A novel wideband linear phase filter and a design method thereof

Abstract

The application belongs to the technical field of radio frequency communication, and discloses a novel wideband linear phase filter and a design method thereof. The novel wideband linear phase filter comprises a first port, a second port, and three first transmission lines connected in series between the first port and the second port. Four connection points are formed between the first port and the adjacent first transmission line, between the three first transmission lines, and between the second port and the adjacent first transmission line. Four short-circuit branch lines are connected at the four connection points one by one. An inductance circuit module is connected at one end of the connection point between the first port and the adjacent first transmission line, and connected at the other end of the connection point between the second port and the adjacent first transmission line.

Description

Technical Field

[0001] This invention belongs to the field of radio frequency communication technology, and specifically relates to a novel broadband linear phase filter and its design method. Background Technology

[0002] As an indispensable component in modern mobile communication, relay communication, satellite communication, and radar systems, filters primarily function to acquire specific frequency bands of the input signal while filtering out unwanted frequency bands. Their performance, such as insertion loss and phase characteristics within the passband, directly impacts the overall quality of the communication system. To obtain a distortion-free phase response, the group delay of each frequency component passing through the filter must be consistent, ensuring waveform consistency before and after signal transmission. Compared to traditional filters, linear phase filters offer advantages in reducing bit error rates and improving signal transmission quality in radar or satellite communication systems. Therefore, linear phase filters hold significant research value in broadband communication.

[0003] With the development of mobile communication technology, users' demands for information quantity and quality are constantly increasing, making it particularly important to improve the bandwidth and group delay flatness of traditional linear phase filters.

[0004] There are two main implementation methods for designing linear-phase filters: external equalization and self-equalization. External equalization, as the name suggests, involves cascading an external equalizer with a bandpass filter to cancel each other out, thus flattening the group delay of the entire system. However, external equalization suffers from drawbacks such as large space requirements and complex circuitry, so self-equalization is generally used. There are three main design methods for linear-phase filters using self-equalization: the first utilizes pole extraction techniques, which typically have a narrow bandwidth and are complex; the second is based on cross-coupling structures, designing linear-phase filters based on dielectric integrated waveguides, which offer a flat passband group delay and compact size; and the third is a cascaded quadruple (CQ) structure. A bandpass filter with self-equalization and a narrow bandwidth was designed using a CQ structure, but it is relatively large. In summary, the above research mainly focuses on how to design linear-phase filters with good group delay. However, these traditional design methods all suffer from the inherent characteristic of narrow bandwidth.

[0005] In recent years, to ensure distortion-free signal transmission, the application of broadband linear-phase filters in modern wireless communication systems has attracted increasing attention. This demand has spurred the development of several broadband linear-phase bandpass filter design methods, such as designing broadband filters by adding square loop filters with stubs and coupling segments, simultaneously obtaining equal ripple amplitude and in-band group delay curves. However, while these methods effectively extend the bandwidth of broadband filters, they all employ optimized design methods and cannot be directly synthesized; that is, the design parameters of the circuit components cannot be directly and quantitatively calculated. Summary of the Invention

[0006] One of the objectives of this invention is to provide a novel broadband linear phase filter that can achieve stable group delay over a wide frequency band.

[0007] Another objective of this invention is to provide a novel design method for broadband linear phase filters, which can improve the bandwidth and group delay flatness of linear filters and enable quantitative calculation of design parameters.

[0008] The technical solution provided by this invention is as follows:

[0009] A novel broadband linear phase filter includes:

[0010] First port;

[0011] Second port;

[0012] Three first transmission lines are connected in series between the first port and the second port;

[0013] Among them, four connection points are formed between the first port and the adjacent first transmission line, between the three first transmission lines, and between the second port and the adjacent first transmission line;

[0014] Four short-circuit stubs are connected one-to-one at the four connection points;

[0015] An inductor circuit module, one end of which is connected to the connection point between the first port and the adjacent first transmission line, and the other end of which is connected to the connection point between the second port and the adjacent first transmission line.

[0016] Preferably, the inductor circuit module consists of an inductor and two second transmission lines;

[0017] The two second transmission lines are respectively connected to the two ends of the inductor.

[0018] Preferably, the first transmission line is a quarter-wavelength transmission line.

[0019] Preferably, the short-circuit stub is a quarter-wavelength short-circuit stub.

[0020] A novel design method for a broadband linear phase filter, comprising the following steps:

[0021] Step 1: Treat each lumped circuit module in the prototype of the fourth-order linear phase low-pass filter as a generalized unit element, and convert the lumped circuit module into a transmission line circuit module.

[0022] The transmission line circuit module consists of a first transmission line and two short-circuit stubs, with the two short-circuit stubs respectively connected to both ends of the first transmission line.

[0023] Step 2: Based on the equivalent relationship between the lumped circuit module and the transmission line circuit module, determine the characteristic impedance of the first transmission line and the short-circuited stub line in the transmission line circuit module to obtain the first filter;

[0024] Step 3: Simplify two adjacent short-circuit stubs in the first filter into one short-circuit stub based on their parallel relationship to obtain the second filter;

[0025] Step 4: Convert the high-impedance transmission line in the second filter into an equivalent inductor circuit module to obtain the novel broadband linear phase filter.

[0026] The high-impedance transmission line refers to a transmission line with an impedance value greater than 150Ω.

[0027] Preferably, the method for determining the impedance of the first transmission line and the short-circuit stub line in the transmission line circuit module is as follows:

[0028] Construct the transmission parameter matrix of the lumped circuit module and the transmission parameter matrix of the transmission line circuit module, and establish the equivalent relationship between the elements in the transmission parameter matrix of the lumped circuit module and the elements in the transmission parameter matrix of the transmission line circuit module based on the equivalence relationship.

[0029] By solving the equality relationship using the frequency mapping function and the constant mapping function, the impedances of the first transmission line and the short-circuit stub line in the transmission line circuit module are obtained.

[0030] Preferably, the transmission parameter matrix of the lumped circuit module is:

[0031] ;

[0032] in, This represents the transmission parameter matrix of the lumped circuit module. This represents the equivalent characteristic impedance of the lumped circuit module. Represents the extended Richard transform of lumped circuit modules. It indicates that it is an imaginary unit. Indicates the converter parameters, This represents the normalized angular frequency parameter. This indicates a capacitor element.

[0033] Preferably, the transmission parameter matrix of the transmission line circuit module is as follows:

[0034] ;

[0035] in, This represents the transmission parameter matrix of the transmission line circuit module. This represents the equivalent characteristic impedance of the transmission line circuit module. This represents the extended Richard transform of the transmission line circuit module. Indicates the input impedance of the transmission line. This represents the reference characteristic impedance of the transmission line. Indicates the electrical length of the transmission line.

[0036] The beneficial effects of this invention are:

[0037] The novel broadband linear phase filter provided by this invention can achieve stable group delay over a wide frequency band.

[0038] The present invention provides a novel design method for broadband linear phase filters, which can improve the bandwidth and group delay flatness of linear filters and enable quantitative calculation of design parameters. Attached Figure Description

[0039] Figure 1 This is a circuit diagram of the novel broadband linear phase filter described in this invention.

[0040] Figure 2 This is a schematic diagram of the prototype (n=4) of the linear phase low-pass filter described in this invention.

[0041] Figure 3 This is a schematic diagram of a unit module consisting of an impedance inverter and a capacitor, as described in this invention.

[0042] Figure 4 This is a circuit diagram of the prototype of the linear phase low-pass filter described in this invention.

[0043] Figure 5 (a) is a schematic diagram of the generalized element unit described in this invention.

[0044] Figure 5 (b) is a lumped coupled LC circuit diagram of the present invention.

[0045] Figure 6 This is a schematic diagram of the transmission line circuit equivalent to the generalized element unit described in this invention.

[0046] Figure 7 This is a structural diagram of the broadband linear phase bandpass filter based on the generalized unit element (GUE) described in this invention.

[0047] Figure 8 This is an equivalent relationship diagram between the transmission line circuit module and the equivalent unit described in this invention.

[0048] Figure 9 This is the equivalent circuit diagram of the broadband linear phase filter described in this invention.

[0049] Figure 10 This is a simplified circuit diagram of the broadband linear phase filter described in this invention.

[0050] Figure 11 (a) is Figure 1 S in the circuit schematic 11 and S 21 Simulation results diagram.

[0051] Figure 11 (b) is Figure 1 The circuit schematic diagram shows the group delay simulation results.

[0052] Figure 12 This is a layout of the novel broadband linear phase filter described in this invention.

[0053] Figure 13 (a) is Figure 12 S in the middle map 11 and S 21 Simulation results and Figure 1 A comparison chart of theoretical circuit simulation results.

[0054] Figure 13 (b) is Figure 12 Simulation results of group delay in the middle layout and Figure 1 A comparison chart of theoretical circuit simulation results. Detailed Implementation

[0055] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0056] like Figure 1 As shown, the present invention provides a novel broadband linear phase filter, which includes: a first port Term1, a second port Term2, three first transmission lines (TL11, TL12, TL13), four short-circuit stub lines (SSL1, SSL2, SSL3, SSL4), and an inductor circuit module.

[0057] Three first transmission lines (TL11, TL12, TL13) are connected in series between the first port Term1 and the second port Term2. Specifically, the first port Term1 forms one connection point with its adjacent first transmission line TL11, the three first transmission lines (TL11, TL12, TL13) form two connection points, and the second port Term2 forms one connection point with its adjacent first transmission line TL13; thus forming a total of four connection points. One end of each of the four short-circuit stub lines (SSL1, SSL2, SSL3, SSL4) is connected to one of the four connection points, and the other end is grounded.

[0058] One end of the inductor circuit module is connected to the connection point of the first port Term1 and its adjacent first transmission line TL11, and the other end is connected to the connection point of the second port Term1 and its adjacent first transmission line TL13.

[0059] The inductor circuit module consists of an inductor L1 and two second transmission lines (TL21, TL22); wherein the two second transmission lines (TL21, TL22) are respectively connected to the two ends of the inductor L1.

[0060] The present invention also provides a novel design method for a broadband linear phase filter. The novel broadband linear phase filter is designed based on the novel design method provided by the present invention. The specific implementation process of the design method is as follows.

[0061] The phase parameters of linear-phase filters are not easy to extract, making their design challenging. It wasn't until 1970 that John David Rhodes first proposed the concept of an equally spaced linear-phase polynomial. This formula allows for the calculation of component values ​​in a prototype low-pass linear-phase filter network, providing a basis for designing linear-phase filters.

[0062] This invention designs a broadband linear-phase filter based on a prototype of a linear-phase low-pass filter (n=4), such as... Figure 2 As shown, the prototype of the linear phase low-pass filter (n=4) consists of two ports and four unit modules. Figure 3As shown, each unit module consists of an impedance inverter and two parallel capacitors. The characteristic admittances of the impedance inverters in the two units are J, J2, J, and J1, respectively. Characteristic admittance J1 represents the cross-coupling between non-adjacent components or resonators, and characteristic admittance J2 represents the direct coupling between two symmetrical parts of the network. The component values ​​are g1 = 0.9046, J1 = 0.1766, g2 = 1.2586, and J2 = 0.7799. Here, 'a' is a variable with a value between 0 and 1. This is based on the normalization of the termination impedance. g 0 = 1Ω (normalized source / load impedance). g 1. g 2. The normalized capacitance value of the parallel capacitor in the corresponding unit module is a parameter that normalizes the actual capacitance value according to the normalized impedance / frequency. It is used to simplify the design, analysis and component value calculation of the network and reflects the susceptance characteristics of the capacitor in the normalized circuit; a is an adjustable variable of the network characteristics.

[0063] In one embodiment, a is taken as 0.5. According to... Figure 2 The capacitance values ​​calculated in the low-pass prototype of the linear phase filter are: ag1=0.4523, (1-a)g1=0.4523, g2-(1-a)g1=0.80638. That is, in the four unit modules, the characteristic admittances of the impedance inverters are: J=1, J2=0.7799, J=1, J1=0.1766; the capacitance value of the capacitor connected in parallel across the impedance inverter with characteristic admittance J=1 is C=(1-a)g1=0.4523F; the capacitance value of the capacitor connected in parallel across the impedance inverter with characteristic admittance J2=0.7799 is C=g2-(1-a)g1=0.80638F; the capacitance value of the capacitor connected in parallel across the impedance inverter with characteristic admittance J1=0.1766 is C=ag1=0.4523F.

[0064] according to Figure 2 The filter structure and component values ​​are used to build a prototype schematic of a linear-phase low-pass filter in the ADS software, as shown below. Figure 4As shown, the system includes ports Term1 and Term2, ABCD matrix controls Chain1, Chain2, Chain3, and Chain4, and capacitors C1, C2, C3, C4, C5, C6, C7, and C8. Specifically, capacitors C1 and C2 are connected in parallel across matrix control Chain1; capacitors C3 and C7 are connected in parallel across matrix control Chain2; capacitors C4 and C8 are connected in parallel across matrix control Chain3; and capacitors C5 and C6 are connected in parallel across matrix control Chain4. The capacitance values ​​of each capacitor are: C1 = C2 = C3 = C5 = C6 = C7 = 0.4523F, C4 = C8 = 0.80638F.

[0065] To physically implement the filter structure, it is necessary to... Figure 4 The lumped circuit in the circuit is transformed into a transmission line circuit through a network of generalized unit elements (GUEs). (The sentence is incomplete and ends abruptly.) Figure 4 The lumped circuit modules in the model are treated as generalized unit elements (GUEs), and then each GUE is converted into a transmission line circuit module. The specific implementation steps are as follows.

[0066] Transmission line circuits are based on λ The 4 / 4 (quarter wavelength) transmission line filtering theory consists of four equivalent transmission line modules. For example... Figure 6 As shown, each transmission line equivalent module consists of λ / 4 Transmission line (impedance) Z t Electrical length θ ) and 2 λ / 4 Short-circuited stub wire (impedance) Z sc Electrical length θ The transmission line consists of two short-circuited stubs connected to its two ends. The main task at present is to determine the relationship between the characteristic impedance of the lumped coupling elements and the transmission line.

[0067] Based on the equivalent characteristic impedance and the extended Richard transform, we can obtain Figure 5 Transmission parameter matrix of the generalized unit element (GUE) shown in (a) As shown in formula (1).

[0068] (1)

[0069] in, This represents the impedance that matches the source and load impedances of a two-port network. This represents the phase constant.

[0070] like Figure 5As shown in (b), the lumped circuit module is a type of GUE. It is defined by a port (admittance g0), two capacitors (C), and components in parallel form with frequency-invariant admittance (B), coupled by an inverter (J).

[0071] The transmission parameter matrix (ABCD matrix) of the lumped circuit is expressed by formula (2):

[0072] (2)

[0073] in, Z LC Represents the equivalent characteristic impedance of a lumped circuit. ;

[0074] λ LC The extended Richard transform represents the lumped circuit. ;

[0075] Represents the imaginary unit. Indicates the converter parameters, This represents the normalized angular frequency parameter. This indicates a capacitor element.

[0076] To facilitate subsequent discussion, θ Defined as the electrical length of a transmission line, it is a frequency-dependent variable. θc This represents the frequency point that indicates the lower cutoff frequency of the passband. In this context, such as... Figure 6 The transmission parameter matrix (ABCD matrix) of the transmission line circuit shown can be represented as:

[0077] ;

[0078] in, Z TL This represents the equivalent characteristic impedance of a transmission line circuit. ;

[0079] λ TL The extended Richard transform represents the transmission line circuit. ;

[0080] , ; ;

[0081] Z t It is the reference characteristic impedance of the transmission line. θ It is the electrical length of the transmission line. g ( θ )and f ( θ (Based on electrical length) θ Auxiliary functions, Z IN Z is the input impedance of the transmission line. sc This is the input impedance of the short-circuited stub.

[0082] To make two GUEs equivalent, their equivalent characteristic impedances ( Z LC and Z TL ) and extended Richard transform ( λ LC and λ TL The conditions for equal sums of squares must be met. The following formulas (3) and (4) must be satisfied:

[0083] (3)

[0084] (4)

[0085] Further analysis of formula (4) reveals that its corresponding equivalent characteristic impedance differs from the extended Richards transform in both expression and frequency variable. Specifically, the constant inverter (J) is associated with a function g(θ), while its frequency variable is ω in the LC circuit and θ in the corresponding transmission line model. To unify them to the same expression and frequency plane, two mapping functions are introduced in this study: the frequency mapping function (Fm) and the constant mapping function (Cm), as shown in formulas (5a) and (5b). According to the two mapping functions, the frequency mapping (Fm) m ) and constant mapping (C m ), which can be used to obtain the characteristic impedance in a transmission line circuit.

[0086] (5a)

[0087] (5b)

[0088] in, σ ω and σ c These are normalization factors related to the filter cutoff frequency. Their calculation formula is Equation (6):

[0089] (6)

[0090] in, t c Defined as t c yes θ cThe tangent value, i.e. The absolute bandwidth is calculated as 2(90- θ c Fractional bandwidth (FBW) is calculated as (90 - θ c ) / 45.

[0091] Based on the derived formula, the final determination is... Figure 6 Transmission line impedance Z t and short-circuit stub impedance Z sc The calculation formula is:

[0092] (7a)

[0093] (7b)

[0094] Through the equivalence of GUE and distributed circuits, Figure 2 The low-pass prototype of the linear phase filter shown (n=4) is equivalent to, as follows: Figure 7 The diagram shows the structure of a broadband linear bandpass filter containing four generalized unit elements (GUEs).

[0095] To illustrate this more intuitively, an ideal transmission line circuit is built in ADS software. Figure 4 The equivalent relationship between the equivalent unit and the transmission line structure is as follows: Figure 8As shown. The equivalent unit composed of matrix control Chain1, capacitors C1 and C2 is equivalent to the transmission line structure composed of the first transmission line TL11 and short-circuit stubs SSL11 and SSL21; wherein, the impedance value of the first transmission line TL11 is 1.0624 Ω, and the impedance values ​​of the short-circuit stubs SSL11 and SSL21 are both 5.9499 Ω. The equivalent unit composed of matrix control Chain4, capacitors C5 and C6 is equivalent to the transmission line structure composed of the first transmission line TL13 and short-circuit stubs SSL41 and SSL32; wherein, the impedance value of the first transmission line TL13 is 1.0624 Ω, and the impedance values ​​of the short-circuit stubs SSL41 and SSL32 are both 5.9499 Ω. The equivalent unit composed of matrix control Chain2, capacitors C3 and C7 is equivalent to the transmission line structure composed of the first transmission line TL14 and short-circuit stubs SSL12 and SSL42; wherein the impedance of the first transmission line TL14 is 6.1266 Ω, and the impedance of the short-circuit stubs SSL12 and SSL42 is 1.0767 Ω each. The equivalent unit composed of matrix control Chain3, capacitors C4 and C8 is equivalent to the transmission line structure composed of the first transmission line TL12 and short-circuit stubs SSL22 and SSL31; wherein the impedance of the first transmission line TL12 is 1.3879 Ω, and the impedance of the short-circuit stubs SSL22 and SSL31 is 0.8155 Ω each.

[0096] The equivalent circuit of the entire broadband linear phase (source impedance z0=1) was finally obtained through the equivalent relationship. Figure 8 The four equivalent transmission line circuit modules, such as Figure 9 As shown. The electrical length variable E of the transmission line in each transmission line circuit module is 90 degrees.

[0097] To achieve the connection between the devices, the impedance values ​​of the first port (Term1) and the second port (Term2) of the circuit are changed to a standard 50 Ω, and... Figure 9 Adjacent λ / 4 Short-circuit stubs are simplified based on their parallel connection. Adjacent short-circuit stubs SSL11 and SSL12 are simplified to one short-circuit stub SSL1, adjacent short-circuit stubs SSL21 and SSL22 are simplified to one short-circuit stub SSL2, adjacent short-circuit stubs SSL31 and SSL32 are simplified to one short-circuit stub SSL3, and adjacent short-circuit stubs SSL41 and SSL42 are simplified to one short-circuit stub SSL4. The simplified broadband linear phase filter circuit is shown below. Figure 10As shown. The simulation frequency is set to 2.4 GHz, the electrical length E is 90 degrees, and the impedance of the first port Term1 and the second port Term2 is 50 Ω. The impedance values ​​of the first transmission lines TL11 and TL13 are both 54.12Ω, the impedance value of the first transmission line TL12 is 69.39Ω, and the impedance value of the first transmission line TL14 is 306.33Ω; the impedance values ​​of the short-circuited stub lines SSL1 and SSL4 are both 45.6Ω, and the impedance values ​​of the short-circuited stub lines SSL2 and SSL3 are both 35.85Ω.

[0098] like Figure 10 As shown, the simplified circuit contains a high-impedance transmission line TL14 with an impedance of 306.33 Ω. However, this high-impedance transmission line (impedance greater than 150 Ω) cannot be practically fabricated. Therefore, this invention replaces the high-impedance line TL14 with an inductor and a short transmission line. The electrical length of the equivalent inductor circuit is... Characteristic impedance Z eq and series reactance X eq The relationship between the parameters of the two circuits before and after the equivalent circuit:

[0099] (8)

[0100] (9)

[0101] in, x It is normalized reactance. z It is the normalized impedance.

[0102] According to formulas (8)-(9), the parameters of the equivalent structure corresponding to the high impedance line 306.33 Ω are calculated as follows: the inductance of inductor L1 is L = 20 nH, the impedance values ​​of the two second transmission lines TL21 and TL22 are both 45.6 Ω, and the electrical lengths are both... =10°. After replacing the high-impedance wire, the result is as follows: Figure 1 The circuit shown is simulated at a frequency of 2.4 GHz. Figure 1 The electrical length of the second transmission line in the diagram is . = 10°, and the electrical length of other transmission lines is 90 degrees.

[0103] Build in ADS software Figure 1 The ideal circuit shown was simulated, and the final simulation results are as follows. Figure 11 As shown. Figure 11 (a) The simulated S-parameter curves of the designed circuit, i.e. Figure 1 S in the circuit schematic 11 and S 21Simulation results are shown. The S-parameter results from the theory and simulation are basically consistent, verifying the effectiveness of the method. The center frequency of both the theory and simulation results is 2.4 GHz, the passband range is 1.8 GHz-3.0 GHz (50% bandwidth), and the return loss is less than -20 dB. Figure 11 (b) Simulation curves of the group delay parameters of the designed circuit. The group delay ranges from 0.982 ns to 1.033 ns. Specifically, the group delay at point m1 (corresponding to a frequency of 1.8 GHz) is 1.033 ns, at point m2 (corresponding to a frequency of 2.4 GHz) is 0.982 ns, and at point m3 (corresponding to a frequency of 3.0 GHz) is 1.033 ns. This basically meets the filter's specifications and performance requirements.

[0104] Example

[0105] The layout was simulated and optimized in ADS, and the resulting layout dimensions, while meeting the filter performance requirements, are as follows: Figure 12 As shown in the diagram. The diagram includes the first port (port 1) and the second port (port 2), and their corresponding... λ The microstrip line circuit employs a compact layout, with optimized lengths and widths for each microstrip line segment. The widths and lengths of the transmission lines are as follows: First transmission line TL11: 16.47 mm long, 1.57 mm wide; First transmission line TL12: 19.09 mm long, 1.03 mm wide; First transmission line TL13: 16.47 mm long, 1.57 mm wide; Short-circuit stub SSL1: 19.19 mm long, 2.09 mm wide; Short-circuit stub SSL2: 16.57 mm long, 2.98 mm wide; Short-circuit stub SSL3: 19.19 mm long, 2.09 mm wide; Short-circuit stub SSL4: 16.57 mm long, 2.98 mm wide; Second transmission line TL21: 1.8 mm long, 2.09 mm wide; Second transmission line TL22: 1.8 mm long, 2.8 mm wide. The diameter is 2.09 mm, and the width is 2.09 mm. The inductance L1 is 20 nH.

[0106] Figure 13 Showing Figure 12 The layout simulation results (actual) of the broadband bandpass linear phase filter shown are compared with... Figure 1 The simulation results of the broadband bandpass linear phase filter (theoretical) shown are compared, and the curves of the two are in good agreement, which verifies the effectiveness of the proposed method. The designed center frequency is 2.4 GHz, and the simulated passband range is 1.8 GHz-3.0 GHz (50%), which proves the effectiveness of the design method provided by the present invention.

[0107] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A novel broadband linear phase filter, characterized in that, include: First port; Second port; Three first transmission lines are connected in series between the first port and the second port; Among them, four connection points are formed between the first port and the adjacent first transmission line, between the three first transmission lines, and between the second port and the adjacent first transmission line; Four short-circuit stubs are connected one-to-one at the four connection points; An inductor circuit module, one end of which is connected to the connection point between the first port and the adjacent first transmission line, and the other end of which is connected to the connection point between the second port and the adjacent first transmission line.

2. The novel broadband linear phase filter according to claim 1, characterized in that, The inductor circuit module consists of an inductor and two second transmission lines; The two second transmission lines are respectively connected to the two ends of the inductor.

3. The novel broadband linear phase filter according to claim 2, characterized in that, The first transmission line is a quarter-wavelength transmission line.

4. The novel broadband linear phase filter according to claim 3, characterized in that, The short-circuit stub line is a quarter-wavelength short-circuit stub line.

5. A novel design method for a broadband linear phase filter, used to design the novel broadband linear phase filter as described in any one of claims 1-4, characterized in that, Includes the following steps: Step 1: Treat each lumped circuit module in the prototype of the fourth-order linear phase low-pass filter as a generalized unit element, and convert the lumped circuit module into a transmission line circuit module. The transmission line circuit module consists of a first transmission line and two short-circuit stubs, with the two short-circuit stubs respectively connected to both ends of the first transmission line. Step 2: Based on the equivalent relationship between the lumped circuit module and the transmission line circuit module, determine the characteristic impedance of the first transmission line and the short-circuited stub line in the transmission line circuit module to obtain the first filter; Step 3: Simplify two adjacent short-circuit stubs in the first filter into one short-circuit stub based on their parallel relationship to obtain the second filter; Step 4: Convert the high-impedance transmission line in the second filter into an equivalent inductor circuit module to obtain the novel broadband linear phase filter. The high-impedance transmission line refers to a transmission line with an impedance value greater than 150Ω.

6. The design method of the novel broadband linear phase filter according to claim 5, characterized in that, In step two, the method for determining the impedance of the first transmission line and the short-circuit stub in the transmission line circuit module is as follows: Construct the transmission parameter matrix of the lumped circuit module and the transmission parameter matrix of the transmission line circuit module, and establish the equivalent relationship between the elements in the transmission parameter matrix of the lumped circuit module and the elements in the transmission parameter matrix of the transmission line circuit module based on the equivalence relationship. By solving the equality relationship using the frequency mapping function and the constant mapping function, the impedances of the first transmission line and the short-circuit stub line in the transmission line circuit module are obtained.

7. The design method of the novel broadband linear phase filter according to claim 6, characterized in that, The transmission parameter matrix of the lumped circuit module is as follows: ； in, This represents the transmission parameter matrix of the lumped circuit module. This represents the equivalent characteristic impedance of the lumped circuit module. Represents the extended Richard transform of lumped circuit modules. It indicates that it is an imaginary unit. Indicates the converter parameters, This represents the normalized angular frequency parameter. This indicates a capacitor element.

8. The design method of the novel broadband linear phase filter according to claim 7, characterized in that, The transmission parameter matrix of the transmission line circuit module is as follows: ； in, This represents the transmission parameter matrix of the transmission line circuit module. This represents the equivalent characteristic impedance of the transmission line circuit module. This represents the extended Richard transform of the transmission line circuit module. Indicates the input impedance of the transmission line. This represents the reference characteristic impedance of the transmission line. Indicates the electrical length of the transmission line.

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