CN103296346B - A kind of micro-band balance filter - Google Patents

Abstract

The invention provides a microstrip balanced filter. The filter is composed of two microstrip bandpass filters with the same structure and connected symmetrically. Under the excitation of the differential mode signal, the source-load coupling structure produces transmission zeros at the edge of the passband and in the stopband, which improves the frequency selectivity in the passband and deepens the stopband; The folded oscillator produces two common-mode signal transmission zeros within the differential-mode passband, thereby improving the common-mode rejection. The balanced filter has the characteristics of high frequency selectivity, low insertion loss, wide stop band and compact structure.

Description

A microstrip balanced filter

technical field

The invention belongs to the field of electronic technology, and in particular relates to a microstrip balanced filter.

Background technique

Balanced circuits can efficiently suppress environmental noise and noise generated by internal active devices in the system, so they are widely used in modern communication systems. The balanced filter requires filtering characteristics when the differential mode signal is input, and has a high common mode rejection capability. The traditional balanced filter is composed of a single-port filter and two balun structures, which not only has a huge circuit area, but also has poor common-mode noise suppression ability.

The use of double-sided parallel striplines can improve the common-mode rejection capability, but this will lead to a large system size; some scholars use a comb-line structure with tapped input, which can effectively suppress common-mode noise, but the resulting balanced filter isolation The density is low, and the circuit design is complex, which is not easy to process.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and propose a microstrip balanced filter. The filter has two common-mode signal transmission zeros in the differential-mode passband, which effectively suppresses common-mode noise, and has very low insertion loss and wide stopband. The circuit is simple and easy to process.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A microstrip balanced filter, composed of a first filter and a second filter, the first filter and the second filter have the same structure;

The first filter includes: a first feeder, a second feeder, a first open-circuit branch line, a second open-circuit branch line, a first resonator, a second resonator, a first folded oscillator, and a first stepped impedance branch; The first feeder line and the second feeder line are arranged in a straight line, and the spacing is a specified value; the first open branch line and the second open branch line are Z-shaped structures, which are respectively connected to one end of the first feeder line and the second feeder line. An open-circuit branch line and a second open-circuit branch line respectively form a first source-load coupling structure at the bending parts close to the first feeder end and the second feeder end; the first resonator and the second resonator are both U-shaped structure, respectively coupled with the first open-circuit branch line and the second open-circuit branch line, and the first resonator and the second resonator are mutually coupled; the first folded oscillator is loaded in the middle of the second feeder line; the first A ladder impedance branch is loaded on the end of the first resonator, and the first resonator is connected to the low impedance line by a high impedance line;

The second filter includes: a third feeder line, a fourth feeder line, a third open-circuit branch line, a fourth open-circuit branch line, a third resonator, a fourth resonator, a second folded oscillator, and a second stepped impedance branch; The third feeder line and the fourth feeder line are distributed in a straight line, and the spacing is a specified value; the third open branch line and the fourth open branch line are both Z-shaped structures, which are respectively connected to one end of the third feeder line and the fourth feeder line. The three open-circuit branch lines and the fourth open-circuit branch line respectively form a second source-load coupling structure at the bending parts close to the third feeder end and the fourth feeder end; the third resonator and the fourth resonator are U-shaped structure, respectively coupled with the third open-circuit branch line and the fourth open-circuit branch line, and the third resonator and the fourth resonator are mutually coupled; the second folded oscillator is loaded in the middle of the fourth feeder line; the first The second ladder impedance branch is loaded on the end of the third resonator, and the third resonator is connected to the low impedance line by a high impedance line;

The first filter is symmetrically connected to the second filter, wherein the first resonator is connected to the third resonator, the second resonator is connected to the fourth resonator, the first ladder impedance branch is connected to the second ladder impedance branch, The first folded oscillator is connected to the second folded oscillator; the other ends of the first feeder and the third feeder are respectively used as the output port or the input port of the microstrip balanced filter; the other ends of the second feeder and the fourth feeder are One end serves as an input port or an output port of the microstrip balanced filter respectively.

The impedances of the first feeder, the second feeder, the third feeder and the fourth feeder are all 50 ohms.

The beneficial effects of the present invention are: the present invention provides a microstrip balanced filter, the filter can generate two common-mode signal transmission zeros in the differential-mode signal passband, has high common-mode rejection, and Multiple transmission zeros are generated at the edge of the passband of the differential mode signal to make the edge of the passband steeper and improve the frequency selectivity of the passband. The balanced filter has low insertion loss, wide stop band, compact structure and is easy to be integrated with other circuits.

Description of drawings

Figure 1 is a schematic diagram of a single-layer printed circuit board.

Figure 2 is a schematic diagram of a balanced filter.

Fig. 3 is the equivalent circuit under common mode signal excitation.

Fig. 4 is the equivalent circuit under the excitation of differential mode signal.

Fig. 5 is the simulation and measurement results of the differential mode signal insertion loss curve S _dd21 and the differential mode signal return loss curve S _dd11 of the balanced filter.

Fig. 6 is the simulation and measurement results of the common-mode signal insertion loss curve S _cc21 of the balanced filter.

Figure 7 is a partially enlarged view of the differential-mode signal insertion loss, return loss, and common-mode signal insertion loss of the balanced filter in the differential-mode passband, which can more clearly display the characteristics in the differential-mode passband.

Explanation of reference numerals: in Fig. 1 to Fig. 4,

1: first filter; 2: second filter;

3: the first feeder; 3’: the third feeder; 4: the second feeder; 4’: the fourth feeder;

5: the first open branch line; 5': the third open branch line; 6: the second open branch line; 6': the fourth open branch line;

7: the first resonator; 7': the third resonator; 8: the second resonator; 8': the fourth resonator;

9: The first ladder impedance branch; 9': The second ladder impedance branch; 10: The first folded oscillator; 10': The second folded oscillator;

11: Upper metal patch; 12: Dielectric substrate; 13: Lower metal patch;

P1: input port 1; P1', input port 1'; P2: output port 1; P2': output port 1'; or use P2 and P2' as input ports, and P1 and P1' as output ports;

Z: first source-load coupling structure; Z': second source-load coupling structure.

Detailed ways

Below in conjunction with accompanying drawing, a kind of microstrip balanced filter that the present invention proposes is described in detail:

The output port and the input port of the balanced filter of the present invention are respectively welded with SMA heads, so as to be connected to testing or practical devices.

In this embodiment, a PCB board with a relative dielectric constant of 2.2 and a thickness of 0.508 mm is used as the substrate, and a PCB board of other specifications may also be used as the substrate. As shown in FIG. 1 , an upper metal patch 11 and a lower metal patch 13 are respectively coated on the upper and lower surfaces of the dielectric substrate 12 of the PCB.

As shown in Figure 2, the balanced filter of the present invention is formed on the PCB board and consists of two microstrip filters, which are respectively the first filter 1 and the second filter 2; the first filter 1 and the second filter The two filters 2 are connected at the dotted line AA' and are symmetrical along the dotted line AA'. The first filter 1 includes: a first feeder 3 and a first open-circuit branch line 5, a second feeder 4 and a second open-circuit branch line 6, a first resonator 7, a second resonator 8, and a first folded oscillator 10 , the first ladder impedance branch 9; the second filter includes: the third feeder 3' and the third open-circuit branch line 5', the fourth feeder line 4' and the fourth open-circuit branch line 6', the third resonator 7' , the fourth resonator 8', the second folded oscillator 10', and the second ladder impedance branch 9'. Taking the first filter 1 as an example, due to the complete symmetry of the structure, it is also applicable to the second filter 2 . In the first filter 1, the first open branch line 5 is connected to the first feeder 3, the second open branch line 6 is connected to the second feeder 4, and the first open branch line 5 and the second open branch line 6 are formed on the length d The first source-load coupling structure Z, the first ladder impedance branch 9 is connected to the first resonator 7, the first resonator 7 and the first open-circuit branch line 5 are mutually coupled on the length L2, the second resonator 8 is connected to the second The open-circuit branch lines 6 are coupled to each other over a length L2, the first resonator 7 and the second resonator 8 are coupled to each other, and the coupling length is L1, and the first folded dipole 10 is connected to the middle of the second feeder line 4 .

When the common-mode signal enters the system from the input port P1 and the input port P1', the balanced filter is equivalent to an open circuit on the axis of symmetry AA', and its equivalent circuit is shown in Figure 3. At this time, the first resonator 7, the second resonator 8, the third resonator 7', and the fourth resonator 8' are all 1/2 of the working wavelength, and the first ladder impedance branch 9 and the second ladder impedance The stubs 9' are loaded at the open-circuit ends of the first resonator 7 and the third resonator 7', respectively. The coupling between the half-wavelength oscillator loaded with the stepped impedance stub and the open stub of the feeder produces a transmission zero point, and the position of the transmission zero point can be changed by adjusting the length of the stepped impedance stub. Therefore, the length of the stepped impedance stub is appropriately selected A value that places the transmission zero within the passband of the differential-mode signal improves rejection of the common-mode signal. In addition, the reduced oscillator loaded on the 50 ohm feeder can also generate a new transmission zero point in the passband of the differential mode signal. Therefore, there are two transmission zero points of the common-mode signal in the passband of the differential-mode signal, which greatly improves the common-mode rejection level.

When the differential mode signal enters the system from the input port P1 and input port P1', the balanced filter is equivalent to a short circuit on the symmetry axis AA', and its equivalent circuit is shown in Figure 4. At this time, the ladder impedance branch is almost ineffective. Under the excitation of the differential mode signal, the lengths of the first resonator 7, the second resonator 8, the third resonator 7' and the fourth resonator 8' are all a quarter of the working wavelength. Both the first filter and the second filter are second-order, and the first filter is taken as an example for description without loss of generality. The first source-load coupling structure Z formed by the input 50 ohm first feeder 3 and the output 50 ohm second feeder 4 can generate a transmission zero at the upper edge of the passband and in the stopband respectively, thereby increasing the upper edge of the passband Frequency selectivity and deepening of the stop band, while harmonics generated by the quarter-wave resonator can also be suppressed. By properly adjusting the distance g1 of the source-load coupling structure and the length d shown in Figure 2 or Figure 4, a filter with a wide stopband and sharp edges can be obtained. In addition, the equivalent oscillator loaded on the 50 ohm output second feeder 4 can generate two new transmission zeros in the differential mode stop band, which further widens and deepens the differential mode signal stop band and improves selectivity.

Fig. 5 is the simulation and measurement results of the differential mode signal insertion loss curve S _dd21 and the differential mode signal return loss curve S _dd11 of the balanced filter. It can be seen from the figure that the simulation results are in good agreement with the measured values, and the slight deviation and error of the center frequency are due to manufacturing errors. The 2dB relative bandwidth of the differential mode signal insertion loss is 7.3%, the measured minimum insertion loss value is -0.9dB, and the return loss values in the differential mode passband are all less than -10dB. The two transmission zeros generated in the upper stopband make the frequency selectivity improved, and widen and deepen the stopband.

Figure 6 is the simulation and measurement results of the common-mode signal insertion loss curve S _cc21 of the balanced filter. It can be seen from the figure that the two are basically consistent. In the differential mode passband, the two common mode transmission zeros located at 2.39GHz and 2.58GHz have an attenuation of -68.5dB, which greatly suppresses the transmission of common mode signals. In the entire differential mode passband, the common mode signal is inserted The losses are all below -51dB, which fully demonstrates that the balanced filter of the present invention has a very high level of common-mode rejection. It can also be seen from the figure that the stopband of the common-mode signal is under the level of rejection of -14.5dB. Up to 11GHz.

From Figure 7, we can see more clearly the values of the insertion loss and return loss of the differential-mode signal and the insertion loss of the common-mode signal in the passband of the differential-mode signal. The insertion loss value is less than -51dB, indicating that the balanced filter of the present invention has a high level of common-mode rejection.

Claims (2)

1. a micro-band balance filter, is characterized in that, be made up of the first filter and the second filter, the first filter is identical with the second filter construction;

Described first filter comprises: the first feeder line, the second feeder line, the first open circuit branch line, the second open circuit branch line, the first resonator, the second resonator, the first folded dipole and the first stepped impedance minor matters; Described first feeder line becomes a word distribution with the second feeder line, spacing is designated value; Described first open circuit branch line and the second open circuit branch line are Z-type structure, be connected to one end of the first feeder line and the second feeder line, the first open circuit branch line and the second open circuit branch line are forming the first source-load coupling structure close to the first feeder line end respectively together with the bending part of the second feeder line end; Described first resonator and the second resonator are U-shaped structure, intercouple respectively, and intercouple between the first resonator and the second resonator with the first open circuit branch line and the second open circuit branch line; Described first folded dipole loads on the middle part of the second feeder line; Described first stepped impedance minor matters load on the end of the first resonator, are connected by the first resonator by high impedance line with low-impedance line;

Described second filter comprises: the 3rd feeder line, the 4th feeder line, the 3rd open circuit branch line, the 4th open circuit branch line, the 3rd resonator, the 4th resonator, the second folded dipole and the second stepped impedance minor matters; Described 3rd feeder line becomes a word distribution with the 4th feeder line, spacing is designated value; Described 3rd open circuit branch line and the 4th open circuit branch line are Z-type structure, be connected to one end of the 3rd feeder line and the 4th feeder line, the 3rd open circuit branch line and the 4th open circuit branch line are forming the second source-load coupling structure close to the 3rd feeder line end respectively together with the bending part of the 4th feeder line end; Described 3rd resonator and the 4th resonator are U-shaped structure, intercouple respectively, and intercouple between the 3rd resonator and the 4th resonator with the 3rd open circuit branch line and the 4th open circuit branch line; Described second folded dipole loads on the middle part of the 4th feeder line; Described second stepped impedance minor matters load on the end of the 3rd resonator, are connected by the 3rd resonator by high impedance line with low-impedance line;

Described first filter is connected with the second filter symmetry, wherein the first resonator is connected with the 3rd resonator, second resonator is connected with the 4th resonator, and the first stepped impedance minor matters are connected with the second stepped impedance minor matters, and the first folded dipole is connected with the second folded dipole; The other end of described first feeder line and the 3rd feeder line is respectively as the output port of described micro-band balance filter or input port; The other end of described second feeder line and the 4th feeder line is respectively as the input port of described micro-band balance filter or output port.

2. a kind of micro-band balance filter as claimed in claim 1, is characterized in that, described first feeder line, the second feeder line, the 3rd feeder line and the 4th feed line impedance are all 50 ohm.