An antenna feeding network system for improving transmission and reception isolation

Abstract

The present invention relates to an antenna feeding network system, comprising a first bridge, a second bridge, a third bridge, and a fourth bridge, wherein the four bridges are interconnected to form two vertically symmetrical paths; wherein the first bridge and the second bridge are both connected to the third bridge and the fourth bridge; the third bridge and the fourth bridge are both connected to antennas; the receiving end of the system is arranged on the first bridge, and the transmitting end of the system is arranged on the second bridge. The present invention forms a cancellation structure through bridge connection, which cancels the leakage signal of the transmitting end while the receiver can receive the signal, and also cancels the reflected signal caused by the mismatch of the antenna port, thereby solving the leakage problem of the transmitting end when the radar is in use and improving the system's transmit-receive isolation.

Description

Technical Field

[0001] The invention relates to an antenna feeding network system, in particular to a 77 GHz antenna feeding network system capable of improving the transmission and reception isolation of an FMCW radar. Background Art

[0002] Early radar technology was mainly based on pulse radar, but pulse radar has obvious disadvantages: it requires a large transmission power, needs to switch between transmission and reception, has a detection blind spot, is bulky, and is easily intercepted. Continuous wave (CW) radar can avoid these disadvantages. It has the advantages of simple structure, small size, and low power, and is therefore increasingly widely used. There are many modulation methods for continuous wave radar, among which frequency modulated continuous wave (FMCW) is the most widely used.

[0003] However, continuous wave radar also has disadvantages: because the transmission and reception work at the same time, when the transmission and reception share the same antenna, or the two antennas are very close to each other, the transmission signal will leak to the receiving end, and the strong leakage signal will cause the sensitivity of the receiver to deteriorate sharply, and even cause the front-end device of the receiver to reach saturation; in addition, the mismatch of the antenna port will cause reflection echoes, and these reflection echoes will enter the receiver and interfere with the sensitivity of the receiver. These are the main factors restricting the development of FMCW radar.

[0004] At present, the on-board 77GHz millimeter wave FMCW radar has become a standard feature of high-end imported cars. The wavelength of 77GHz is only 3.9mm. Circuit design in this frequency band will face problems such as small wavelength, small size, unknown material properties, and difficult processing. Many technologies that are already mature in low-frequency band circuit design cannot be directly used in 77GHz circuit design, which increases the difficulty of circuit design of FMCW radar working in this frequency band.

[0005] In the prior art, such as Figure 1A-Figure 1D As shown, in order to achieve the isolation of transmission and reception, the antenna feed network system usually adopts the following solutions: Figure 1A The 3dB bridge shown; Figure 1B The circulator shown; Figure 1C The coupler shown; Figure 1D Dual antennas shown.

[0006] In the 3dB bridge and coupler solution, under normal processing accuracy, the 3dB bridge and coupler can only achieve -30dB isolation within a very narrow bandwidth, and this can only be achieved under the premise that the antenna port is fully matched to 50 ohms. The reflected wave of the general antenna port is about -20dB, so the overall transmit-receive (Tx-Rx) isolation can only reach about -20dB. Moreover, in the 77GHz frequency band, the antenna and the feed circuit are both processed using traditional PCB processing technology, and it is very difficult to achieve a very small processing accuracy error, resulting in a large difference between the isolation required by the design and the isolation obtained by actual processing. In addition, in the 3dB bridge and coupler solution, it is necessary to terminate the 50-ohm matching load to the fourth unused port. However, at 77GHz, it is difficult to achieve a 50-ohm resistor using traditional PCB technology. The smallest size 0402 chip resistor that can be used in the low-frequency band is 1mm in size, which is basically the order of 1 / 4 wavelength of 77GHz. Therefore, it will produce serious parasitic effects and cannot be used at 77GHz.

[0007] In the circulator solution, the properties of ferrite materials will change dramatically at the 77GHz frequency band, making it difficult to implement using PCB technology.

[0008] In the dual-antenna solution, one antenna is used for transmission and one for reception. Theoretically, as long as the two antennas are far enough apart, the isolation can be very good; however, in terms of product design, two antennas will inevitably require twice the space; especially in the design of 77GHz radar, in order to achieve a longer communication distance, the size of the antenna is large, and two antennas will inevitably occupy a larger space, making the product structure more bloated. Summary of the invention

[0009] In order to overcome the problems existing in the related art, the antenna feed network is designed under the current PCB process level to improve the transmit-receive isolation. The embodiment of the present invention provides an antenna feed network system for improving the transmit-receive isolation. The technical solution is as follows:

[0010] An antenna feeding network system comprises a first bridge, a second bridge, a third bridge and a fourth bridge, wherein the four bridges are interconnected to form two vertically symmetrical paths; wherein the first bridge is connected to the third bridge and the fourth bridge, and the second bridge is connected to the third bridge and the fourth bridge; antennas are connected to the third bridge and the fourth bridge; a receiving end of the system is arranged on the first bridge, and a transmitting end of the system is arranged on the second bridge.

[0011] Among them, the fourth port of the third bridge and the first port of the fourth bridge are connected to the second port and the fourth port of the second bridge through the first node N1 and the second node N2 respectively; the first port of the third bridge and the fourth port of the fourth bridge are connected to the second port and the fourth port of the first bridge through the seventh node N7 and the eighth node N8 respectively.

[0012] The first bridge and the second bridge are 180° bridges, and the third bridge and the fourth bridge are 90° bridges; both the 180° bridge and the 90° bridge have four ports.

[0013] The third port of the first bridge is a sum port; the first port of the second bridge is a differential port; the receiving end of the system is arranged on the third port of the first bridge, and the transmitting end of the system is arranged on the first port of the second bridge.

[0014] The first port of the first bridge is a differential port; the third port of the second bridge is a sum port; the first port of the first bridge and the third port of the second bridge are both connected to an open line, and the length of the open line is adjustable.

[0015] The antennas of the system include a first antenna, a second antenna, a third antenna, and a fourth antenna, which correspond to a first antenna port, a second antenna port, a third antenna port, and a fourth antenna port, respectively; the first antenna and the second antenna are connected to the second port and the third port of the third bridge through a fourth node N4 and a third node N3, respectively; the third antenna and the fourth antenna are connected to the second port and the third port of the fourth bridge through a fifth node N5 and a sixth node N6, respectively.

[0016] The antenna impedances at the first antenna port, the second antenna port, the third antenna port, and the fourth antenna port are the same.

[0017] The system also includes a 180° phase shift line, which can be provided on either antenna.

[0018] The system is applied to frequency modulated continuous wave radar;

[0019] The operating frequency of the system is 77 GHz.

[0020] The technical solution provided by the embodiment of the present invention has the following beneficial effects:

[0021] In the present invention, reflection cancellation is performed in the antenna feed network. Unlike the cancellation technology in the circuit, it does not introduce additional noise, and thus does not reduce the sensitivity of the system. Compared with other cancellation technologies in the feed network, this cancellation scheme can not only cancel the transmission leakage, but also cancel the reflection caused by the mismatch of the antenna port. At the same time, the receiver can also receive the signal without canceling all the signals.

[0022] The present invention introduces a 180° bridge and a 90° bridge to form a cancellation structure, thereby canceling the leakage signal and the reflected signal of the port; at the same time, a 180° phase shift line is introduced to prevent the received signal from being cancelled at the same time; an open line is introduced on the port that is not used by the 180° bridge to form a tiny reflection, and the isolation of the entire network is fine-tuned by using the open line, thereby solving the leakage problem of the transmitting end Tx when the FMCW radar is in use. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0024] Figure 1A It is a schematic diagram of an antenna feeding network system implemented by using a 3dB bridge in the prior art.

[0025] Figure 1B The figure is a schematic diagram of an antenna feeding network system realized by using a circulator in the prior art.

[0026] Figure 1C The figure is a schematic diagram of realizing an antenna feeding network system by using a coupler in the prior art.

[0027] Figure 1D The figure is a schematic diagram of an antenna feeding network system implemented by using dual antennas in the prior art.

[0028] Figure 2 It is a structural schematic diagram of the antenna feeding network system in the present invention.

[0029] Figure 3 It is a schematic diagram of the principle of the 90° bridge used in the present invention.

[0030] Figure 4 It is a schematic diagram of the principle of the 180° bridge used in the present invention. DETAILED DESCRIPTION

[0031] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] The antenna feeding network involved in the present invention is as follows Figure 2 As shown:

[0033] The antenna feeding network includes a first bridge 21 , a second bridge 22 , a third bridge 23 , and a fourth bridge 24 .

[0034] The first bridge 21 and the second bridge 22 are both 180° bridges, the third bridge 23 and the fourth bridge 24 are both 90° bridges, and both the 180° bridge and the 90° bridge have four ports.

[0035] See also Figure 3 The 90° bridge characteristic is as follows: the 90° bridge has four ports, among which, when one end is used as the input end 31 and the other end is used as the isolation end 34, the other two ports can output signals with a phase difference of 90°, that is, when the input signal phase of the input end is 0°, the signal phase of one output end 32 of the two output ends is -90°, and the signal phase of the other output end 33 is -180°, and the output signal amplitude has a -3dB attenuation.

[0036] See also Figure 4 The characteristics of the 180° bridge are as follows: the 180° bridge has four ports, a first port 41, a second port 42, a third port 43, and a fourth port 44, including a differential port and a sum port, and the other two ports can be used as both output ports and input ports; when a signal is input from the differential port, the phase difference of the signals at the two output ports is 180°; when a signal is input from the sum port, the signals at the two output ports are in phase; when signals are input from two input ports at the same time, if the two signals are in phase, there is an output signal only at the sum port, and if the two signals are in reverse phase, there is an output signal only at the differential port.

[0037] The scattering matrix S describing the 180° bridge characteristics is as follows:

[0038]

[0039] In the present invention, the upper and lower paths of the antenna feed network have a symmetrical structure. The antenna feed network includes a first antenna 25, a second antenna 26, a third antenna 27, and a fourth antenna 28, which correspond to a first antenna port 251, a second antenna port 261, a third antenna port 271, and a fourth antenna port 281, respectively.

[0040] The first antenna 25 and the second antenna 26 are connected to the second port 232 and the third port 233 of the third bridge 23 through the fourth node N4 and the third node N3 respectively. The third antenna 27 and the fourth antenna 28 are connected to the second port 242 and the third port 243 of the fourth bridge 24 through the fifth node N5 and the sixth node N6 respectively.

[0041] The fourth port 234 of the third bridge 23 and the first port 241 of the fourth bridge 24 are connected to the second port 222 and the fourth port 224 of the second bridge 22 through the first node N1 and the second node N2 respectively. The first port 221 of the second bridge 22 is a differential end connected to the transmitting end Tx of the antenna feeding network.

[0042] According to the characteristics of the 180° bridge, when the transmitting end Tx has a signal input, a phase distribution of the corresponding nodes is formed on the antenna feed network. The transmitting end Tx inputs a signal through the differential end of the second bridge 22, and the output signal amplitudes of the two ports of the second bridge 22: the second port 222 and the fourth port 224 at the first node N1 and the second node N2 are equal but have a phase difference of 180°, wherein the phase at the first node N1 is 0° and the phase at the N2 node is -180°. The two output signals reach the two 90° bridges, namely the third bridge 23 and the fourth bridge 24, through the same path. According to the characteristics of the 90° bridge, the output signal amplitudes of the output end of the third bridge 23 at the third node N3 and the fourth node N4 and the output end of the fourth bridge at the fifth node N5 and the sixth node N6 are the same, and the phases at the third node N3, the fourth node N4, the fifth node N5, and the sixth node N6 are -90°, -180°, -270°, and -360°, respectively. When all four antenna ports use the same antenna, the reflected signals caused by the four antenna ports will not produce any additional phase difference, and the phase remains unchanged. At this time, the phases at the second antenna port 261, the first antenna port 251, the third antenna port 271, and the fourth antenna port 281 are -90°, -180°, -270°, and -360°, respectively.

[0043] The signal at the seventh node N7 connecting the first port 231 of the third bridge 23 and the second port 212 of the first bridge 21 can be divided into two parts: the signal S1 directly leaking from the first node N1 to the seventh node N7 due to limited isolation; and the reflected signal S2 formed due to the non-ideal matching of the antenna port. The signal at the eighth node N8 connecting the fourth port 244 of the fourth bridge 24 and the fourth port 214 of the first bridge 21 is also composed of two parts, including the signal S3 directly leaking from the second node N2 to the eighth node N8 and the reflected signal S4 formed due to the non-ideal matching of the antenna port. Since the signals of the first node N1 and the second node N2 have a phase difference of 180°, and the upper and lower structures are symmetrical, S1 = -S3; and since the same antenna is used, the reflected signal is also consistent with the phase of the signals of the first node N1 and the second node N2, that is, S2 = -S4.

[0044] In the first bridge 21, except for the other two ports of the sum end and the differential port, the second port 212 of the first bridge 21 is connected to the first port 231 of the third bridge 23 through the seventh node N7, and the fourth port 214 of the first bridge 21 is connected to the fourth port 244 of the fourth bridge 24 through the eighth node N8, and the receiving end Rx of the antenna feeding network is located at the first port 211 of the first bridge 21, which is the sum end of the first bridge 21, so Rx = N7 + N8 = (S1 + S2) + (S3 + S4) = 0, that is, the transmission signal of the transmitting end Tx cannot leak to the receiving end Rx through the feeding network. It can be seen that the above antenna feeding network structure has a high isolation.

[0045] Furthermore, the antenna feed network also includes a 180° phase shift line, which can be set on any antenna. The phase shift line can ensure that the radar receives the echo signal reflected by the distant object, as follows:

[0046] The total electromagnetic wave signal emitted by the four antenna ports is in They represent the antenna vectors of the second antenna port 261, the first antenna port 251, the third antenna port 271 and the fourth antenna port 281 respectively. After the transmitted signal is reflected by a distant object and received by the four antenna ports, the signals received by each antenna port are:

[0047]

[0048]

[0049]

[0050]

[0051] After the above four receiving signals pass through the two 90° bridges of the third bridge 23 and the fourth bridge 24, the signals at the seventh node N7 connected to the third bridge 23 and the eighth node N8 connected to the fourth bridge 24 are respectively:

[0052] Rx N7 =-Rx2-jRx1

[0053] Rx N8 =Rx4-jRx3

[0054] Finally, to the receiving end The signal is:

[0055]

[0056] As long as the antenna vectors of the first antenna port 251 and the third antenna port 271, and the second antenna port 261 and the fourth antenna port 281 are not perpendicular to each other at the same time, that is, the inner product is equal to zero, then Rx is not zero, that is, the echo signal reflected by the distant object can be received by Rx, and the Rx-Tx isolation can still be guaranteed. On the contrary, if there is no 180° phase shift line, the signal at the receiving end Rx is always zero, that is, no signal can be received.

[0057] Furthermore, since there are no corresponding mature resistors and capacitors available in the 77 GHz frequency band, matching loads cannot be used at the sum port of the second bridge 22 where the transmitting end Tx is located and the differential port of the first bridge 21 where the receiving end Rx is located, that is, a 50 ohm load cannot be used. In the present invention, the mutual isolation between the sum port and the differential port of the 180° bridge is utilized, and an open line is directly connected to the above-mentioned port, and the small reflection introduced by the open line is utilized to cancel the leaked signal again. Among them, the length of the open line is set to L2 on the differential port of the first bridge 21, and the length of the open line is set to L1 on the sum port of the second bridge 22. By adjusting the lengths L1 and L2, the overall leakage level of Tx-Rx can be fine-tuned, so that the entire feeding network can have sufficient transmit and receive isolation at the 77 GHz frequency.

[0058] The type of antenna connected to each antenna port can be determined according to specific circumstances. The present invention does not limit the specific type, as long as the antenna impedance of each antenna port is the same.

[0059] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art who is familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. An antenna feeding network system, characterized in that: The system comprises a first bridge, a second bridge, a third bridge and a fourth bridge, wherein the four bridges are interconnected to form two vertically symmetrical paths; wherein the first bridge is connected to the third bridge and the fourth bridge, and the second bridge is connected to the third bridge and the fourth bridge; the third bridge and the fourth bridge are both connected to antennas; the receiving end of the system is arranged on the first bridge, and the transmitting end of the system is arranged on the second bridge; The first bridge and the second bridge are 180° bridges, and the third bridge and the fourth bridge are 90° bridges; the 180° bridge and the 90° bridge both have four ports; a cancellation structure is formed by the 180° bridge and the 90° bridge to cancel the leakage signal and the reflected signal of the port; Among them, the fourth port of the third bridge and the first port of the fourth bridge are connected to the second port and the fourth port of the second bridge through the first node and the second node respectively; the first port of the third bridge and the fourth port of the fourth bridge are connected to the second port and the fourth port of the first bridge through the seventh node and the eighth node respectively.

2. The system according to claim 1, wherein: The third port of the first bridge is a sum port; the first port of the second bridge is a differential port; the receiving end of the system is arranged on the third port of the first bridge, and the transmitting end of the system is arranged on the first port of the second bridge.

3. The system according to claim 1, wherein: The first port of the first bridge is a differential port; the third port of the second bridge is a sum port; an open line is connected to the first port of the first bridge and the third port of the second bridge, and the length of the open line can be adjusted.

4. According to the system of claim 1, the antenna includes a first antenna, a second antenna, a third antenna, and a fourth antenna, which respectively correspond to a first antenna port, a second antenna port, a third antenna port, and a fourth antenna port; the first antenna and the second antenna are connected to the second port and the third port of the third bridge through a fourth node and a third node, respectively; the third antenna and the fourth antenna are connected to the second port and the third port of the fourth bridge through a fifth node and a sixth node, respectively.

5. The system according to claim 4, wherein: The antenna impedances at the first antenna port, the second antenna port, the third antenna port, and the fourth antenna port are the same.

6. The system according to claim 4, further comprising a 180° phase shift line, wherein the phase shift line is provided on any one of the antennas.

7. The system according to claim 1, wherein the system is applied in a frequency modulated continuous wave radar. The system according to claim 1 , wherein the operating frequency of the system is 77 GHz.

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