonator exhibits maximum admittance, while the shunt resonators have minimum admittance, enabling effective signal tra… view at source ↗
Figure 11
Figure 11. Figure 11: Extracted resonator parameters: (a) and (b) fs vs. θ with simulation-based predictions (dotted lines), (c) and (d) k 2 vs. θ with fitted curves (dashed lines), and (e) and (f) Qp vs. θ from study sets 1 and 2, respectively [PITH_FULL_IMAGE:figures/full_fig_p007_11.png] view at source ↗
Figure 19
Figure 19. Figure 19: Measured and fitted (dotted lines) TCF measurements of (a) the three-element prototype and its resonators and (b) the eight-element prototype and its resonators [PITH_FULL_IMAGE:figures/full_fig_p010_19.png] view at source ↗
read the original abstract

This article presents an approach to control the operating frequency and fractional bandwidth (FBW) of miniature acoustic filters in thin-film lithium niobate (TFLN). More specifically, we used first-order antisymmetric (A1) mode lateral-field-excited bulk acoustic wave resonators (XBARs) to achieve efficient operation at 20.5 GHz. Our technique leverages the thickness-dependent resonant frequency of A1 XBARs, combined with the in-plane anisotropic properties of 128$^\circ$ Y-cut TFLN, to customize filter characteristics. The implemented three-element ladder filter prototype achieves an insertion loss (IL) of only 1.79 dB and a controlled 3-dB FBW of 8.58% at 20.5 GHz, with an out-of-band (OoB) rejection greater than 14.9 dB across the entire FR3 band, while featuring a compact footprint of 0.90 $\times$ 0.74 mm2. Moreover, an eight-element filter prototype shows an IL of 3.80 dB, an FBW of 6.12% at 22.0 GHz, and a high OoB rejection of 22.97 dB, demonstrating the potential for expanding to higher-order filters. As frequency allocation requirements become more stringent in future FR3 bands, our technique showcases promising capability in enabling compact and monolithic filter banks toward next-generation acoustic filters for 6G and beyond.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript presents an approach to control operating frequency and fractional bandwidth (FBW) of miniature acoustic filters in thin-film lithium niobate (TFLN) for the FR3 band. It uses first-order antisymmetric (A1) mode lateral-field-excited bulk acoustic wave resonators (XBARs) in 128° Y-cut material, exploiting thickness-dependent resonance for frequency setting and in-plane anisotropy for FBW control. Fabricated prototypes are demonstrated: a three-element ladder filter achieves 1.79 dB insertion loss and 8.58% 3-dB FBW at 20.5 GHz with >14.9 dB out-of-band rejection in a 0.90 × 0.74 mm² footprint; an eight-element filter achieves 3.80 dB IL, 6.12% FBW at 22.0 GHz, and 22.97 dB rejection. The work positions these as enabling compact monolithic filter banks for 6G.

Significance. If the reported measured performance holds, the work is significant for providing direct experimental evidence of functional A1 XBAR-based filters at 20–22 GHz with competitive insertion loss, controlled bandwidth, and high out-of-band rejection in compact footprints. The physical fabrication and RF measurement results (rather than purely simulated) strengthen the demonstration of the thickness-and-anisotropy customization technique and its relevance to FR3-band requirements in next-generation wireless systems.

major comments (2)
  1. [Results section (prototypes)] Results section (prototypes): The headline metrics (IL = 1.79 dB, FBW = 8.58% at 20.5 GHz for the three-element filter; IL = 3.80 dB, FBW = 6.12% at 22.0 GHz for the eight-element filter) are reported as single-point values without error bars, measurement uncertainty, simulation-to-measurement overlay, or statistics across multiple devices. This weakens assessment of whether the thickness-dependent A1 resonance and anisotropy tuning are robust enough to support the claimed customization technique beyond these specific prototypes.
  2. [Design technique description] Design technique description: No sensitivity analysis, tolerance budget, or discussion of fabrication variations (e.g., film thickness spread of ~5–10 nm or orientation misalignment of 1–2°) is provided, even though the abstract and introduction present thickness dependence and 128° Y-cut anisotropy as the means to reliably customize frequency and FBW. Without such quantification, it remains unclear whether the achieved specs reflect general applicability or required exceptional process control.
minor comments (3)
  1. [Figures] Figure captions should explicitly state whether traces are measured data, simulated, or both, and include scale bars or annotations for the reported footprints.
  2. [Results] Consider adding a summary table comparing the two prototypes' key parameters (IL, FBW, rejection, center frequency, size) for clarity.
  3. [Introduction] Ensure all acronyms (XBAR, FBW, OoB, TFLN) are defined at first use in the main text.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments on statistical robustness and fabrication sensitivity. We address each major comment point by point below and indicate the planned revisions.

read point-by-point responses

  1. Referee: Results section (prototypes): The headline metrics (IL = 1.79 dB, FBW = 8.58% at 20.5 GHz for the three-element filter; IL = 3.80 dB, FBW = 6.12% at 22.0 GHz for the eight-element filter) are reported as single-point values without error bars, measurement uncertainty, simulation-to-measurement overlay, or statistics across multiple devices. This weakens assessment of whether the thickness-dependent A1 resonance and anisotropy tuning are robust enough to support the claimed customization technique beyond these specific prototypes.

    Authors: We agree that the presentation would benefit from additional context on variability. In the revised manuscript we will include simulation-to-measurement overlay plots for both filters and describe the RF measurement setup. We will also report the observed device-to-device variation from the measured samples in the current fabrication run. Comprehensive multi-device statistics were not collected in this proof-of-concept effort; this limitation will be stated explicitly. revision: partial

  2. Referee: Design technique description: No sensitivity analysis, tolerance budget, or discussion of fabrication variations (e.g., film thickness spread of ~5–10 nm or orientation misalignment of 1–2°) is provided, even though the abstract and introduction present thickness dependence and 128° Y-cut anisotropy as the means to reliably customize frequency and FBW. Without such quantification, it remains unclear whether the achieved specs reflect general applicability or required exceptional process control.

    Authors: We will add a dedicated paragraph in the design section that quantifies the effect of typical fabrication variations. Using wafer-level thickness uniformity data from our process (standard deviation ~5 nm), we will present simulated frequency and FBW shifts and show that the anisotropy-based bandwidth control remains effective within 1–2° orientation misalignment. This will demonstrate that the reported performance is achievable under standard process control. revision: yes

Circularity Check

0 steps flagged

No circularity: results are experimental measurements from fabricated devices

full rationale

The paper reports measured insertion loss, FBW, and rejection from physical three-element and eight-element ladder filter prototypes fabricated in 128° Y-cut TFLN using A1 XBARs. Performance numbers (1.79 dB IL at 20.5 GHz, 8.58% FBW, etc.) are direct RF measurement outcomes, not outputs of any derivation, equation, or model that reduces to its own inputs by construction. No self-definitional steps, fitted parameters renamed as predictions, or load-bearing self-citations appear in the provided text; the technique description relies on material properties and fabrication rather than circular logic.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central claims rest on standard domain knowledge of lithium niobate acoustic resonators and measured device behavior; no new entities are introduced and only routine design parameters are chosen to meet target frequencies.

free parameters (1)
  • Piezoelectric film thickness
    Thickness is selected to place the A1 resonance at the desired operating frequency; this is a standard design choice rather than a fitted constant in a derivation.
axioms (2)
  • domain assumption A1-mode lateral-field-excited XBARs exhibit resonant frequency that depends on piezoelectric film thickness.
    Invoked in the abstract to explain frequency control; this is a well-established property of the resonator geometry.
  • domain assumption 128° Y-cut TFLN possesses usable in-plane anisotropy that can be leveraged for filter response shaping.
    Stated as part of the customization technique; treated as a given material property.

pith-pipeline@v0.9.0 · 5825 in / 1656 out tokens · 70150 ms · 2026-05-19T12:33:26.324354+00:00 · methodology

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Reference graph

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