A cross-coupled narrowband filter

Abstract

This invention provides a cross-coupled narrowband filter, relating to the field of filter design, comprising: a metal microstrip layer, a dielectric substrate, and a metal ground layer arranged sequentially from top to bottom; a signal input terminal of the metal microstrip layer connected to a first stub line; a signal output terminal connected to a second stub line; multiple bow-shaped resonators arranged in a staggered, looping pattern; cross-coupling formed between two staggered bow-shaped resonators; multiple bow-shaped resonators positioned between the signal input terminal and the signal output terminal; the signal input terminal is matched to one bow-shaped resonator and located outside the bow-shaped resonator matched to the signal input terminal; the signal output terminal is matched to another bow-shaped resonator and located outside the bow-shaped resonator matched to the signal output terminal. This invention can reduce the filter size and improve the filter's out-of-band rejection capability.

Description

Technical Field

[0001] This invention relates to the field of filter design, and in particular to a cross-coupled narrowband filter. Background Technology

[0002] Radio frequency (RF) filters are frequency-selective devices and crucial components of modern wireless communication and defense systems. Their performance directly impacts the overall quality of the communication system. As communication systems become increasingly miniaturized, higher demands are placed on the performance and size of filters. Rapid advancements in new materials, processes, and semiconductor technology have led to increasingly higher integration levels and smaller device sizes in RF microwave circuits. Therefore, designing high-performance, miniaturized filters is currently one of the key aspects of RF microwave circuit design.

[0003] Currently, the relative bandwidth of conventional microstrip filters is typically above 15%. When using traditional hairpin or interdigital designs for narrowband filters, the smaller the required passband range, the smaller the coupling coefficient. This necessitates a larger coupling gap to achieve weak coupling between resonators, resulting in a large filter size and hindering device miniaturization. Therefore, exploring new filter structures and coupling methods to achieve both high performance and miniaturization is essential.

[0004] Traditional fourth-order hairpin microstrip bandpass filters treat the transmission line as a distributed inductor or capacitor at specific frequencies. Since tanβl (β is the phase shift constant and l is the resonator length) is periodic, the overall performance of the filter will also exhibit periodicity, resulting in parasitic passbands at the high end of the passband and out-of-band rejection. Furthermore, the absence of transmission zeros at low frequencies in traditional fourth-order hairpin microstrip bandpass filters leads to poor out-of-band rejection at the low end of the passband. Summary of the Invention

[0005] The purpose of this invention is to provide a cross-coupled narrowband filter to solve the problems of large size and poor out-of-band suppression of conventional microstrip bandpass filters.

[0006] To achieve the above objectives, the present invention provides the following solution:

[0007] A cross-coupled narrowband filter includes: a metal microstrip layer, a dielectric substrate, and a metal ground layer arranged sequentially from top to bottom;

[0008] The metal microstrip layer includes a signal input terminal, a signal output terminal, a first stub line, a second stub line, and multiple bow-shaped resonators;

[0009] The signal input terminal is connected to the first stub line; the signal output terminal is connected to the second stub line.

[0010] Multiple bow-shaped resonators are arranged in a staggered, ring-like pattern; cross-coupling is formed between two staggered bow-shaped resonators; multiple bow-shaped resonators are arranged between the signal input terminal and the signal output terminal; the signal input terminal is matched with one bow-shaped resonator and is located outside the bow-shaped resonator matched with the signal input terminal; the signal output terminal is matched with another bow-shaped resonator and is located outside the bow-shaped resonator matched with the signal output terminal.

[0011] Optionally, a first coupling gap is provided between the signal input terminal and the bow-shaped resonator, and between the signal output terminal and the bow-shaped resonator;

[0012] The first coupling gap is determined by the derivative of the external loss tangent.

[0013] Optionally, a second coupling gap is provided between the two interleaved bow-shaped resonators;

[0014] The second coupling gap is determined by the coupling coefficient between two interlaced bow-shaped resonators.

[0015] Optionally, the equivalent electrical length of each bow-shaped resonator is equal to half the wavelength.

[0016] Optionally, the size of each bow-shaped resonator is determined by the center frequency of each bow-shaped resonator.

[0017] Optionally, the metal microstrip layer and the metal ground layer are made of immersion gold metal.

[0018] Optionally, the thickness of both the metal microstrip layer and the metal ground layer is 0.004 mm.

[0019] Optionally, the dielectric substrate is an alumina ceramic substrate.

[0020] Optionally, the relative permittivity of the dielectric substrate is 9.8.

[0021] Optionally, the thickness of the dielectric substrate is 0.254 mm.

[0022] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0023] This invention employs four bow-shaped resonators arranged in a staggered, encircling pattern. The staggered bow-shaped resonators form cross-coupling, resulting in transmission zeros on both sides of the passband of the cross-coupled narrowband filter, which improves the low-end out-of-band suppression capability of the passband. Furthermore, it pushes the parasitic passband to a higher frequency band, widening the high-end out-of-band suppression width of the passband, effectively solving the out-of-band suppression problem of traditional fourth-order hairpin microstrip bandpass filters.

[0024] Furthermore, by setting the resonator to a bow-shaped structure, the energy coupling region between the resonators is reduced, thus achieving weak coupling between the resonators. This reduces the coupling gap between the resonators, shrinks the size of the filter, and enables device miniaturization. The bow-shaped resonator makes full use of space, achieving narrow bandwidth while meeting the requirement of small size. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 A three-dimensional view of the cross-coupled narrowband filter provided by the present invention;

[0027] Figure 2 This is a top view of the metal microstrip layer provided by the present invention;

[0028] Figure 3 This is a schematic diagram of other forms of the bow-shaped resonator provided by the present invention; wherein, Figure 3 (a) in the diagram is a schematic diagram of the widened version of one end of the bow-shaped resonator; Figure 3 (b) is a schematic diagram of a bow-shaped resonator with one end grounded.

[0029] Figure 4 This is a schematic diagram of a traditional fourth-order hairpin microstrip bandpass filter structure.

[0030] Figure 5 This is a schematic diagram of the S-parameter response of the fourth-order cross-coupled narrowband filter provided by the present invention;

[0031] Figure 6 A comparison of the S-parameter responses of a traditional fourth-order hairpin microstrip bandpass filter and a fourth-order cross-coupled narrowband filter provided by this invention;

[0032] Figure 7 This is a schematic diagram of the multi-order cross-coupled narrowband filter provided by the present invention;

[0033] Figure 8 This is a schematic diagram of a multi-order cross-coupled narrowband filter that modifies the curvature of the bow-shaped structure, as provided by the present invention. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] The purpose of this invention is to provide a cross-coupled narrowband filter that can reduce the filter size and improve the filter's out-of-band rejection capability.

[0036] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0037] Taking a fourth-order cross-coupled narrowband filter as an example, such as Figures 1-2 As shown, the present invention provides a cross-coupled narrowband filter, comprising: a metal microstrip layer, a dielectric substrate 101, and a metal ground layer arranged sequentially from top to bottom; the metal microstrip layer includes a signal input terminal 104, a signal output terminal 111, a first stub line 103, a second stub line 110, a first bow-shaped resonator 105, a second bow-shaped resonator 108, a third bow-shaped resonator 102, and a fourth bow-shaped resonator 109; the signal input terminal 104 is connected to the first stub line 103; the signal output terminal 111 is connected to the second stub line 110; the first bow-shaped resonator 105... The resonator 105, the third bow-shaped resonator 102, the fourth bow-shaped resonator 109, and the second bow-shaped resonator 108 are arranged in a clockwise, interlaced manner in a ring-like pattern; cross-coupling is formed between the two interlaced bow-shaped resonators; multiple bow-shaped resonators are arranged between the signal input terminal 104 and the signal output terminal 111; the signal input terminal 104 is matched with the first bow-shaped resonator 105 and is located outside the first bow shape; the signal output terminal 111 is matched with the fourth bow shape and is located outside the fourth bow shape.

[0038] The signal input terminal 104 is connected to an external device. The energy sent by the external device is transmitted to the first stub line 103 through the signal input terminal 104. The energy is coupled to the first bow-shaped resonator 105 through the first stub line 103. After being selected by the first bow-shaped resonator 105, the energy is transmitted to the second bow-shaped resonator 108 and the third bow-shaped resonator 102. After being selected, the energy is coupled to the fourth bow-shaped resonator 109 and then coupled to the second stub line 110 to the signal output terminal 111.

[0039] Filter coupling topologies can be broadly classified into two categories: cascaded coupling and cross-coupling. A cascaded-coupled filter has only one coupling path, and coupling exists only between adjacent resonators. Cross-coupling, on the other hand, involves multiple coupling paths from the signal source to the load, including a main coupling path and relatively weaker secondary coupling paths. Compared to cascaded coupling, the biggest advantage of cross-coupling is its ability to generate transmission zeros at finite frequencies near the passband, thus significantly improving the filter's out-of-band rejection capability.

[0040] In practical applications, the entire filter is centrally symmetrical.

[0041] In practical applications, a first coupling gap 106 exists between the signal input terminal 104 and the bow-shaped resonator, and between the signal output terminal 111 and the bow-shaped resonator; the first coupling gap 106 is determined by the derivative value of the external loss tangent. External energy is coupled from the signal input terminal 104 and the first stub line 103 to the first bow-shaped resonator 105 through the first coupling gap 106 via indirect feeding, thus achieving the high external Q value requirement of the cross-coupled narrowband filter.

[0042] In practical applications, a first coupling gap 107 is formed between two interlaced bow-shaped resonators; the first coupling gap 107 is determined by the coupling coefficient between the two interlaced bow-shaped resonators. Based on the traditional hairpin resonator, a new coupling characteristic is achieved by setting the bow-shaped resonators into a cross-coupled structure to achieve a smaller coupling coefficient, thereby achieving the goal of filter miniaturization.

[0043] In practical applications, each bow-shaped resonator has a uniform width. Bow-shaped resonators with uniform width are uniform impedance resonators, forming a uniform impedance. Therefore, the equivalent electrical length of each bow-shaped resonator is equal to half the wavelength.

[0044] In practical applications, the size of each bow-shaped resonator is determined by the center frequency of each bow-shaped resonator.

[0045] In practical applications, the shape of each bow-shaped resonator can be changed according to the actual situation.

[0046] This invention alters the filter's filtering characteristics by changing the size of the resonator, the thickness of the dielectric substrate 101, and the coupling gaps between the resonators, thereby improving the filter's selectivity. By widening the width of one end (either one or both ends) of the bow-shaped resonator to create a step impedance, the equivalent electrical length of the resonator is made less than half the wavelength, thus achieving device miniaturization.

[0047] In practical applications, the metal microstrip layer and the metal ground layer are made of immersion gold metal.

[0048] In practical applications, the thickness of both the metal microstrip layer and the metal ground layer is 0.004 mm.

[0049] In practical applications, the dielectric substrate 101 is an alumina ceramic substrate.

[0050] In practical applications, the relative permittivity of the dielectric substrate 101 is 9.8.

[0051] In practical applications, the thickness of the dielectric substrate 101 is 0.254 mm.

[0052] In addition to setting the resonator as such Figure 2 Besides the bow shape shown, they can also be made to intersect each other to form a shape like... Figure 3 (a) and Figure 3 The form shown in (b) is shown in the diagram. Figure 3 In (a), the width of one end (either one end or both ends) of the bow-shaped resonator is widened to form a step impedance, so that the equivalent electrical length of the resonator is less than half the wavelength, thereby reducing the size of the resonator and achieving the purpose of miniaturization of the device.

[0053] Figure 3 In (b), one end of the bow-shaped resonator is grounded, and the equivalent electrical length of the bow-shaped resonator is one-quarter wavelength. This structure allows cross-coupling between the resonators, generating a transmission zero at the low end of the filter's passband, thereby improving the out-of-band rejection capability at the low end of the passband; the parasitic passband of the filter appears near the third harmonic of the center frequency, thus widening the out-of-band rejection width at the high end of the passband.

[0054] The design method for the aforementioned cross-coupled narrowband filter is as follows:

[0055] Step 1: Simulate the center frequency of a single bow-shaped resonator to determine its dimensions.

[0056] Step 2: Simulate the resonant frequencies f1 and f2 of two adjacent bow-shaped resonators, and then use the formula... Determine its coupling coefficient, based on the coupling coefficient K 12 Determine the size of the coupling gap.

[0057] Step 3: Simulate the signal input terminal 104, the first stub line 103 and its adjacent bow-shaped resonator, calculate the time delay through the external Q value, and determine the size of the coupling gap.

[0058] Based on the above method, an implementation example of a fourth-order cross-coupled narrowband filter designed in this invention is shown below. Figure 2As shown, alumina ceramic material with a thickness of 0.254 mm is used as the dielectric substrate 101 of the microstrip bandpass filter. The simulation results of the S-parameters (scattering parameters) of the narrowband filter with high selectivity are as follows. Figure 5 As shown, S11 represents return loss and S21 represents gain.

[0059] As can be seen, the filter operates in the 9.3GHz-9.6GHz frequency band, corresponding to a relative bandwidth of 3.17%, exhibiting narrowband characteristics. The passband insertion loss is less than 1.5dB, and the return loss is better than -20dB. The low-frequency and high-frequency transmission zeros in the passband are located at 7.14GHz and 16.25GHz, respectively, resulting in a significantly improved out-of-band suppression level. In the 0-7.97GHz low-frequency stopband, the suppression level is better than -40dB, and in the 12.64-16.88GHz high-frequency stopband, the suppression level is also better than -40dB. The filter measures 5.25mm*4.5mm, demonstrating miniaturization.

[0060] To facilitate performance comparison, Figure 4 The diagram shows a conventional fourth-order hairpin microstrip bandpass filter structure involved in this invention. It consists of a metal ground layer, a dielectric substrate 101, a signal input terminal 104, a signal output terminal 111, and four hairpin resonators. The equivalent electrical length of the hairpin resonator is half the wavelength. Figure 6 The paper presents a comparison of the S-parameter responses of a traditional hairpin microstrip bandpass filter of the same frequency band and the fourth-order cross-coupled narrowband filter of this invention.

[0061] This invention employs a fourth-order narrowband filter with a bow-shaped, cross-coupled resonator structure, generating a transmission zero at the low end of the passband (7.14 GHz). This significantly improves the low-end out-of-band rejection compared to a traditional fourth-order hairpin microstrip bandpass filter with the same passband frequency. Furthermore, regarding parasitic passband rejection, this cross-coupled narrowband filter pushes the parasitic passband to a higher frequency band than 15 GHz. In contrast, the traditional fourth-order hairpin microstrip bandpass filter achieves a -40 dB out-of-band rejection of only 13.79 GHz, while the cross-coupled narrowband filter provided by this invention achieves a -40 dB out-of-band rejection of 16.88 GHz, broadening the high-frequency out-of-band rejection range and demonstrating excellent out-of-band rejection capability.

[0062] like Figures 7-8 As shown, this invention forms cross-coupling between bow-shaped resonators, which is not limited to fourth-order cross-coupled narrowband filters, but can also be extended to multiple orders. Furthermore, the curvature of the bow-shaped structure can be changed. As long as cross-coupling can be formed, the filter size can be reduced and the out-of-band rejection capability of the filter can be improved.

[0063] By arranging the resonators in a bow shape, the structural layout of traditional hairpin filters is adjusted, making efficient use of space. The bow shape also reduces the energy coupling region between resonators, achieving weak coupling and thus minimizing coupling gaps. Employing a cross-coupling method generates transmission zeros at finite frequencies near the passband, improving out-of-band rejection and reducing the number of required resonators. This facilitates filter size reduction and miniaturization. The filter design avoids short-circuit vias, simplifying manufacturing and reducing costs, making it highly valuable in engineering applications.

[0064] The filter's filtering characteristics can be altered by changing the size of the resonator, the thickness of the dielectric substrate 101, and the coupling gaps between the resonators, thereby improving the filter's selectivity. The width of one end (either one or both ends) of the bow-shaped resonator can be widened to create a step impedance, making the equivalent electrical length of the resonator less than half the wavelength, thus achieving device miniaturization.

[0065] Alternatively, one end of the bow-shaped resonator can be grounded, making its equivalent electrical length a quarter wavelength. This structure, with one end of the bow-shaped resonator grounded, allows for cross-coupling between the resonators, creating a transmission zero at the low end of the filter's passband, thus improving the low-end out-of-band rejection capability. The parasitic passband of the filter appears near the third harmonic of the center frequency, thereby widening the high-end out-of-band rejection width. Therefore, the cross-coupling structure provided by this invention offers flexible and versatile design options for narrowband filters.

[0066] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0067] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A cross-coupled narrowband filter, characterized in that, include: The metal microstrip layer, dielectric substrate, and metal ground layer are arranged sequentially from top to bottom; The metal microstrip layer includes a signal input terminal, a signal output terminal, a first stub line, a second stub line, and multiple bow-shaped resonators; The signal input terminal is connected to the first stub line; the signal output terminal is connected to the second stub line. Multiple bow-shaped resonators are arranged in a staggered, ring-like pattern. Cross-coupling is formed between the two intersecting bow-shaped resonators; Multiple bow-shaped resonators are arranged between the signal input terminal and the signal output terminal; the signal input terminal is matched with a bow-shaped resonator and is located outside the bow-shaped resonator matched with the signal input terminal; The signal output terminal is matched with another bow-shaped resonator and is located outside the bow-shaped resonator that matches the signal output terminal; The plurality of bow-shaped resonators include a first bow-shaped resonator, a second bow-shaped resonator, a third bow-shaped resonator, and a fourth bow-shaped resonator; wherein the first bow-shaped resonator, the third bow-shaped resonator, the fourth bow-shaped resonator, and the second bow-shaped resonator are arranged in a clockwise staggered manner in a ring-like pattern. There is a second coupling gap between the two interlaced bow-shaped resonators; The second coupling gap is determined by the coupling coefficient between two interlaced bow-shaped resonators; A first coupling gap exists between the signal input terminal and the bow-shaped resonator, and between the signal output terminal and the bow-shaped resonator; The first coupling gap is determined by the derivative of the external loss tangent angle; external energy is coupled from the signal input terminal and the first stub line to the first bow-shaped resonator through the first coupling gap via indirect feeding. The equivalent electrical length of each bow-shaped resonator is equal to half the wavelength; The size of each bow-shaped resonator is determined by the center frequency of each bow-shaped resonator.

2. The cross-coupled narrowband filter according to claim 1, characterized in that, The metal microstrip layer and the metal ground layer are made of immersion gold metal.

3. The cross-coupled narrowband filter according to claim 1, characterized in that, The thickness of both the metal microstrip layer and the metal ground layer is 0.004 mm.

4. The cross-coupled narrowband filter according to claim 1, characterized in that, The dielectric substrate is an alumina ceramic substrate.

5. The cross-coupled narrowband filter according to claim 1, characterized in that, The relative permittivity of the dielectric substrate is 9.

8.

6. The cross-coupled narrowband filter according to claim 1, characterized in that, The thickness of the dielectric substrate is 0.254 mm.

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