Millimeter wave band-pass filter

Abstract

A coplanar waveguide based (CPW based) millimeter wave band pass filter is disclosed. The filter may include a substrate, first and second ground metal plates, an input signal transmission line and an output signal transmission line, and four half-wave CPW resonators. A T-slot or I-slot is optionally embedded in each CPW resonator to improve suppression of the upper stop band. Optionally, the filter may also include two T-stubs connected to the first and second ground metal plates, respectively, to reduce the filter size and produce transmission zeros in the lower stop band. Optionally, the filter may also include a cross-shaped dual-mode resonator in the central region of the CPW plane to increase the bandwidth of the filter. The millimeter wave band-pass filter based on the CPW can realize low passband insertion loss, high out-of-band rejection, small size and low cost.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to and the benefit of U.S. Provisional Patent Application Serial No. 63 / 636,122, filed April 19, 2024, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to the field of millimeter wave (mmWave) bandpass filters. Background Art

[0004] With data rates increasing to 1Tbps, latency less than 0.1 milliseconds, and large capacity, fifth- to sixth-generation (5G-6G) wireless communication systems are expected to be commercially available around 2030. In the era of big data, 5G-6G wireless communication systems can ultimately enable the Internet of Everything (IoE). Portable smart terminals, especially smartphones, are widely used in 5G-6G wireless communication systems. Modern smartphones typically integrate dozens of filters in their circuits. Portable smart terminals or IoE devices require miniaturized millimeter-wave filters with strict size constraints. However, millimeter-wave filter chips with low passband insertion loss, high stopband suppression, high roll-off, and small size remain challenging and are not available globally. Summary of the Invention

[0005] In one aspect of the disclosure, a coplanar waveguide (CPW) based millimeter wave bandpass filter is provided. The CPW based millimeter wave bandpass filter includes a substrate, a first ground metal plate and a second ground metal plate each coupled to a ground and disposed on a rectangular CPW plane of a top surface of the substrate, an input signal transmission line and an output signal transmission line each located at two ends of a signal region in a longitudinal direction and each aligned with a first midline, a first and a second CPW resonator each longitudinally arranged between the first ground metal plate and the intermediate signal region, and a third and a fourth CPW resonator each longitudinally arranged between the second ground metal plate and the intermediate signal region. The first and the second ground metal plates continuously extend along the longitudinal direction of the CPW plane from a first end of the CPW plane to a second end of the CPW plane and are located on opposite sides of the CPW plane in a transverse direction of the CPW plane, thereby defining the signal region between the first ground metal plate and the second ground metal plate. The signal region continuously extends in the longitudinal direction. The first midline is a midline of the CPW plane in the longitudinal direction. The input and the output signal transmission lines are symmetrically arranged about a second midline. The second midline is a midline of the CPW plane in the transverse direction. The input signal transmission line is separated from the output signal transmission line by a central transverse gap. The intermediate signal region is composed of the input signal transmission line, the output signal transmission line, and the central transverse gap. The first CPW resonator is separated from the second CPW resonator by a first transverse gap. The first transverse gap is aligned with the second midline. In the transverse direction, the first and the second CPW resonators are separated from the intermediate signal region by a first longitudinal gap and from the first ground metal plate by a second longitudinal gap. The third CPW resonator is separated from the fourth CPW resonator by a second transverse gap. The second transverse gap is aligned with the second midline. In the transverse direction, the third and the fourth CPW resonators are separated from the intermediate signal region by a third longitudinal gap and from the second ground metal plate by a fourth longitudinal gap. The first CPW resonator and the third CPW resonator are symmetrically arranged about the first midline. The second CPW resonator and the fourth CPW resonator are symmetrically arranged about the first midline. The first CPW resonator and the second CPW resonator are symmetrically arranged about the second midline. The third CPW resonator and the fourth CPW resonator are symmetrically arranged about the second midline.

[0006] Additionally or alternatively, in the longitudinal direction, a total length of the first and the second CPW resonators and the first transverse gap is greater than a length of the central transverse gap but less than a length of the CPW plane. In the longitudinal direction, a total length of the third and the fourth CPW resonators and the second transverse gap is greater than the length of the central transverse gap but less than the length of the CPW plane.

[0007] Additionally or alternatively, the first, second, third, and fourth T-shaped slots are embedded into the first, second, third, and fourth CPW resonators, respectively. The first and third T-shaped slots are symmetrically arranged about the first midline. The second and fourth T-shaped slots are symmetrically arranged about the first midline. The first and second T-shaped slots are symmetrically arranged about the second midline. The third and fourth T-shaped slots are symmetrically arranged about the second midline. A height of each of the first, second, third, and fourth T-shaped slots embedded into its respective CPW resonator is equal to a thickness of the respective CPW resonator. Each T-shaped slot includes a longitudinal slot extending along a longitudinal midline of the respective CPW resonator and a transverse slot extending from a midpoint of a long side of the longitudinal slot to a midpoint of a long side of the respective CPW resonator oriented away from the intermediate signal region. Each T-shaped slot shapes the respective CPW resonator into a rectangular ring including a notch at a midpoint of a long side of the rectangular ring and the notch is oriented away from the intermediate signal region.

[0008] Additionally or alternatively, each T-shaped slot is symmetric about a transverse midline of the respective CPW resonator.

[0009] Additionally or alternatively, transmission zeros in the upper stopband are achievable by the first, second, third, and fourth T-shaped slots.

[0010] Additionally or alternatively, the CPW-based millimeter-wave bandpass filter further includes a first T-shaped stub located in the second longitudinal gap and connected to the first ground metal plate and a second T-shaped stub located in the fourth longitudinal gap and connected to the second ground metal plate. In the transverse direction, the first T-shaped stub is separated from the first and second CPW resonators by a fifth longitudinal gap. In the transverse direction, the second T-shaped stub is separated from the third and fourth CPW resonators by a sixth longitudinal gap. The first and second T-shaped stubs are symmetrically arranged about the first midline. Each of the first and second T-shaped stubs includes a longitudinal segment and a transverse segment. The transverse segment of the first T-shaped stub extends from a midpoint of a long side of the first ground metal plate oriented closer to the first and second CPW resonators to a midpoint of a long side of the longitudinal segment of the first T-shaped stub. The transverse segment of the second T-shaped stub extends from a midpoint of a long side of the second ground metal plate oriented closer to the third and fourth CPW resonators to a midpoint of a long side of the longitudinal segment of the second T-shaped stub.

[0011] Additionally or alternatively, in the longitudinal direction, a length of the longitudinal segment of the first T-shaped stub is greater than a total length of the first and second CPW resonators and the first transverse gap but less than a length of the CPW plane. In the longitudinal direction, a length of the longitudinal segment of the second T-shaped stub is greater than a total length of the third and fourth CPW resonators and the second transverse gap but less than a length of the CPW plane.

[0012] Additionally or alternatively, each T-shaped stub is symmetric about the second midline.

[0013] Additionally or alternatively, the first and second T-shaped stubs can achieve two transmission zeros in the lower stopband.

[0014] Additionally or alternatively, the CPW-based millimeter-wave bandpass filter further includes a cross-shaped dual-mode resonator. The cross-shaped dual-mode resonator includes a longitudinal arm located in a central lateral gap and extending along a first midline and a lateral arm extending along a second midline. The longitudinal arm is separated from the input signal transmission line by a third lateral gap and separated from the output signal transmission line by a fourth lateral gap. The longitudinal arm is symmetric about the first midline and the second midline. The third lateral gap and the fourth lateral gap are each symmetric about the first midline. The third lateral gap and the fourth lateral gap are symmetrically arranged about the second midline. The lateral arm is separated from the first ground metal plate by a seventh longitudinal gap and separated from the second ground metal plate by an eighth longitudinal gap. The lateral arm is symmetric about the first midline and the second midline. The seventh longitudinal gap and the eighth longitudinal gap are each symmetric about the second midline. The seventh longitudinal gap and the eighth longitudinal gap are symmetrically arranged about the first midline. The seventh longitudinal gap extends deep into the interior of the first ground metal plate such that a first inward recess is formed near the midpoint of the longer side of the first ground metal plate oriented closer to the lateral arm. The eighth longitudinal gap extends deep into the interior of the second ground metal plate such that a second inward recess is formed near the midpoint of the longer side of the second ground metal plate oriented closer to the lateral arm.

[0015] Additionally or alternatively, the first, second, third, and fourth I-shaped slots are respectively embedded into the first, second, third, and fourth CPW resonators. The first I-shaped slot and the third I-shaped slot are symmetrically arranged about the first midline. The second I-shaped slot and the fourth I-shaped slot are symmetrically arranged about the first midline. The first I-shaped slot and the second I-shaped slot are symmetrically arranged about the second midline. The third I-shaped slot and the fourth I-shaped slot are symmetrically arranged about the second midline. The height of each of the first, second, third, and fourth I-shaped slots embedded into its respective CPW resonator is equal to the thickness of the respective CPW resonator. Each I-shaped slot includes a single lateral slot extending from the midpoint of a first longer side of the respective CPW resonator oriented away from the intermediate signal region toward the midpoint of a second longer side of the respective CPW resonator oriented closer to the intermediate signal region, but not extending through the second longer side. Each I-shaped slot shapes the respective CPW resonator into a notched rectangular strip with the notch located at the midpoint of the longer side. The notch is oriented away from the intermediate signal region.

[0016] Additionally or alternatively, the bandwidth of the filter is increased by adding the cross-shaped dual-mode resonator.

[0017] Additionally or alternatively, the substrate is a lithium niobate (LiNbO3) substrate.

[0018] Other example embodiments are discussed herein. BRIEF DESCRIPTION OF DRAWINGS

[0019] Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings:

[0020] Figure 1 An RF system including a millimeter-wave filter for dividing a frequency band according to an embodiment of the present disclosure is illustrated.

[0021] Figure 2 A side view of a CPW-based millimeter-wave bandpass filter according to an embodiment of the present disclosure is illustrated.

[0022] Figure 3A A top view of a CPW-based millimeter-wave bandpass filter according to an embodiment of the present disclosure is illustrated.

[0023] Figure 3B Simulation S parameters of a filter in Figure 3A

[0024] A top view of a third T-shaped slot in Figure 4A

[0025] A perspective view of a filter in Figure 4B Figure 4A A top view of a third T-shaped slot in

[0026] Figure 4C Figure 4A Simulation S parameters of a filter in

[0027] Figure 5A A top view of a CPW-based millimeter-wave bandpass filter according to an embodiment of the present disclosure is illustrated.

[0028] Figure 5B A perspective view of a filter in Figure 5A

[0029] A symbol of a size of each element of a filter in Figure 5C Figure 5A Simulation results of a frequency response varying with a length (L3) of a longitudinal slot of each T-shaped slot in a filter of

[0030] Figure 5D Simulation results of a center frequency varying with a length (L2) of each CPW resonator in a longitudinal direction in a filter of Figure 5A

[0031] Simulation results of a center frequency varying with a length (L2) of each CPW resonator in a longitudinal direction in a filter of Figure 5E Figure 5A

[0032] Figure 5F Simulation results of a center frequency varying with a length (L2) of each CPW resonator in a longitudinal direction in a filter of​​​​​Figure 5A Simulation results of passband bandwidth in the filter of

[0033] Figure 5G Simulation results of frequency response in the filter of Figure 5A

[0034] Figure 5H Simulation results of frequency response in the filter of Figure 5A

[0035] Figure 5I Simulation and measured S-parameters of the filter in Figure 5A

[0036] Group delay of the filter in Figure 5J Figure 5A

[0037] Figure 6A Top view of a CPW-based millimeter-wave bandpass filter according to certain embodiments of the present disclosure.

[0038] Figure 6B Top view of a third I-shaped slot in Figure 6A

[0039] Figure 6C Simulation S-parameters of the filter in Figure 6A

[0040] In the drawings, like reference numerals are used to refer to like elements throughout the specification and the drawings, to help understand. DETAILED DESCRIPTION

[0041] The present disclosure will now be described with reference to the following examples, which should be regarded in an illustrative and non-limiting capacity. In the drawings, corresponding features in the same embodiments or common features in different embodiments are given the same or similar reference numerals.

[0042] Throughout the specification and claims, the word “comprise” or “comprising”, and other forms such as “comprises” or “comprising”, and include and the like are not intended to exclude the possibility of any additionally item, parameter or the like. That is, the meaning of “comprising” includes that of “including but not limited to” or equivalently “has as an element”.

[0043] ​​​​​​Also, as used herein, the use of the ordinal adjectives “first,” “second,” etc., to describe a common object simply indicates that different instances of like objects are being referred to and does not imply that the objects must appear in the given order, whether spatially, temporally, rank-wise, etc.

[0044] A. Technical terms and notations

[0045] The following terms and notations are used in the specification and appended claims.

[0046] “Microwave,” “radio frequency (RF),” “millimeter wave (mmWave):” Microwave is a form of electromagnetic (EM) radiation with a wavelength ranging from about 1 meter to 1 millimeter, corresponding to frequencies between 300 MHz and 300 GHz, broadly understood. In RF engineering, the more common definition is the range between 20 k and 300 GHz (wavelength between 1 millimeter and 15 kilometers). Millimeter wave is a type of electromagnetic wave with a wavelength ranging from 1 millimeter to 10 millimeters, and a frequency between 30 GHz and 300 GHz.

[0047] “Electromagnetic filter (EM filter):” An EM filter is a device or circuit designed to selectively pass or block specific frequency components of an EM signal. It is used to filter out unwanted frequencies while allowing desired frequencies to pass. Commonly used manufacturing techniques for RF filters include monolithic microwave integrated circuit (MMIC), low-temperature co-fired ceramic (LTCC), and printed circuit board (PCB). A wide variety of manufacturing process technologies are used, including silicon microelectromechanical systems (Si MEMS), gallium arsenide (GaAs), GaAs MEMS, silicon benzocyclobutene (Si BCB), silicon germanium (SiGe), integrated passive device (IPD), liquid crystal polymer (LCP), and complementary metal-oxide-semiconductor (CMOS).

[0048] Bipolar CMOS (Bi-CMOS) is a semiconductor technology that integrates two semiconductor technologies (bipolar junction transistors (BJT) and CMOS logic gates) into a single integrated circuit (IC).

[0049] “Coplanar waveguide (CPW):” A CPW is an electrical plane transmission line that can be manufactured using PCB technology for the transmission of microwave frequency signals. It is a transmission line that allows high-frequency signals to propagate with minimal loss and interference. A CPW consists of a central signal conductor (strip) flanked by two ground planes on the same plane. All three conductors are on the same side of the substrate, hence coplanar. The signal conductor and ground planes are separated by a dielectric substrate.

[0050] “Substrate Integrated Waveguide (SIW)”: SIW is a modern waveguide technology that combines the advantages of traditional rectangular waveguides and planar transmission lines (such as microstrip or CPW). It is implemented within a dielectric substrate, making it compatible with standard PCB manufacturing processes. SIW structures are widely used in microwave and millimeter-wave applications due to their high performance, compact size, and ease of integration.

[0051] “Silicon on Insulator (SOI)”: SOI is a semiconductor fabrication technology that involves creating a layered structure consisting of a thin silicon layer on top of an insulating layer (typically silicon dioxide (SiO2)), which is then placed on a silicon substrate. This unique structure offers significant advantages over traditional bulk silicon technology, particularly in terms of performance, power efficiency, and integration capabilities. SOI is widely used in the fabrication of ICs and MEMS.

[0052] “Lithium Niobate (LiNbO3)”: LiNbO3 or LN is widely used as a substrate material in photonics, acousto-optics, and microwave applications due to its excellent electro-optic, piezoelectric, and nonlinear optical properties. After crystal growth, it is cut into wafers with different orientations relative to the crystal axes. Common orientations include Z-cut, X-cut, Y-cut, and cuts at rotation angles relative to the aforementioned axes.

[0053] “Piezoelectric Effect”: The piezoelectric effect or piezoelectricity is a property of certain materials that allows them to generate an electric charge (positive piezoelectric effect) in response to an applied mechanical stress, or to undergo mechanical deformation (inverse piezoelectric effect) in response to an applied electric field. This effect is reversible and can be observed in materials with non-centrosymmetric crystal structures, such as quartz, LiNbO3, and lead zirconate titanate (PZT).

[0054] “Acoustic Filter”: An acoustic filter is a device that selectively passes or blocks certain frequency components of a signal using acoustic waves. These filters are widely used in telecommunications, signal processing, and sensing applications. Acoustic filters are typically implemented using piezoelectric materials (such as quartz, LiNbO3, or aluminum nitride (AlN)) that convert electrical signals into mechanical vibrations (acoustic waves) and vice versa. There are several types of acoustic filters: surface acoustic wave (SAW) filters, which use acoustic waves propagating along the surface of a piezoelectric substrate; bulk acoustic wave (BAW) filters, which use acoustic waves propagating through the bulk of a piezoelectric material, and can offer higher frequencies compared to SAW filters.

[0055] “Hybrid Microwave Filter”: A hybrid microwave filter is an innovative development through the joint design of EM and acoustic filters. Hybrid filters achieve wideband performance while retaining the advantages of compact size and high quality factor inherent in the acoustic domain.

[0056] “Passband”, “Upper stopband”, “Lower stopband”: Passband is the range of frequencies or wavelengths that can pass through a filter. Stopband is the range of frequencies where a filter significantly reduces or suppresses a signal. Upper stopband refers to the range of frequencies above the passband where a filter attenuates a signal. Lower stopband refers to the range of frequencies below the passband where a filter attenuates a signal.

[0057] “Center frequency”: Center frequency refers to the middle frequency of a passband or resonant frequency range in a filter, oscillator, or any frequency-selective system. Mathematically, the center frequency (f c ) can be calculated as the geometric mean of the lower cutoff frequency (f1) and the upper cutoff frequency (f2) in a bandpass filter:

[0058]

[0059] “Fractional bandwidth (FBW)”, “3-dB FBW”: FBW is a dimensionless measure used to describe the bandwidth of a system or component relative to its center frequency. It is expressed as a ratio or percentage and is particularly useful for comparing the bandwidth performance of systems operating at different center frequencies. FBW provides insight into the relative width of the frequency range over which a system operates effectively. 3-dB FBW (or 3-dB relative bandwidth) is a measure used to describe the width of a frequency range in a filter or resonant system. The term “3-dB” refers to the point at which the signal power reduces to half of its maximum value, as a 3-dB drop corresponds to a 50% reduction in power. Mathematically, FBW is calculated as follows:

[0060]

[0061] where f1 and f2 are the lower and upper cutoff frequencies, respectively.

[0062] “Insertion loss (IL)”: IL is a key performance indicator used to describe the reduction in signal power when a component such as a filter is inserted into a transmission line or system. It is typically measured in dB and represents the loss in signal strength due to the introduction of the component. Mathematically, IL is represented as:

[0063]

[0064] where P in is the input power, and P out is the output power.

[0065] “Stopband rejection level”: Stopband rejection level refers to the degree of attenuation or reduction of signals within the stopband of a filter or frequency-selective system. It quantifies how effectively a filter blocks or suppresses unwanted frequencies outside its passband, typically measured in dB.

[0066] “Return loss”: Return loss quantifies the amount of signal power that is reflected back to the source due to impedance mismatch at the input or output port of a filter. It is typically expressed in decibels (dB) as the ratio of the input signal power to the reflected signal power. The higher the return loss value (closer to infinity), the better the performance.

[0067]

[0068] where P 入射 represents the power of the incident signal, and P 反射 represents the power of the reflected signal.

[0069] “Transmission zero (TZ)”: TZ is the frequency at which the transfer function of a filter or network exhibits a sharp drop in signal transmission, resulting in almost complete attenuation of the signal. In other words, it is the frequency point at which the output signal power decreases significantly, ideally to zero, indicating that the filter effectively blocks or rejects the signal at that particular frequency.

[0070] “Group delay”: Group delay is a measure of the time delay experienced by different frequency components of a signal as it passes through a system, such as a filter, amplifier, or communication channel. It is defined as the negative derivative of the phase response with respect to the angular frequency, and is typically measured in seconds. Group delay provides insight into the phase distortion characteristics of a system, which is crucial for maintaining signal integrity. Mathematically, group delay (τ g ) is expressed as:

[0071]

[0072] where, is the phase response of the system as a function of angular frequency (w).

[0073] “Roll-off rate”: Roll-off rate refers to the rate at which the frequency response of a filter or system decreases outside its passband. It is a measure of how quickly the signal attenuates as the frequency moves away from the cutoff frequency, typically expressed in dB per decade (dB / decade) or dB per octave (dB / octave). Roll-off rate is a critical parameter in filter design, as it determines the filter's ability to distinguish between the desired signals within the passband and the unwanted signals in the stopband.

[0074] “Scattering parameters (S-parameters)”: S-parameters are fundamental tools used in filter simulation and analysis, especially in high-frequency and microwave engineering. They describe how electrical signals propagate through a network (such as a filter) by quantifying the relationship between the incident and reflected waves at each port of the device. S-parameters are a set of complex numbers representing the amplitude and phase of the output signal relative to the input signal. For a two-port network (such as a filter), S-parameters are typically represented as a matrix:

[0075]

[0076] S 11 is the reflection coefficient of the input port. S 12 is the transmission coefficient from the output port to the input port. S 21 is the transmission coefficient from the input port to the output port. S 22 is the reflection coefficient of the output port. S parameters are usually depicted in magnitude (in dB) and phase (in degrees) versus frequency. Common depictions include S 21 for visualizing the frequency response (passband and stopband) of a filter, S 11 for evaluating input matching and reflection characteristics.

[0077] B. Research objectives

[0078] Wireless communication terminals require filter chips with low IL on the passband, high stopband rejection level, high roll-off degree, and small size. Acoustic filters are much smaller than traditional EM filters, making them ideal for small devices such as integrated into smartphones, wearable devices, and Internet of Things (IoT) devices. The small size of acoustic filters is achieved by taking advantage of the slow speed of sound wave propagation, which results in shorter wavelengths at a given frequency. From 2 GHz to 5 GHz, 6 GHz below, SAW and BAW filter chips are used for signal selection. However, SAW and BAW filter chips are relatively narrow in width, with frequency limitations generally around 6 GHz. The millimeter wave frequency band above 6 GHz has a large bandwidth, which can improve communication capacity and data rate, and therefore has significantly broader application potential. Although the frequency limit of SAW and BAW filter chips can be improved by methods such as reducing filter size or using high-order modes, there are still problems of narrow bandwidth and large IL on the passband. Therefore, EM millimeter wave filter chips will become the main solution for 5G-6G wireless communication terminals.

[0079] Millimeter wave filters using PCB technology have been widely studied. L. Zhu et al. proposed a millimeter wave SIW bandpass filter using PCB technology in 2023 [1]. The center frequency is 25.88 GHz, the FBW is 4.48%, the minimum IL is 4.36 dB, and the size is 0.84λ0x 0.53λ 0。To achieve size reduction, LTCC technology and IPD technology have been used to design millimeter-wave filters. Y. Li et al. proposed a millimeter-wave SIW bandpass filter using LTCC technology in 2017 [2]. The center frequency is 174 GHz, the FBW is 13.8%, and the IL measured at the center frequency is 1.9 dB. In 2019, M. G. Bautista et al. proposed a millimeter-wave bandpass filter using 0.13-micron Bi-CMOS technology [3]. The center frequency is 29 GHz, the FBW is 26.7%, and the minimum IL is 3.5 dB. In 2021, G. Shen et al. proposed a MMIC bandpass filter for 5G millimeter-wave applications [4]. The center frequency is 28.4 GHz, the FBW is 10.6%, and the minimum IL is 1.8 dB. In 2021, Z. Ge et al. proposed a millimeter-wave wideband bandpass filter using CMOS technology [5]. The center frequency is 34.5 GHz, the FBW is 61.2%, and the minimum IL is 1.6 dB. In 2021, L. Gao et al. proposed a millimeter-wave bandpass filter using 45-nanometer CMOS SOI [6]. The center frequency is 33 GHz, the FBW is 66.7%, and the minimum IL is 1.5 dB.

[0080] To improve the stopband rejection level and roll-off degree, hybrid microwave filters are proposed. In 2020, H. Wu et al. proposed a hybrid wideband bandpass filter integrated with FBARs to achieve high roll-off degree [7]. The center frequency is 2.14 GHz, the FBW is 24.9%, and the minimum IL is 1.87 dB. In 2020, R. Zhang et al. proposed a hybrid bandpass filter integrated with SAW resonators [8]. The center frequency is 418 MHz, the FBW is 0.03%, and the minimum IL is 4.3 dB. In 2020, T. Cai et al. proposed a hybrid dual-passband filter integrated with SAW resonators [9]. The center frequency of the first passband is 869 MHz, the FBW is 0.37%, and the minimum IL is 0.89 dB. The center frequency of the second passband is 916 MHz, the FBW is 0.25%, and the minimum IL is 1.57 dB. So far, the working frequency of hybrid filters is below 6 GHz, and hybrid millimeter-wave filters are still unavailable.

[0081] In summary, millimeter-wave filter chips with low IL on passband, high stopband rejection level, high roll-off degree, and small size in wireless communication terminals are still challenging and unavailable worldwide. The present application proposes a CPW-based millimeter-wave bandpass filter with wide bandwidth, low IL on passband, high stopband rejection level, and small size.

[0082] The inventors of the present disclosure aim to design narrowband and wideband millimeter-wave bandpass filter chips with center frequencies in the range of 30-60 GHz. The 3-dB FBW of the narrowband filter chip will not exceed 30%, while the 3-dB FBW of the wideband filter chip will not be less than 50%. The minimum IL on the passband will be less than 1.5 dB, while the minimum stopband rejection level will be greater than 20 dB. The area of the narrowband and wideband filter chips will be less than 4 mm 2 and 6 mm 2 .

[0083] C. Details of embodiments of the present disclosure

[0084] Figure 1 An RF system including a millimeter-wave filter for dividing a frequency band according to certain embodiments of the present disclosure is shown. To meet the requirements of 5G-6G wireless communication systems, millimeter-wave filters are key elements to achieve frequency selectivity in RF systems. Figure 1 It is shown that a large number of millimeter-wave filters are required to divide the frequency band.

[0085] C.1 CPW-based millimeter wave filter with four half-wavelength resonators

[0086] Figure 2 A side view of a CPW-based millimeter-wave bandpass filter 200 according to certain embodiments of the present disclosure is shown. The CPW-based millimeter-wave bandpass filter employs a LN piezoelectric substrate design. LN is an anisotropic piezoelectric material with a relative permittivity when cut at 128°Y, as shown in the following matrix expression. LN crystals are characterized by low cost, low loss tangent, and high permittivity. The high permittivity of the LN substrate is beneficial to reduce the size of the filter, while the low loss tangent is beneficial to achieve high selectivity. Micro-nanofabrication technology can achieve high precision in the processing of the millimeter-wave bandpass filter 200. Other substrate materials, such as lithium tantalate (LiTaO3 or LT), can be applied. The selected substrate material should have a relatively high permittivity. The LN substrate thickness employed is about 0.15 millimeters. The thickness of the LN substrate is critical for LN-based millimeter-wave filters, so the tolerance of the LN thickness needs to be small enough. A surface metal layer is provided in the area above the LN substrate where wiring is required to facilitate signal transmission and ground arrangement. In a preferred embodiment, the metal layer can be a copper (Cu) layer with a thickness of 10 μm. In the drawings, the Cu layer is represented by a shaded grid, while the LN substrate is represented by white color. These legends are consistently applied to all subsequent drawings. The dimensions in all figures are only schematic and do not represent actual dimensions or relative proportions of the elements. Figure 2 In the drawings, the Cu layer is represented by a shaded grid, while the LN substrate is represented by white color. These legends are consistently applied to all subsequent drawings. The dimensions in all figures are only schematic and do not represent actual dimensions or relative proportions of the elements.

[0087]

[0088] Figure 3AA top view of a CPW-based millimeter-wave bandpass filter 300 according to certain embodiments of the present disclosure is shown. From the top view, the horizontal cross-sectional shape of the LN substrate is approximately rectangular. A Cu layer will be disposed on the LN substrate. The plane on which the Cu layer lies is defined as the CPW plane. In the context of the specification and claims, the direction along the long side of the rectangular CPW plane is defined as the longitudinal direction, while the direction along the short side of the rectangular CPW plane is defined as the transverse direction. For ease of description, the midline along the longitudinal direction of the rectangular CPW plane is defined as the first midline 301, while the midline along the transverse direction of the rectangular CPW plane is defined as the second midline 302. The first and second ground metal plates 311, 312 are each coupled to ground and disposed on the rectangular CPW plane. In the longitudinal direction, the first and second ground metal plates 311, 312 continuously extend along the longitudinal direction from one end of the CPW plane to the other. In the transverse direction, the first and second ground metal plates 311, 312 are respectively located on opposite sides of the CPW plane along the transverse direction, leaving a signal region 303 between the first and second ground metal plates 311, 312. The first and second ground metal plates 311, 312 are each symmetric about the second midline 302, and they are symmetrically arranged about the first midline 301. In certain embodiments, the first and second ground metal plates 311, 312 are each rectangular in shape from the top view. In the present disclosure, the thickness and height directions are both perpendicular to the CPW plane.

[0089] The input and output signal transmission lines 321, 322 are respectively located at the two ends of the signal region 303 in the longitudinal direction and aligned with the first midline 301. The input and output signal transmission lines 321, 322 are each symmetric about the first midline 301, and they are symmetrically arranged about the second midline 302. The input and output signal transmission lines 321, 322 are separated by a central transverse gap 340. An intermediate signal region 350 is composed of the input and output signal transmission lines 321, 322 and the central transverse gap 340. Signals propagate from a CPW input port composed of the input signal transmission line 321 and the first and second ground metal plates 311, 312 to a CPW output port composed of the output signal transmission line 322 and the first and second ground metal plates 311, 312. For all embodiments of the present disclosure, the input impedance of the CPW input port and the output impedance of the CPW output port are each 50 Ω. In certain embodiments, the input and output signal transmission lines 321, 322 are each rectangular in shape from the top view.

[0090] Four half-wavelength CPW resonators are provided between the intermediate signal region 350 and the first and second ground metal plates 311, 312. Specifically, the first and second CPW resonators 331, 332 are each arranged longitudinally between the first ground metal plate 311 and the intermediate signal region 350. The third and fourth CPW resonators 333, 334 are each arranged longitudinally between the second ground metal plate 312 and the intermediate signal region 350. The first CPW resonator 331 is separated from the second CPW resonator 332 by a first lateral gap 341. The third CPW resonator 333 is separated from the fourth CPW resonator 334 by a second lateral gap 342. The first and second lateral gaps 341, 342 each extend in the lateral direction along the second center line 302. The first CPW resonator 331 and the third CPW resonator 333 are symmetrically arranged about the first center line 301, and the second CPW resonator 332 and the fourth CPW resonator 334 are symmetrically arranged about the first center line 301. The first CPW resonator 331 and the second CPW resonator 332 are symmetrically arranged about the second center line 302, and the third CPW resonator 333 and the fourth CPW resonator 334 are symmetrically arranged about the second center line 302. In certain embodiments, the four CPW resonators 331, 332, 333, 334 are each rectangular in shape as viewed from the top.

[0091] In the lateral direction, the first and second CPW resonators 331, 332 are separated from the intermediate signal region 350 by a first longitudinal gap 351, and from the first ground metal plate 311 by a second longitudinal gap 352. In the lateral direction, the third and fourth CPW resonators 333, 334 are separated from the intermediate signal region 350 by a third longitudinal gap 353, and from the second ground metal plate 312 by a fourth longitudinal gap 354.

[0092] Figure 3B Simulated S-parameters of the filter 300 are shown. The center frequency is 30 GHz, and the FBW is 27%. The minimum passband IL is 1.04 dB, and the return loss at 30 GHz is 32 dB. However, the suppression at the upper stopband is low, and needs to be improved.

[0093] C.2 CPW-based millimeter wave filter including slotted resonators

[0094] The reason for the low suppression at the upper stopband is due to the parasitic passband. By integrating a T-shaped slot in each half-wavelength resonator, the parasitic passband can be suppressed. Figure 4AA top view of a CPW-based millimeter-wave bandpass filter 400 according to an embodiment of the present disclosure is shown. Specifically, a first T-slot 461, a second T-slot 462, a third T-slot 463, and a fourth T-slot 464 are embedded in the first CPW resonator 431, the second CPW resonator 432, the third CPW resonator 433, and the fourth CPW resonator 434, respectively. The first T-slot 461 and the third T-slot 463 are symmetrically arranged about the first centerline 401, the second T-slot 462 and the fourth T-slot 464 are symmetrically arranged about the first centerline 401, the first T-slot 461 and the second T-slot 462 are symmetrically arranged about the second centerline 402, and the third T-slot 463 and the fourth T-slot 464 are symmetrically arranged about the second centerline 402. The height of each T-slot 461, 462, 463, 464 is equal to the thickness of its corresponding CPW resonator 431, 432, 433, 434.

[0095] like Figure 4B As shown in FIG, taking the third T-slot 463 as an example, each T-slot includes a longitudinal slot 4631 and a transverse slot 4632. The longitudinal slot 4631 extends along the longitudinal centerline 4331 of the third CPW resonator 433. In alternative embodiments, the longitudinal slot 4631 need not be strictly aligned with the longitudinal centerline 4331 of the third CPW resonator 433. The transverse slot 4632 extends from the midpoint 4633 of the long side of the longitudinal slot 4631 along the transverse centerline 4332 of the third CPW resonator 433 to the midpoint 4334 of the long side 4333 of the third CPW resonator 433 that is oriented away from the intermediate signal region 450. Thus, the third T-slot 463 shapes the third CPW resonator 433 into a rectangular ring that includes a notch located at the midpoint 4334 of the long side 4333 of the rectangular ring. The long side of the longitudinal slot 4631 can be either of its two long sides. The notch is oriented away from the intermediate signal region 450. Although the third T-slot 463 is shown as an example, the first, second, and fourth T-slots 461 , 462 , 464 have similar configurations.

[0096] Each T-slot 461, 462, 463, 464 is equivalent to a capacitor and an inductor connected in parallel, which can reduce the size of the filter and generate a TZ in the upper stop band, such as Figure 4C As shown in FIG, which shows the simulated S parameters of the filter 400. The center frequency is 30 GHz and the FBW is 29.9%. The minimum passband IL is 1.03 dB, and the return loss at 30 GHz is 30 dB. The suppression in the upper stopband has been improved to more than 20 dB. The total size is 1.78 mm × 1.65 mm. Other reasonably configured slot shapes are also within the scope of the present disclosure, for example, Figure 6A The I-type groove shown in , which will be discussed later.

[0097] C.3 CPW-based millimeter wave filter including resonators and T-shaped stubs

[0098] A dual-mode T-shaped stub structure extending from each ground metal plate 511, 512 can further improve the performance of the filter. Two dual-mode T-shaped stubs are equivalent to two quarter-wavelength resonators coupled in parallel, which can reduce the size and create two TZs in the lower stopband. Figure 5A A top view of a CPW-based millimeter-wave bandpass filter 500 according to certain embodiments of the present disclosure is shown. Specifically, a first T-shaped stub 571 is located in the second longitudinal gap 552 and connected with the first ground metal plate 511. A second T-shaped stub 572 is located in the fourth longitudinal gap 554 and connected with the second ground metal plate 512. In the transverse direction, the first T-shaped stub 571 is separated from the first and second CPW resonators 531, 532 by the fifth longitudinal gap 555, and the second T-shaped stub 572 is separated from the third and fourth CPW resonators 533, 534 by the sixth longitudinal gap 556. The first and second T-shaped stubs 571, 572 are symmetrically arranged about the first midline 501.

[0099] Each T-shaped stub 571, 572 includes a longitudinal segment 5711, 5721 and a transverse segment 5712, 5722. The transverse segment 5712 of the first T-shaped stub 571 extends from a midpoint 5112 of a longer side 5111 of the first ground metal plate 511 that is oriented closer to the first and second CPW resonators 531, 532 to a midpoint 5713 of a longer side of the longitudinal segment 5711 of the first T-shaped stub 571. The longer side of the longitudinal segment 5711 can be either of its two longer sides. The transverse segment 5722 of the second T-shaped stub 572 extends from a midpoint 5122 of a longer side 5121 of the second ground metal plate 512 that is oriented closer to the third and fourth CPW resonators 533, 534 to a midpoint 5723 of a longer side of the longitudinal segment 5721 of the second T-shaped stub 572. The longer side of the longitudinal segment 5721 can be either of its two longer sides.

[0100] Although Figure 5A The filter 500 with T-shaped slots and T-shaped stubs 571, 572 is shown, but a filter including only T-shaped stubs is also within the scope of the present disclosure.

[0101] To achieve optimal performance, the inventors conducted extensive simulations to study the relationship between the size of each element of the filter 500 and the performance characteristics. In discussing Figure 5D , Figure 5E , Figure 5F , Figure 5G and Figure 5H the simulations shown, reference is made to the symbols shown in Figure 5B and Figure 5C .

[0102] Figure 5D Simulation results showing the variation of the frequency response in the filter 500 with the length (L3) of the longitudinal slot of each T-shaped slot in the longitudinal direction are shown. The simulation results show that the frequency of the TZ in the upper stopband decreases if L3 increases. The simulation results also show that the center frequency of the passband of the filter 500 decreases as L3 increases.

[0103] Figure 5E Simulation results showing the variation of the center frequency in the filter 500 with the length (L2) of each CPW resonator 531, 532, 533, 534 in the longitudinal direction are shown. The simulation results show that the center frequency decreases as L2 increases.

[0104] Figure 5F Simulation results showing the variation of the bandwidth of the passband in the filter 500 with the width (g2) of the first and second lateral gaps 541, 542 in the longitudinal direction are shown. The simulation results show that the bandwidth narrows as g2 increases.

[0105] Figure 5G Simulation results showing the frequency response in the filter 500 with and without the two T-shaped stubs 571, 572 are shown. The simulation results show that when the two T-shaped stubs 571, 572 are present in the filter 500, two TZs are generated in the lower stopband, which significantly improves the frequency selectivity, and at the same time, the presence of the two T-shaped stubs 571, 572 can decrease the center frequency of the passband, thereby the size of the filter 500 can be reduced.

[0106] Figure 5H Simulation results showing the variation of the frequency of the two TZs in the lower stopband in the filter 500 with the length (L4) of the longitudinal section 5711, 5721 of each T-shaped stub 571, 572 in the longitudinal direction are shown. The simulation results show that the frequency of the two TZs in the lower stopband decreases as L4 increases.

[0107] With the above simulations, the optimized dimensions of each element of the filter 500 can be obtained. On a qualitative scale, the following arrangements are suggested: (1) each T-shaped slot is symmetric about the transverse midline of the corresponding CPW resonator 531, 532, 533, 534; (2) each T-shaped stub 571, 572 is symmetric about the second midline 502; (3) in the longitudinal direction, the total length of the first and second CPW resonators 531, 532 and the first transverse gap 541 is greater than the length of the central transverse gap 540 but less than the length of the CPW plane; (4) in the longitudinal direction, the total length of the third and fourth CPW resonators 533, 534 and the second transverse gap 542 is greater than the length of the central transverse gap 540 but less than the length of the CPW plane; (5) in the longitudinal direction, the length of the longitudinal segment 5711 of the first T-shaped stub 571 is greater than the total length of the first and second CPW resonators 531, 532 and the first transverse gap 541 but less than the length of the CPW plane; (6) in the longitudinal direction, the length of the longitudinal segment 5721 of the second T-shaped stub 572 is greater than the total length of the third and fourth CPW resonators 533, 534 and the second transverse gap 542 but less than the length of the CPW plane.

[0108] In a preferred embodiment of the present application, the various elements of the filter 500 take the following dimensions, with reference to the above-described exemplary filter 500: Figure 5A 、 Figure 5B and Figure 5C(1) the thickness (h) of the LN substrate is about 0.15 mm, the length (Lo) of the LN substrate in the longitudinal direction is about 2 mm, and the width (Wo) of the LN substrate in the lateral direction is about 1.7 mm; (2) the thickness (t) of the CPW plane is about 0.01 mm; (3) the length (Li) of the input and output signal transmission lines 521, 522 in the longitudinal direction is about 0.542 mm, and the width (Wi) of the input and output signal transmission lines 521, 522 in the lateral direction is about 0.2 mm; (4) the length (L2) of each CPW resonator 531, 532, 533, 534 in the longitudinal direction is about 0.591 mm, and the width (W2) of each CPW resonator 531, 532, 533, 534 in the lateral direction is about 0.15 mm; (5) the length (L3) of the longitudinal slot of each T-shaped slot in the longitudinal direction is about 0.4 mm, and the width (W3) of the longitudinal slot of each T-shaped slot in the lateral direction is about 0.05 mm; (6) the length (L4) of the longitudinal slot 5711, 5721 of each T-shaped stub 571, 572 in the longitudinal direction is about 1.7 mm, the width (W4) of the longitudinal slot 5711, 5721 of each T-shaped stub 571, 572 in the lateral direction is about 0.1 mm, the length (L5) of the lateral segment 5712, 5722 of each T-shaped stub 571, 572 in the lateral direction is about 0.05 mm, and the width (W5) of the lateral segment 5721, 5722 of each T-shaped stub 571, 572 in the longitudinal direction is about 0.4 mm; (7) the width (gl) of the first and third longitudinal gaps 551, 553 in the lateral direction is about 0.015 mm; (8) the width (g2) of the first and second lateral gaps 541, 542 in the longitudinal direction is about 0.08 mm; (9) the length of the filter 500 is about 2 mm, the width of the filter 500 is about 1.7 mm, and the thickness of the filter 500 is about 0.16 mm.

[0109] Figure 5I The simulations and measured S-parameters of the filter 500 when the above-mentioned suggested dimensions are applied are shown. Figure 5J The simulations and measured group delay of the filter 500 when the above-mentioned suggested dimensions are applied are shown. As Figure 5I and Figure 5J As shown in FIGS. 1-4, the filter 500 achieves the following performance in simulations: (1) a center frequency of 30 GHz; (2) a 3-dB relative bandwidth of 22.7%; (3) a minimum IL on the passband of 0.91 dB; (4) a TZ in the upper stopband at a frequency of 47.76 GHz; (5) two TZs in the lower stopband at frequencies of 16.84 GHz and 25.08 GHz; and (6) a maximum group delay of 183 picoseconds. Reference is made to FIGS. 1-4. Figure 5I and Figure 5JThe actual test results are as follows: (1) the center frequency is 30.26 GHz; (2) the 3-dB relative bandwidth is 17%; (3) the minimum IL on the passband is 1.12 dB; (4) the frequency of the TZ in the upper stopband is 45.04 GHz; (5) the frequencies of the two TZs in the lower stopband are 15.19 GHz and 25.14 GHz; and (6) the maximum group delay is 232 picoseconds.

[0110] Table 1 shows the performance comparison results between the filter 500 and the following filters: (A) a millimeter-wave piezoelectric acoustic filter with large IL

[10] ; (B) a millimeter-wave filter with large IL and large size using PCB technology [1]; (C) a millimeter-wave filter with large IL and large size using LTCC technology

[11] ; and (D) a millimeter-wave filter with large IL and high cost using IPD technology

[12] .

[0111] Table 1. Comparison of the performance of the proposed filter 500 and other filters in the references

[0112]

[0113]

[0114] As indicated by the comparison results above, the filter 500 successfully implements a millimeter-wave bandpass filter with the features of high selectivity, miniaturization, and cost-effectiveness. The introduction of one TZ in the upper stopband using the T-shaped slot and the introduction of two TZs in the lower stopband using the T-shaped stubs 571, 572 can significantly improve the frequency selectivity of the filter. The low-cost LN crystal, as the substrate, is characterized by low loss tangent and high dielectric constant, which realizes low IL on the passband and miniaturization of the size. Therefore, by using the LN crystal as the substrate and introducing multiple TZs in the upper stopband and the lower stopband, a millimeter-wave bandpass filter with high selectivity, miniaturization, and low cost can be achieved.

[0115] C.4 CPW-based millimeter wave filter with cross-shaped dual-mode resonators

[0116] In some embodiments of the present disclosure, a cross-shaped dual-mode resonator is integrated onto the CPW plane of the filter, thereby increasing the order of the filter to four, which is beneficial to increasing the bandwidth. Figure 6AA top view of a CPW-based millimeter-wave bandpass filter 600 according to an embodiment of the present disclosure is shown. An I-shaped slot (instead of a T-shaped slot) is embedded in each of the CPW resonators 631, 632, 633, and 634. Specifically, a first I-shaped slot 661, a second I-shaped slot 662, a third I-shaped slot 663, and a fourth I-shaped slot 664 are embedded in the first CPW resonator 631, the second CPW resonator 632, the third CPW resonator 633, and the fourth CPW resonator 634, respectively. The first I-shaped slot 661 and the third I-shaped slot 663 are symmetrically arranged about the first centerline 601, the second I-shaped slot 662 and the fourth I-shaped slot 664 are symmetrically arranged about the first centerline 601, the first I-shaped slot 661 and the second I-shaped slot 662 are symmetrically arranged about the second centerline 602, and the third I-shaped slot 663 and the fourth I-shaped slot 664 are symmetrically arranged about the second centerline 602. The height of each I-slot 661 , 662 , 663 , 664 is equal to the thickness of its corresponding CPW resonator 631 , 632 , 633 , 634 .

[0117] Each I-type groove, Figure 6B Taking the third I-shaped slot 663 shown in FIG as an example, it includes a single transverse slot 6632. The single transverse slot 6632 extends from a midpoint 6334 of a first long side 6333 of the third CPW resonator 633, which is oriented away from the intermediate signal region 650, toward a midpoint 6336 of a second long side 6335 of the third CPW resonator 633, which is oriented closer to the intermediate signal region 650, without extending through the second long side 6335. The single transverse slot 6632 extends along the transverse centerline 6332 of the third CPW resonator 633. Thus, the third I-shaped slot 663 forms the third CPW resonator 633 into a rectangular strip with a notch located at a midpoint 6334 of a long side 6333 of the rectangular strip. The notch is oriented away from the intermediate signal region 650. Although the third I-shaped slot 663 is shown as an example, the first, second, and fourth I-shaped slots 661, 662, and 664 have similar configurations.

[0118] The cross-type dual-mode resonator 680 is disposed in the central area of ​​the signal region 603. The cross-type dual-mode resonator 680 includes a longitudinal arm 681 and a transverse arm 682.

[0119] Longitudinal arm 681 is located in central transverse gap 640 and extends along first centerline 601. Longitudinal arm 681 is separated from input signal transmission line 621 by a third transverse gap 643 and from output signal transmission line 622 by a fourth transverse gap 644. Longitudinal arm 681 is symmetrical about first centerline 601 and second centerline 602. Third and fourth transverse gaps 643 and 644 are each symmetrical about first centerline 601. Third and fourth transverse gaps 643 and 644 are symmetrically arranged about second centerline 602.

[0120] The transverse arm 682 is also symmetrical about the first and second midlines 601, 602, extending along the second midline 602. A first end 6821 of the transverse arm 682 is separated from the first ground metal plate 611 by a seventh longitudinal gap 657. A second end 6822 of the transverse arm 682 is separated from the second ground metal plate 612 by an eighth longitudinal gap 658. The seventh and eighth longitudinal gaps 657, 658 are each symmetrical about the second midline 602.

[0121] The seventh longitudinal gap 657 extends into the interior of the first ground metal plate 611, forming an inward recess at a midpoint of a long side 6111 of the first metal plate 611 oriented closer to the transverse arm 682. An edge 6823 of the first end 6821 can be substantially flush with the long side 6111 or extend into the inward recess. The second end 6822 is arranged symmetrically about the first midline 601 relative to the first end 6821, and the eighth longitudinal gap 658 is arranged symmetrically about the first midline 601 relative to the seventh longitudinal gap 657.

[0122] Figure 6C Simulated S-parameters of the filter 600 are shown. The center frequency is 31.73 GHz with an FBW of 53.3%. The minimum passband IL is 0.9 dB, while the return loss at 31.73 GHz is 43 dB. The stopband rejection level is greater than 20 dB. The overall size is 3.14 mm x 1.65 mm.

[0123] In summary, the proposed CPW-based EM millimeter-wave bandpass filter can achieve low passband IL, high out-of-band rejection, and small size.

[0124] It will also be understood that any features of the above-described embodiments of this disclosure can be combined together without necessarily being mutually exclusive. Similar combinations of two or more features from the above-described embodiments or preferred forms of this disclosure can be readily devised by persons skilled in the art.

[0125] Unless otherwise defined, technical and scientific terms used herein have the ordinary meaning commonly used in the art to which the example embodiments pertain. The examples are shown by way of non-limiting examples. Various modifications that can be suggested to one skilled in the art based on the above-described disclosure of embodiments will be within the spirit of the example embodiments.

[0126] Cited References

[0127] [1] L. Zhu, S. Yang, T. Gao, et al., "A design method to realize manufacture-friendly millimeter-wave folded substrate integrated waveguide bandpass filters", IEEE Transactions on Circuits and Systems II: Express Briefs, 70(3), 979-983, 2023.

[0128] [2] Y. Li, L. A. Yang, L. Du, et al., "Design of millimeter-wave resonant cavity and filter using 3-D substrate-integrated circular waveguide", IEEE Microwave and Wireless Components Letters, 27(8), 706-708, 2017.

[0129] [3] M. G. Bautista, H. Zhu, X. Zhu, et al., "Compact millimeter-wave bandpass filters using quasi-lumped elements in 0.13-um (Bi)-CMOS technology for 5G wireless systems", IEEE Transactions on Microwave Theory and Techniques, 67(7), 3064-3073, 2019.

[0130] [4] G. Shen, W. Che, W. Feng, et al., "Low insertion-loss MMIC bandpass filter using lumped-distributed parameters for 5G millimeter-wave application", IEEE Transactions on Components, Packaging and Manufacturing Technology, 11(1), 98-108, 2021.

[0131] [5] Z. Ge, L. Chen, R. Gómez-García, et al., “Millimeter-wave wide-band bandpass filter in CMOS technology using a two-layered highpass-type approach with embedded upper stopband”, IEEE Transactions on Circuits and Systems II: Express Briefs, 68(5), 1586-1590, 2021.

[0132] [6] L. Gao, G. M. Rebeiz, “Wideband bandpass filter for 5G millimeter-wave application in 45-nm CMOS silicon-on-insulator”, IEEE Electron Device Letters, 42(8), 1244-1247, 2021.

[0133] [7] H. Wu, Y. Wu, Z. Lai, et al. A hybrid film-bulk-acoustic-resonator / coupled-line / transmission-line high selectivity wideband bandpass FBAR filter. IEEE Transactions on Microwave Theory and Techniques, 2020, 68(8): 3389-3396.

[0134] [8] R. Zhang, M. Abdelfattah, L. Yang, et al. A hybrid low-cost bandpass filter with SAW resonators and external lumped inductors using a dual-coupling scheme. IEEE Transactions on Microwave Theory and Techniques, 2020, 68(6): 2289-2299.

[0135] [9] T. Cai, C. Chen, X. Zhang, et al. A hybrid transmission-line / SAW-resonator analog signal-interference dual-band bandpass filter. IEEE Microwave and Wireless Components Letters, 2020, 30(1): 27-30.

[0136]

[10] O. Barrera et al.,“Thin-film lithium niobate acoustic filter at 23.5 GHz with 2.38 dB IL and 18.2% FBW,” J. Microelectromech. Syst., vol. 32, no. 6, pp. 622-625, Dec. 2023.

[0137]

[11] K. S. Chin et al.,“LTCC multilayered substrate-integrated waveguide filter with enhanced frequency selectivity for system-in-package applications,” IEEE Trans. Compon. Packag. Manuf. Technol., vol. 4, no. 4, pp. 664-672, Apr. 2014.

[0138]

[12] Q. Lu et al.,“W-band compact triple-mode bandpass filter using multistub resonator in 65-nm CMOS technology,” IEEE Microw. Wireless Technol. Lett., vol. 34, no. 12, pp. 1319-1322, Dec. 2024.

Claims

1. A coplanar waveguide (CPW) based millimeter wave bandpass filter, comprising: a substrate; a first ground metal plate and a second ground metal plate, each coupled to ground and disposed on a rectangular CPW plane of a top surface of the substrate, wherein the first and second ground metal plates continuously extend along a longitudinal direction of the CPW plane from a first end to a second end of the CPW plane and are located on opposite sides of the CPW plane in a transverse direction of the CPW plane, thereby defining a signal region between the first and second ground metal plates, and wherein the signal region continuously extends along the longitudinal direction; an input signal transmission line and an output signal transmission line, each located at opposite ends of the signal region in the longitudinal direction and each aligned with a first midline that is a midline of the CPW plane in the longitudinal direction, wherein the input and output signal transmission lines are symmetrically arranged about a second midline that is a midline of the CPW plane in the transverse direction, wherein the input and output signal transmission lines are separated by a central transverse gap, and wherein an intermediate signal region is comprised of the input signal transmission line, the output signal transmission line, and the central transverse gap; a first CPW resonator and a second CPW resonator each longitudinally arranged between the first ground metal plate and the intermediate signal region, wherein the first and second CPW resonators are separated by a first transverse gap that is aligned with the second midline, and wherein in the transverse direction, the first and second CPW resonators are separated from the intermediate signal region by a first longitudinal gap and from the first ground metal plate by a second longitudinal gap; and a third CPW resonator and a fourth CPW resonator each longitudinally arranged between the second ground metal plate and the intermediate signal region, wherein the third and fourth CPW resonators are separated by a second transverse gap that is aligned with the second midline, and wherein in the transverse direction, the third and fourth CPW resonators are separated from the intermediate signal region by a third longitudinal gap and from the second ground metal plate by a fourth longitudinal gap, wherein the first and third CPW resonators are symmetrically arranged about the first midline, the second and fourth CPW resonators are symmetrically arranged about the first midline, the first and second CPW resonators are symmetrically arranged about the second midline, and the third and fourth CPW resonators are symmetrically arranged about the second midline.

2. The filter of claim 1, wherein: a total length of the first and second CPW resonators and the first transverse gap in the longitudinal direction is greater than a length of the central transverse gap but less than a length of the CPW plane, and a total length of the third and fourth CPW resonators and the second transverse gap in the longitudinal direction is greater than the length of the central transverse gap but less than the length of the CPW plane. In the longitudinal direction, the total length of the third and fourth CPW resonators and the second lateral gap is greater than the length of the central lateral gap but less than the length of the CPW plane.

3. The filter of claim 1, wherein: the first, second, third, and fourth T-shaped slots are respectively embedded into the first, second, third, and fourth CPW resonators, the first and third T-shaped slots are symmetrically arranged about the first midline, the second and fourth T-shaped slots are symmetrically arranged about the first midline, the first and second T-shaped slots are symmetrically arranged about the second midline, and the third and fourth T-shaped slots are symmetrically arranged about the second midline, the height of each of the first, second, third, and fourth T-shaped slots embedded into its respective CPW resonator is equal to the thickness of the respective CPW resonator, and each T-shaped slot comprises: a longitudinal slot extending along a longitudinal midline of the respective CPW resonator; and a lateral slot extending from a midpoint of a long side of the longitudinal slot to a midpoint of a long side of the respective CPW resonator oriented away from the intermediate signal region, wherein each T-shaped slot shapes the respective CPW resonator into a rectangular ring comprising a notch at a midpoint of a long side of the rectangular ring, and wherein the notch is oriented away from the intermediate signal region.

4. The filter of claim 3, wherein each T-shaped slot is symmetric about a lateral midline of the respective CPW resonator.

5. The filter of claim 3, wherein transmission zeros in an upper stopband are achievable by the first, second, third, and fourth T-shaped slots.

6. The filter of claim 1, further comprising: a first T-shaped stub in the second longitudinal gap and connected to the first ground metal plate, wherein, in the lateral direction, the first T-shaped stub is separated from the first and second CPW resonators by a fifth longitudinal gap; and a second T-shaped stub in the fourth longitudinal gap and connected to the second ground metal plate, wherein, in the lateral direction, the second T-shaped stub is separated from the third and fourth CPW resonators by a sixth longitudinal gap, wherein: the first and second T-shaped stubs are symmetrically arranged about the first midline, each of the first and second T-shaped stubs comprises a longitudinal segment and a lateral segment, the lateral segment of the first T-shaped stub extends from a midpoint of a long side of the first ground metal plate oriented closer to the first and second CPW resonators to a midpoint of a long side of the longitudinal segment of the first T-shaped stub, and the lateral segment of the second T-shaped stub extends from a midpoint of a long side of the second ground metal plate oriented closer to the third and fourth CPW resonators to a midpoint of a long side of the longitudinal segment of the second T-shaped stub.

7. The filter of claim 6, wherein: In the longitudinal direction, the length of the longitudinal segment of the first T-shaped stub is greater than the total length of the first CPW resonator, the second CPW resonator and the first lateral gap, but less than the length of the CPW plane, and In the longitudinal direction, a length of the longitudinal segment of the second T-shaped stub is greater than a total length of the third CPW resonator, the fourth CPW resonator and the second lateral gap, but less than a length of the CPW plane. The filter according to claim 6 , wherein each T-stub is symmetrical about the second center line. 9 . The filter according to claim 6 , wherein two transmission zeros in the lower stop band can be realized by the first T-type stub and the second T-type stub.

10. The filter according to claim 1, further comprising a cross-type dual-mode resonator, wherein the cross-type dual-mode resonator comprises: a longitudinal arm located in the central transverse gap and extending along the first centerline, and a transverse arm extending along the second midline, in: The longitudinal arm is separated from the input signal transmission line by a third transverse gap, and is separated from the output signal transmission line by a fourth transverse gap. The longitudinal arm is symmetrical about the first centerline and the second centerline, the third transverse gap and the fourth transverse gap are each symmetrical about the first centerline, and the third transverse gap and the fourth transverse gap are arranged symmetrically about the second centerline, The transverse arm is separated from the first ground metal plate by a seventh longitudinal gap and is separated from the second ground metal plate by an eighth longitudinal gap. The transverse arm is symmetrical about the first centerline and the second centerline, the seventh longitudinal gap and the eighth longitudinal gap are each symmetrical about the second centerline, and the seventh longitudinal gap and the eighth longitudinal gap are arranged symmetrically about the first centerline, The seventh longitudinal gap extends deep into the interior of the first ground metal plate so that a first inward recess is formed near a midpoint of a long side of the first ground metal plate that is oriented closer to the transverse arm, and The eighth longitudinal gap extends deep into the interior of the second ground metal plate so that a second inward recess is formed near a midpoint of a long side of the second ground metal plate oriented closer to the transverse arm.

11. The filter according to claim 10, wherein: The first I-type slot, the second I-type slot, the third I-type slot, and the fourth I-type slot are respectively embedded in the first CPW resonator, the second CPW resonator, the third CPW resonator, and the fourth CPW resonator. The first I-shaped groove and the third I-shaped groove are symmetrically arranged about the first center line, the second I-shaped groove and the fourth I-shaped groove are symmetrically arranged about the first center line, the first I-shaped groove and the second I-shaped groove are symmetrically arranged about the second center line, and the third I-shaped groove and the fourth I-shaped groove are symmetrically arranged about the second center line. The height of each of the first I-type slot, the second I-type slot, the third I-type slot, and the fourth I-type slot embedded in its corresponding CPW resonator is equal to the thickness of the corresponding CPW resonator, Each Type I slot includes a single transverse slot that extends from a midpoint of a first long side of the respective CPW resonator oriented away from the intermediate signal region toward a midpoint of a second long side of the respective CPW resonator oriented closer to the intermediate signal region, but does not extend across the second long side, and Each Type I slot shapes the respective CPW resonator as a notched rectangular strip with the notch at a midpoint of a long side of the rectangular strip and the notch oriented away from the intermediate signal region.

12. The filter of claim 10, wherein the bandwidth of the filter is increased by adding the cross-type dual-mode resonator.

13. The filter of claim 1, wherein the substrate is a lithium niobate (LiNbO3) substrate.

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