CN114639932B - Microstrip differential band-pass filter - Google Patents

Abstract

The invention provides a microstrip differential band-pass filter, which belongs to the technical field of radio frequency microwave filters and comprises a PCB top layer, a dielectric substrate and a PCB bottom layer in sequence, wherein the PCB top layer and the PCB bottom layer are correspondingly provided with microstrip structures, and the microstrip differential band-pass filter and the dielectric substrate positioned between the two microstrip structures jointly form a microstrip differential circuit; the microstrip differential circuit comprises a fourth capacitor, a first differential transformer, a fifth capacitor, a second differential transformer and a sixth capacitor in sequence; the first differential transformer and the second differential transformer are formed by sequentially connecting a plurality of microstrip lines alternately positioned on the top layer and the bottom layer of the PCB in series through metalized through holes penetrating through the dielectric substrate. The invention utilizes the differential transformer to convert the inductive coupling differential band-pass filter into the differential band-pass filter with simpler and symmetrical structure, is suitable for being realized on a PCB board through the design of a microstrip line, and has the advantages of convenient debugging and large adjustable range of the frequency band of the filter.

Description

A Microstrip Differential Bandpass Filter

technical field

The invention belongs to the technical field of radio frequency microwave filters, and in particular relates to a microstrip differential bandpass filter.

Background technique

The filter is the basic device in electronics and one of the key components in the wireless communication transceiver. The performance of the filter directly affects the performance of the system in which it is located. The main function of the filter is to pass specific frequency components in the signal, while greatly attenuating other frequency components. Using this frequency selection function of the filter, interference noise can be filtered out or spectrum analysis can be performed. When it comes to communication systems, differential circuits always provide better performance than single-ended circuits. For example, higher signal amplitudes can be achieved with differential circuits than with single-ended circuits. Differential signals provide twice the amplitude of single-ended signals at the same supply voltage, as well as better linearity and SNR (signal-to-noise ratio) performance. In recent years, with the research of differential circuits, effective suppression of harmful signals in the system has been achieved. Compared with single-ended circuits, differential circuits have inherent suppression effects on environmental noise and electromagnetic interference in the system. More and more microwave devices are designed in a differential structure, such as power amplifiers, mixers, and low-noise amplifiers. Among them, the differential filter plays a pivotal role. It not only needs to have good differential mode response and common mode rejection in the required frequency band, but also has good frequency selectivity, appearing on both sides of the differential mode passband as much as possible. Transmission zero, and wider and better out-of-band rejection are also must-have features. Therefore, differential filters have received extensive attention and research.

As a common differential filter, the inductively coupled differential bandpass filter has a circuit structure as shown in FIG. 1 , including a first inductor L ₁ , a second inductor L ₂ , a third inductor L ₃ , a fourth inductor L ₄ , the fifth inductor L ₅ , the sixth inductor L ₆ , the seventh inductor L ₇ , the first capacitor C ₁ , the second capacitor C ₂ and the third capacitor C ₃ ; wherein the first inductor L ₁ and the second inductor L ₂ are connected in series connected to the differential positive line; the _third inductance L3 and the _fourth inductance L4 are connected in series on the differential negative line; the _fifth inductance L5 and the first capacitor _C1 form a first resonant circuit connected in parallel to the differential input end; the sixth The inductor L ₆ and the second capacitor C ₂ form a second resonant circuit, and the two ends are respectively connected between the first inductor L ₁ and the second inductor L ₂ and between the third inductor L ₃ and the fourth inductor L ₄ ; The inductor L ₇ and the third capacitor C ₃ form a third resonant circuit connected in parallel with the differential output terminal. Ideally, the above parameters should satisfy: L ₁ =L ₂ =L ₃ =L ₄ , L ₅ =L ₆ =L ₇ , C ₁ =C ₃ .

In recent years, the development of microstrip differential filters has been extremely rapid. Scholars at home and abroad have researched and proposed a variety of microstrip-based differential filter design methods, which have strongly promoted the development of microwave differential filters. At present, the microstrip differential filter structures mainly include a double-sided parallel stripline structure, a coupling-based microstrip stepped impedance line structure, a cross resonator-based and a microstrip-slotline-based structure.

For the inductively coupled differential bandpass filter composed of multiple inductors and capacitors, the microstrip line is used on the PCB (printed circuit board) board to design it as a microstrip differential bandpass filter, which has a complex structure and a narrow bandwidth. , as well as the shortcomings of not easy to debug, and the application is limited.

SUMMARY OF THE INVENTION

Aiming at the problems existing in the inductively coupled differential bandpass filter in the above-mentioned background technology, the present invention proposes a microstrip differential bandpass filter. Compared with the microstrip circuit based on the inductively coupled differential bandpass filter, it has the advantages of simple structure, Symmetrical, easy-to-implement advantage.

For achieving the above object, the technical scheme adopted in the present invention is as follows:

A microstrip differential bandpass filter is characterized in that it comprises a top layer of a PCB board, a dielectric substrate and a bottom layer of the PCB board arranged in sequence, and the top layer of the PCB board and the bottom layer of the PCB board include two correspondingly arranged floors and a middle of the two floors. The microstrip structure, the microstrip structure on the top layer of the PCB board and the bottom layer of the PCB board and the dielectric substrate between them together form a microstrip differential circuit;

The microstrip differential circuit has a symmetrical structure, including a fourth capacitor C ₄ , a first differential transformer, a fifth capacitor C ₅ , a second differential transformer and a sixth capacitor C ₆ arranged in sequence; the first differential transformer and the sixth Two differential transformers are formed by a plurality of microstrip lines alternately located on the top layer of the PCB board and the bottom layer of the PCB board in series through metallized vias penetrating the dielectric substrate; the fourth capacitor C ₄ is connected to the differential input terminal, and the sixth capacitor C ₆ is connected to the differential output terminal.

Further, the capacitance values of the fourth capacitor C ₄ and the sixth capacitor C ₆ are equal, and are equal to the capacitance values of the first capacitor C ₁ and the third capacitor C ₃ corresponding to the inductively coupled differential bandpass filter in an ideal state.

Further, the parameters of the first differential transformer and the second differential transformer are the same, and the self-inductances L _a , L _b and the mutual inductance M satisfy:

Wherein, Z ₁ , Z ₅ and Z ₆ are the impedances of the first inductance L ₁ , the fifth inductance L ₅ and the sixth inductance L ₆ in the ideal state corresponding to the inductively coupled differential bandpass filter respectively;

Since the inductively coupled differential bandpass filter ideally satisfies: Z ₅ =Z ₆ , so

Among them, ω is the angular frequency of the transmitted signal, and j is the imaginary unit.

Further, under the condition that the self-inductances _La , _Lb and mutual inductance M of the first differential transformer and the second differential transformer, as well as the capacitance values of the fourth capacitor _C4 and the sixth capacitor _C6 have been determined, it is obtained through simulation optimization. The capacitance value of the fifth capacitor _C5 .

Further, the fourth capacitor C ₄ , the first differential transformer and the second differential transformer and the sixth capacitor C ₆ are symmetrical with respect to the fifth capacitor C ₅ .

Further, the fourth capacitor C ₄ , the fifth capacitor C ₅ and the sixth capacitor C ₆ are all plate capacitors, which are composed of a microstrip line on the top layer of the PCB board, a dielectric substrate and a microstrip line on the bottom layer of the PCB board.

Further, the two floors on the top layer of the PCB board are connected to the corresponding floors on the bottom layer of the PCB board through metallized vias penetrating through the dielectric substrate.

Further, the self-inductance and mutual inductance of the first differential transformer and the second differential transformer are adjusted by controlling the line width of the microstrip line, the geometric shape of the microstrip line, and the position and size of the metallized via.

The beneficial effects of the present invention are:

The invention proposes a new type of microstrip differential bandpass filter, which utilizes a differential transformer to convert the inductively coupled differential bandpass filter into a differential bandpass filter with a simpler and symmetrical structure. The structure is suitable to be realized by microstrip line design on the PCB board, and has the advantages of simple and symmetrical structure, easy debugging, and a large adjustable range of filter frequency band.

Description of drawings

Figure 1 is a schematic diagram of the circuit structure of an inductively coupled differential bandpass filter;

Fig. 2 is the simulation diagram of the inductively coupled differential bandpass filter in the comparative example;

3 is a schematic diagram of a circuit structure of a microstrip differential bandpass filter proposed in an embodiment of the present invention;

4 is a simulation diagram of a microstrip differential bandpass filter proposed in an embodiment of the present invention;

5 is a schematic diagram of a plate capacitor adopted by the fourth capacitor C ₄ , the fifth capacitor C ₅ and the sixth capacitor C ₆ in the embodiment of the present invention;

6 is a schematic structural diagram of a first differential transformer and a second differential transformer in an embodiment of the present invention;

7 is a top perspective structural view of a microstrip differential bandpass filter proposed by an embodiment of the present invention;

FIG. 8 is a bottom perspective structural diagram of a microstrip differential bandpass filter proposed in an embodiment of the present invention.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only some, but not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

Comparative ratio

This comparative example proposes an inductively coupled differential bandpass filter. The structure is shown in FIG. 1 , including a first inductor LL ₁ , a second inductor L ₂ , a third inductor L ₃ , a fourth inductor L ₄ , and a fifth inductor L ₅ , the sixth inductor L ₆ , the seventh inductor L ₇ , the first capacitor C ₁ , the second capacitor C ₂ and the third capacitor C ₃ ; wherein the first inductor L ₁ and the second inductor L ₂ are connected in series to the differential On the positive line; the _third inductance L3 and the _fourth inductance L4 are connected in series on the differential negative line; the _fifth inductance L5 and the first capacitor _C1 form a first resonant circuit connected in parallel with the differential input terminal; the _sixth inductance L6 and the second capacitor C ₂ to form a second resonant circuit, the two ends are respectively connected between the first inductor L ₁ and the second inductor L ₂ and between the third inductor L ₃ and the fourth inductor L ₄ ; the seventh inductor L ₇ and the _third capacitor C3 form a third resonant circuit connected in parallel with the differential output terminal.

In the ideal state, the parameters of the inductively coupled differential bandpass filter are as follows: L ₁ =L ₂ =L ₃ =L ₄ =4.034nH, L ₅ =L ₆ =L ₇ =4.216nH, C ₁ =C ₃ = 1.525 pF, C ₂ = 2.048 pF.

The inductively coupled differential bandpass filter proposed in this comparative example is brought into the ADS (Advanced Design System) software for simulation. The simulation results are shown in Figure 2. It can be seen that its passband range is about 1.8GHz to 2.9GHz, and its bandwidth is about 1.1 GHz.

Example

Aiming at the inductively coupled differential bandpass filter proposed in the comparative example, a microstrip differential bandpass filter is improved by using a differential transformer in this embodiment. And the bottom layer of the PCB board, the top layer of the PCB board and the bottom layer of the PCB board include two correspondingly arranged floors and a microstrip structure located in the middle of the two floors.

The floors on the top layer of the PCB board and the bottom layer of the PCB board are made of large copper sheets, and the two floors on the top layer of the PCB board are connected to the corresponding floors on the bottom layer of the PCB board through metallized vias penetrating the dielectric substrate.

The microstrip differential circuit includes a fourth capacitor C ₄ , a first differential transformer, a fifth capacitor C ₅ , a second differential transformer and a sixth capacitor C ₆ , and the fourth capacitor C ₄ , the first differential transformer and the sixth capacitor C 6 are arranged in sequence. The second differential transformer and the sixth capacitor C ₆ are symmetrical with respect to the fifth capacitor C ₅ . The differential signal is input from the differential input terminal, and is output from the differential output terminal after sequentially passing through the fourth capacitor C ₄ , the first differential transformer, the fifth capacitor C ₅ , the second differential transformer and the sixth capacitor C ₆ .

The fourth capacitor C ₄ , the fifth capacitor C ₅ and the sixth capacitor C ₆ are all plate capacitors. The structure is shown in FIG. 5 . Constructed with lines. Among them, the dielectric substrate is made of FR4 material, the dielectric constant is 9.6, and the dielectric thickness is 0.508mm; in this embodiment, the capacitance values of the fourth capacitor _C4 and the sixth capacitor _C6 are equal, which are equal to the inductive coupling differential bandpass filter in the comparative example. The capacitance value of the first capacitor C ₁ and the third capacitor C ₃ of the device is 1.525pF; the size C of the plate capacitor is determined by the area and shape of the microstrip line, and the calculation formula is:

Among them, d is the plate spacing, which is the thickness of the dielectric substrate here; ε ₀ is the vacuum dielectric constant, and its value is 8.85×10 ¹² (F/m); ε _r is the relative permittivity, where its value is 9.6; When the capacitance value is 1.525pF, the area S of the microstrip line is:

That is, a square microstrip line with a side length of about 3mm can be equivalent to the required plate capacitance. The above calculation is only for the case of ideal capacitance. In practice, the shape and size of the microstrip line need to be debugged.

As shown in FIG. 6 , the first differential transformer and the second differential transformer are formed of a plurality of microstrip lines alternately located on the top layer of the PCB board and the bottom layer of the PCB board in series through metallized vias penetrating the dielectric substrate. Fig. 7 is the top perspective structure diagram of the microstrip differential bandpass filter, that is, the structure view from the top layer of the PCB board to the bottom layer of the PCB board; Fig. 8 is the bottom perspective structure diagram of the microstrip differential bandpass filter, that is, from the PCB board The bottom layer looks at the structure diagram of the top layer of the PCB board.

Due to the use of metallized vias, it is possible to route back and forth between the top layer of the PCB board and the bottom layer of the PCB board, so as to complete the design of the first differential transformer and the second differential transformer between the microstrip lines on the top layer of the PCB board and the bottom layer of the PCB board , and adjust the self-inductance L _a of the first differential transformer and the second differential transformer by controlling the line width of the microstrip line on the top layer of the PCB board and the bottom layer of the PCB board, the geometry of the trace and the position and size of the metallized via. L _b and mutual inductance M.

In this embodiment, the parameters of the first differential transformer and the second differential transformer are the same, and the self-inductances L _a , L _b and the mutual inductance M satisfy:

Wherein, Z ₁ , Z ₅ and Z ₆ are the impedances of the first inductance L ₁ , the fifth inductance L ₅ and the sixth inductance L ₆ in the ideal state corresponding to the inductively coupled differential bandpass filter respectively;

Since the inductively coupled differential bandpass filter in the comparative example satisfies in an ideal state: L ₅ =L ₆ =4.216nH, the ideal conditions of the first differential transformer and the second differential transformer in this embodiment satisfy:

Among them, ω is the angular frequency of the transmitted signal, and j is the imaginary unit.

Under the condition that the self-inductances _La , _Lb and mutual inductance M of the first differential transformer and the second differential transformer, as well as the capacitance values of the fourth capacitor _C4 and the sixth capacitor _C6 have been determined, the fifth capacitor _C5 is used as Adjustable variable, debugging of microstrip differential bandpass filter through simulation optimization. When the capacitance value of the fifth capacitor _C5 is 2.7902pF, the simulation results are shown in Figure 4. It can be seen that its passband range is about 1.875GHz to 3.575GHz, and the bandwidth is about 1.7GHz, which is similar to the inductive coupling differential bandpass in the comparative example. Compared with the simulation structure of the filter, the bandwidth has a certain increase, and the structure is simple and symmetrical, which is convenient for debugging.

Those of ordinary skill in the art will appreciate that the embodiments described herein are intended to assist readers in understanding the principles of the present invention, and it should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. Various modifications and variations of the present invention are possible for those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the scope of the claims of the present invention.

Claims (4)

1. a microstrip differential bandpass filter, it is characterized in that, comprise the PCB board top layer, the dielectric substrate and the PCB board bottom layer that are arranged successively, the PCB board top layer and the PCB board bottom layer respectively comprise correspondingly set two floors and be located in two The microstrip structure in the middle of the floor, the two microstrip structures and the dielectric substrate between them together form a microstrip differential circuit; The microstrip differential circuit has a symmetrical structure, including a fourth capacitor C ₄ , a first differential transformer, a fifth capacitor C ₅ , a second differential transformer and a sixth capacitor C ₆ arranged in sequence; the first differential transformer and the sixth Two differential transformers are formed by a plurality of microstrip lines alternately located on the top layer of the PCB board and the bottom layer of the PCB board in series through metallized vias penetrating the dielectric substrate; the fourth capacitor C ₄ is connected to the differential input terminal, and the sixth capacitor C ₆ is connected to the differential output terminal. 2. The microstrip differential band-pass filter according to claim 1, wherein the fourth capacitor C ₄ , the fifth capacitor C ₅ and the sixth capacitor C ₆ are all plate capacitors, and are formed by a capacitor located on the top layer of the PCB. The microstrip line, the dielectric substrate and the microstrip line on the bottom layer of the PCB are composed. 3 . The microstrip differential bandpass filter according to claim 1 , wherein the two floors on the top layer of the PCB board are connected to the corresponding floors on the bottom layer of the PCB board through metallized vias penetrating through the dielectric substrate. 4 . 4. The microstrip differential bandpass filter according to claim 1, wherein the first differential transformer and the Self and mutual inductance of the second differential transformer.

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