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arxiv: 2606.29790 · v1 · pith:TG4HDK5Snew · submitted 2026-06-29 · 🌌 astro-ph.IM · physics.ins-det

Design Method of Quasi-Lumped Element Bandpass Filters Using Superconducting Coplanar Waveguide for Millimeter-Wave Multichroic Imaging

Shinsuke Uno , Kah Wuy Chin , Tai Oshima , Satoshi Ono , Takeshi Sakai , Kazuki Watanabe , Shuhei Inoue , Tatsuya Takekoshi
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Kotaro Kohno
This is my paper

Pith reviewed 2026-06-30 04:28 UTC · model grok-4.3

classification 🌌 astro-ph.IM physics.ins-det
keywords bandpass filterquasi-lumped elementcoplanar waveguidesuperconducting filtermillimeter-waveChebyshev filteron-chip multiplexermultichroic imaging
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The pith

Quasi-lumped element filters can be designed with elements sized up to a quarter wavelength for accurate 150-270 GHz bandpass performance in superconducting coplanar waveguide.

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

The paper develops a design method for bandpass filters that relaxes the strict lumped-element size limit to a quarter wavelength to work within the constraints of photolithography and coplanar waveguide geometry. This approach still treats the elements as lumped for circuit synthesis while suppressing harmonics to limit crosstalk in multiplexers. The method produces practical 8th-order Chebyshev designs at 150, 220, and 270 GHz plus a triplexer, all intended for on-chip integration with photon detectors in multichroic millimeter-wave cameras. Validation comes from measurements on a scaled model of one filter.

Core claim

By defining a quasi-lumped element regime in which the largest circuit element reaches one-quarter wavelength, the authors obtain closed-form design solutions for 8th-order Chebyshev bandpass filters at 150, 220, and 270 GHz and for a triplexer, all realized in superconducting coplanar waveguide and shown to meet the inductance and capacitance values required after fabrication limits are applied.

What carries the argument

The quasi-lumped element approximation that treats circuit elements up to quarter-wavelength size as lumped for Chebyshev synthesis while retaining harmonic suppression.

If this is right

  • Compact on-chip bandpass filters become feasible for 150, 220, and 270 GHz bands.
  • An 8th-order triplexer can be realized on the same chip without excessive band-to-band crosstalk.
  • Large-format detector arrays can incorporate these filters while keeping the overall pixel footprint small.
  • Harmonic suppression from the quasi-lumped topology reduces the need for additional filtering stages.

Where Pith is reading between the lines

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

  • The same sizing rule could be tested at higher frequencies where the quarter-wavelength limit becomes even more restrictive.
  • Direct integration with transition-edge sensors or kinetic inductance detectors would test whether the filter footprint actually fits within a multichroic pixel.
  • Electromagnetic simulation of the full triplexer layout would quantify any additional parasitic coupling not captured by the lumped model.

Load-bearing premise

Parasitic distributed effects remain small enough that the filter response and crosstalk levels stay close to the lumped-element prediction even when elements reach a quarter wavelength.

What would settle it

A measured S-parameter response of a fabricated 150 GHz filter that deviates from the target Chebyshev passband shape or shows higher than expected transmission in the harmonic bands.

Figures

Figures reproduced from arXiv: 2606.29790 by Kah Wuy Chin, Kazuki Watanabe, Kotaro Kohno, Satoshi Ono, Shinsuke Uno, Shuhei Inoue, Tai Oshima, Takeshi Sakai, Tatsuya Takekoshi.

Figure 1
Figure 1. Figure 1: Equivalent lumped element circuit of an Nth-order BPF. The transformation follows the method of Nomura & Kobayashi (1996) [12]. TABLE I IDEAL LUMPED ELEMENT VALUES FOR THE 8TH-ORDER BPFS f0 BW Lr Ca01 Cb01 Cc01 Ca12 Cb12 Cc12 Ca23 Cb23 Cc23 Ca34 Cb34 Cc34 Ca45 Cb45 (GHz) (GHz) (pH) (fF) (fF) (fF) (fF) (fF) (fF) (fF) (fF) (fF) (fF) (fF) (fF) (fF) (fF) 150 45 50.3 4.9 44.1 22.6 62.6 44.1 16.8 35.5 17.8 32.6 … view at source ↗
Figure 2
Figure 2. Figure 2: Layout of the 8th-order 150 GHz quasi-lumped element BPF in Sonnet, reproduced from [ [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Equivalent lumped element circuits. (a) Short transmission line for a [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figure 5
Figure 5. Figure 5: Simulated frequency responses of the 150, 220, and 270 GHz BPFs. [PITH_FULL_IMAGE:figures/full_fig_p004_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: (a) Layout of the manifold multiplexer for the triplexer. (b) Simulated [PITH_FULL_IMAGE:figures/full_fig_p005_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: (a) Cross-sectional schematic of the scaled model chip. (b) Photograph [PITH_FULL_IMAGE:figures/full_fig_p006_7.png] view at source ↗
read the original abstract

An on-chip band-defining filter coupled with a superconducting photon detector is a promising technology for developing multi-band imaging cameras at millimeter and submillimeter wavelengths. In this paper, we present the design of on-chip bandpass filters based on coplanar waveguide geometry, which can be easily integrated into large-format multi-band detector arrays. A lumped element filter design is suitable not only for achieving a compact footprint but also for suppressing harmonics to reduce band-to-band crosstalk in a multiplexer. However, the coplanar waveguide geometry and the photolithography process rule limit the maximum available inductance and capacitance of lumped elements, which does not sufficiently meet the requirements of filter circuits. To overcome this limitation, we have established a design method for quasi-lumped element filters, in which the maximum element size is relaxed to a quarter wavelength, exceeding the ideal lumped element size. We achieved design solutions for 150, 220, and 270 GHz 8th-order Chebyshev bandpass filters and a triplexer. We also report on the measurement results of a scaled model of the bandpass filter, demonstrating the validity of our proposed filter design.

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 / 2 minor

Summary. The manuscript presents a design method for quasi-lumped element bandpass filters in superconducting coplanar waveguide (CPW) geometry for millimeter-wave multichroic imaging. By relaxing the maximum element size limit to λ/4 (exceeding conventional lumped-element constraints imposed by photolithography), the authors derive 8th-order Chebyshev bandpass filter solutions at 150, 220, and 270 GHz together with a triplexer implementation. They report a scaled-model measurement to support the validity of the approach for on-chip integration with superconducting detectors.

Significance. If the quasi-lumped approximation remains accurate at the target frequencies, the method would enable more compact on-chip filters that satisfy fabrication limits while maintaining harmonic suppression and low crosstalk, directly benefiting large-format multichroic detector arrays in millimeter-wave astronomy. The explicit designs for three bands and a triplexer illustrate practical applicability, though the absence of quantitative metrics limits immediate assessment of performance gains over existing approaches.

major comments (2)
  1. [Abstract and measurement results] Abstract and measurement section: the claim of 'successful designs' and 'demonstrating the validity' is not supported by reported quantitative metrics such as measured insertion loss, return loss, fractional bandwidth deviation from Chebyshev targets, or direct simulation-to-measurement comparison for the scaled model; without these, the central assertion that the λ/4 relaxation meets filter specifications cannot be evaluated.
  2. [Design method and scaled-model measurement] Design method and validation: the quasi-lumped approximation (elements up to λ/4) is load-bearing for the 150–270 GHz designs, yet the scaled-model data are acquired at lower frequencies where kinetic inductance variation, distributed phase shift, and parasitic radiation are weaker; no section provides a quantitative error analysis or full-wave simulation confirming that these effects remain negligible at the design frequencies.
minor comments (2)
  1. [Abstract] The abstract would be strengthened by inclusion of at least one key performance number (e.g., achieved return loss or crosstalk level) from the scaled model.
  2. [Design method] Notation for the size-relaxation rule and the mapping from lumped to quasi-lumped element values should be defined explicitly with an equation or table to improve reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments on our manuscript. We agree that strengthening the quantitative support for our claims will improve the paper and will revise accordingly. Below we respond point by point to the major comments.

read point-by-point responses

  1. Referee: [Abstract and measurement results] Abstract and measurement section: the claim of 'successful designs' and 'demonstrating the validity' is not supported by reported quantitative metrics such as measured insertion loss, return loss, fractional bandwidth deviation from Chebyshev targets, or direct simulation-to-measurement comparison for the scaled model; without these, the central assertion that the λ/4 relaxation meets filter specifications cannot be evaluated.

    Authors: We acknowledge that the abstract and measurement section do not currently include the specific quantitative metrics requested. In the revised manuscript we will add these values, including measured insertion loss and return loss at the scaled-model frequencies, the observed fractional bandwidth, its deviation from the target Chebyshev response, and direct overlay plots comparing measurement, electromagnetic simulation, and ideal lumped-element targets. These additions will allow readers to evaluate the performance of the λ/4-relaxed designs. revision: yes

  2. Referee: [Design method and scaled-model measurement] Design method and validation: the quasi-lumped approximation (elements up to λ/4) is load-bearing for the 150–270 GHz designs, yet the scaled-model data are acquired at lower frequencies where kinetic inductance variation, distributed phase shift, and parasitic radiation are weaker; no section provides a quantitative error analysis or full-wave simulation confirming that these effects remain negligible at the design frequencies.

    Authors: The referee is correct that the manuscript does not presently contain a quantitative error budget or full-wave simulations at the target frequencies. We will add a dedicated subsection that presents full-wave simulations of the 150/220/270 GHz filter layouts, quantifies the deviations caused by distributed effects, kinetic inductance, and radiation, and compares these to the quasi-lumped circuit model. This analysis will be performed at the design frequencies to demonstrate that the approximation remains within acceptable bounds for the intended application. revision: yes

Circularity Check

0 steps flagged

No significant circularity; design follows standard synthesis with independent validation

full rationale

The paper describes a design method extending standard 8th-order Chebyshev bandpass filter synthesis to allow element sizes up to λ/4 (quasi-lumped regime) for CPW geometry at 150-270 GHz, then reports achieving explicit designs for three bands plus a triplexer and validating via scaled-model measurements. No equations, parameters, or claims reduce the filter responses or crosstalk performance to quantities defined by the same data or by self-citation chains. The scaled-model results constitute external empirical support rather than a fitted input renamed as prediction. The derivation chain is therefore self-contained against external benchmarks (standard filter theory plus physical prototype data).

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The method rests on standard microwave filter synthesis (Chebyshev prototype) plus the quasi-lumped size relaxation rule. No new entities are postulated. Free parameters are the target center frequencies, bandwidths, and the quarter-wavelength size limit chosen to satisfy fabrication constraints.

free parameters (1)
  • maximum element size limit
    Set to quarter wavelength to meet required L and C values within photolithography rules.
axioms (1)
  • domain assumption Quasi-lumped element model remains valid when physical size reaches quarter wavelength at the design frequency
    Invoked to justify relaxing the lumped-element size constraint while preserving filter response.

pith-pipeline@v0.9.1-grok · 5773 in / 1160 out tokens · 13039 ms · 2026-06-30T04:28:54.985456+00:00 · methodology

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Reference graph

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