Self-calibration method for four-quadrant active phased-array antenna system

Abstract

The invention discloses a self-calibration method for a four-quadrant active phased-array antenna system, and belongs to the technical field of active phased-array antennae, an active phased-array antenna is divided into four quadrants, an excitation signal is transmitted to pass through a switch chip, any quadrant can be selectively excited, and the power amplifier switch chip controls receiving and transmitting of each quadrant. According to the method, the receiving phase difference between adjacent channels is obtained by controlling the receiving and transmitting of different channels in different quadrants, carrying out mutual coupling receiving and transmitting measurement between the channels and processing measured phase information, so that the self-calibration of each channel of the phased-array antenna is realized. An additional calibration antenna does not need to be added on an antenna array plane, so that the size, the cost and the design complexity of the phased-array antenna are reduced; compared with the traditional mutual coupling calibration measurement method, the method has the advantages that transceiving control is realized by adopting a plurality of TR components instead of transceiving control on four quadrants, so that the test method is simplified.

Description

Technical Field

[0001] The invention belongs to the technical field of active phased array antennas, and in particular relates to a self-calibration method for a four-quadrant active phased array antenna system. Background Art

[0002] Each channel in the active phased array antenna contains active devices. In order to eliminate the amplitude and phase inconsistency caused by the active devices at the back end of the transceiver port, it is necessary to obtain amplitude and phase compensation data through channel calibration to ensure the accuracy of the antenna beam pointing. During long-term storage or changes in the surrounding environment, the active device will drift in phase, resulting in the original compensation data being unable to guarantee the phase consistency of each channel. Therefore, in order to achieve the long-term sustainable operation of the phased array antenna and meet the requirements of beam pointing accuracy, it is necessary to recalibrate each channel of the phased array antenna and correct the compensation data. The current solutions are mainly divided into external calibration and internal calibration. External calibration requires adding additional calibration antennas to the antenna array surface, which increases the volume, cost and design complexity of the phased array antenna; internal calibration can detect the amplitude and phase consistency of the active channel through a calibration network composed of a power divider and a coupler, but the calibration network in large-scale phased array antennas is relatively complex and difficult to integrate; self-calibration can also be achieved by controlling the transceiver of each channel in multiple TR components, but the test method is relatively complex. Therefore, how to effectively realize channel self-calibration of active phased array antenna systems has become a hot topic of research and concern in the field of phased array antennas. Summary of the invention

[0003] The purpose of the present invention is to provide a self-calibration method for a four-quadrant active phased array antenna system to solve the problems of large size, high cost or complex testing methods in current methods for calibrating phased array antennas.

[0004] To achieve the above object, the present invention adopts the following technical solution:

[0005] In one aspect, the present specification provides a self-calibration method for a four-quadrant active phased array antenna system, comprising:

[0006] Step 102, based on adjacent channels in each quadrant, measuring and obtaining a receiving phase difference between adjacent channels in each quadrant;

[0007] Step 104, based on adjacent channels in adjacent quadrants, measuring and obtaining a receiving phase difference between adjacent channels in adjacent quadrants;

[0008] Step 106, based on the receiving phase difference between adjacent channels in each quadrant and the receiving phase difference between adjacent channels in adjacent quadrants, calculate and obtain the receiving phase difference between adjacent channels in the active phased array antenna;

[0009] Step 108: Perform phase compensation based on the receiving phase difference between adjacent channels in the active phased array antenna to obtain a self-calibrated active phased array antenna.

[0010] In another aspect, the present specification provides a self-calibration network for a four-quadrant active phased array antenna system, comprising:

[0011] A transmitting excitation module, a two-stage switch chip and an active phased array antenna; the active phased array antenna is divided into four quadrants; the transmitting excitation signal output by the transmitting excitation module passes through the two-stage switch chip, selects any quadrant for excitation, and performs transmission and reception control on the four quadrants.

[0012] Based on the above technical solution, this specification can achieve the following technical effects:

[0013] This method divides the active phased array antenna into four quadrants. The transmitting excitation signal passes through the switch chip, and any quadrant can be selected to be excited. The power amplifier switch chip controls the reception and transmission of each quadrant respectively. By controlling the transmission and reception of different channels in different quadrants, the mutual coupling transmission and reception between channels is measured, and the measured phase information is processed to obtain the phase difference between adjacent channels in each quadrant. Then, by measuring the mutual coupling phase between adjacent transceiver channels in adjacent quadrants, the phase difference between adjacent quadrants is obtained, thereby obtaining the phase difference between adjacent channels in the entire array, and realizing self-calibration of each channel in the phased array antenna system, thereby solving the problems of large size, high cost or complex testing methods in the current method of calibrating phased array antennas. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The figure is a schematic diagram of the structure of a self-calibration network for a four-quadrant active phased array antenna system according to an embodiment of the present invention.

[0015] Figure 2 The figure is a flow chart of a self-calibration method for a four-quadrant active phased array antenna system according to an embodiment of the present invention.

[0016] Figure 3 Schematic diagram of the measurement of the transmitting and receiving mutual coupling between adjacent channels in each quadrant in one embodiment of the present invention.

[0017] Figure 4 Schematic diagram of the measurement of the transceiver mutual coupling between adjacent channels in adjacent quadrants in one embodiment of the present invention. DETAILED DESCRIPTION

[0018] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become more apparent from the following description and claims. It should be noted that the drawings are all in very simplified form and are not in precise proportions, and are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0019] It should be noted that, in order to clearly explain the content of the present invention, the present invention specifically cites multiple embodiments to further illustrate different implementations of the present invention, wherein the multiple embodiments are enumerated rather than exhaustive. In addition, for the sake of brevity of explanation, the contents mentioned in the previous embodiments are often omitted in the subsequent embodiments. Therefore, the contents not mentioned in the subsequent embodiments can refer to the previous embodiments accordingly.

[0020] Example 1

[0021] Please refer to Figure 1 , Figure 1 The figure shows a self-calibration network for a four-quadrant active phased array antenna system provided by this embodiment. In this embodiment, the network includes:

[0022] A transmitting excitation module, a two-stage switch chip and an active phased array antenna; the active phased array antenna is divided into four quadrants; the transmitting excitation signal output by the transmitting excitation module passes through the two-stage switch chip, selects any quadrant for excitation, and performs transmission and reception control on the four quadrants.

[0023] In this embodiment, the quadrant includes a power amplifier switch chip, an RF transceiver port and an RF receiving port; the transmission excitation signal output by the transmission excitation module passes through a two-stage switch chip, selects any quadrant for excitation, passes through a power amplifier switch chip for transmission selection and power amplification, and is output from the RF transceiver port, and then transmitted by the transmission channel selected in the quadrant. The transmission signal will be coupled to the receiving channel selected in another quadrant. The signal is input into another quadrant from the RF transceiver port, passes through the power amplifier switch chip for reception selection, and is finally output from the RF receiving port. By measuring the transmission characteristics from the transmission channel to the receiving channel, the transmission and reception mutual coupling phase between the channels in different quadrants can be obtained.

[0024] In this embodiment, the two-stage switch chip includes a primary switch chip and two secondary switch chips; the input end of the primary switch chip is connected to the output end of the transmitting excitation module, and its output end is respectively connected to the input ends of the two secondary switch chips; the output ends of the two secondary switch chips are respectively connected to the power amplifier switch chips of the two quadrants.

[0025] Based on this, this network divides the active phased array antenna into four quadrants. By controlling the transceiver of the four quadrants instead of the traditional method of using multiple TR components to achieve transceiver control, the test method can be simplified. The transmission excitation signal passes through a two-stage switch chip, and any quadrant can be selected for excitation. The power amplifier switch chip controls the reception and transmission of each quadrant separately, which is equivalent to adding a level of switch isolation to each transceiver link, which can increase the isolation of the transceiver port and achieve effective self-calibration.

[0026] Example 2

[0027] Please refer to Figure 2 , Figure 2 The figure shows a self-calibration method for a four-quadrant active phased array antenna system provided by this embodiment. In this embodiment, the method includes:

[0028] Step 102, based on adjacent channels in each quadrant, measuring and obtaining a receiving phase difference between adjacent channels in each quadrant;

[0029] In this embodiment, one implementation of step 102 is:

[0030] Step 202, selecting two adjacent channels in the first target quadrant as a first receiving channel and a second receiving channel;

[0031] Step 204, based on the positions of the first receiving channel and the second receiving channel, two adjacent channels in the second target quadrant are selected as the first transmitting channel and the second transmitting channel;

[0032] Step 206, controlling the first transmitting channel to transmit a signal, the first receiving channel and the second receiving channel to receive a signal in a time-sharing manner, and measuring and obtaining a first mutual coupling phase and a second mutual coupling phase between the first receiving channel and the second receiving channel and the first transmitting channel respectively;

[0033] Step 208, controlling the second transmitting channel to transmit a signal, the first receiving channel and the second receiving channel to receive a signal in a time-sharing manner, and measuring and obtaining a third mutual coupling phase and a fourth mutual coupling phase between the first receiving channel and the second receiving channel and the second transmitting channel respectively;

[0034] Step 210: Calculate and obtain a receiving phase difference between the first receiving channel and the second receiving channel based on the first mutual coupling phase, the second mutual coupling phase, the third mutual coupling phase, and the fourth mutual coupling phase.

[0035] In this embodiment, the mutual coupling phase between the transmitting channel and the receiving channel is equal to the sum of the phase change of the receiving channel, the phase change of the transmitting channel, and the phase difference generated by the coupling path between the transceiver units.

[0036] In this embodiment, the receiving phase difference between the first receiving channel and the second receiving channel is equal to the difference between the phase changes of the first receiving channel and the second receiving channel.

[0037] In this embodiment, one implementation of step 210 is:

[0038] Step 2101, calculating the difference between the first mutual coupling phase and the fourth mutual coupling phase to obtain a first difference;

[0039] Step 2102, calculating the difference between the second mutual coupling phase and the third mutual coupling phase to obtain a second difference;

[0040] Step 2103, calculating half of the difference between the first difference and the second difference to obtain a receiving phase difference between the first receiving channel and the second receiving channel.

[0041] Step 104, based on adjacent channels in adjacent quadrants, measuring and obtaining a receiving phase difference between adjacent channels in adjacent quadrants;

[0042] In this embodiment, one implementation of step 104 is:

[0043] Step 302, selecting two adjacent channels in two adjacent quadrants as a third receiving channel and a fourth receiving channel;

[0044] Step 304, based on the positions of the third receiving channel and the fourth receiving channel, two adjacent channels in the other two adjacent quadrants are selected as the third transmitting channel and the fourth transmitting channel;

[0045] Step 306, controlling the third transmitting channel to transmit signals, the third receiving channel and the fourth receiving channel to receive signals in time-sharing, and measuring and obtaining the fifth mutual coupling phase and the sixth mutual coupling phase between the third receiving channel and the fourth receiving channel and the third transmitting channel respectively;

[0046] Step 308, controlling the fourth transmitting channel to transmit signals, the third receiving channel and the fourth receiving channel to receive signals in time-sharing, and measuring and obtaining the seventh mutual coupling phase and the eighth mutual coupling phase between the third receiving channel and the fourth receiving channel and the fourth transmitting channel respectively;

[0047] Step 310 , based on the fifth mutual coupling phase, the sixth mutual coupling phase, the seventh mutual coupling phase and the eighth mutual coupling phase, calculate and obtain the receiving phase difference between the third receiving channel and the fourth receiving channel.

[0048] In this embodiment, one implementation of step 310 is:

[0049] Step 3101, calculating the difference between the fifth mutual coupling phase and the eighth mutual coupling phase to obtain a third difference;

[0050] Step 3102, calculating the difference between the sixth mutual coupling phase and the seventh mutual coupling phase to obtain a fourth difference;

[0051] Step 3103: Calculate half of the difference between the third difference and the fourth difference to obtain a receiving phase difference between the third receiving channel and the fourth receiving channel.

[0052] Step 106, based on the receiving phase difference between adjacent channels in each quadrant and the receiving phase difference between adjacent channels in adjacent quadrants, calculate and obtain the receiving phase difference between adjacent channels in the active phased array antenna;

[0053] Step 108: Perform phase compensation based on the receiving phase difference between adjacent channels in the active phased array antenna to obtain a self-calibrated active phased array antenna.

[0054] Specifically, the self-calibration method for a four-quadrant active phased array antenna system comprises the following specific steps:

[0055] The first step is to establish a self-calibration network for the four-quadrant active phased array antenna system.

[0056] like Figure 1 , the active phased array antenna is divided into four quadrants, and the traditional method of using multiple TR components to achieve transceiver control is replaced by transceiver control of the four quadrants, which can simplify the test method. The transmission excitation signal passes through the two-stage switch chip, and any quadrant can be selected for excitation. The power amplifier switch chip controls the reception and transmission of each quadrant separately, which is equivalent to adding a level of switch isolation to each transceiver link, which can increase the isolation of the transceiver port, thereby achieving effective self-calibration.

[0057] The second step is to measure the mutual coupling phase between the transmit and receive channels in different quadrants.

[0058] By controlling the reception and transmission of the corresponding channels in different quadrants, the mutual coupling between the channels in different quadrants can be measured. The transmission excitation signal excites a certain quadrant, and after the transmission selection and power amplification by the power amplifier switch chip, it is output from the RF transceiver port, and then transmitted by the selected transmission channel in the quadrant. The transmission signal will be coupled to the selected receiving channel in another quadrant. The signal is input from the RF transceiver port, received through the power amplifier switch, and finally output from the RF receiving port. By measuring the transmission characteristics from the transmission excitation port to the RF receiving port, the phase of the mutual coupling between the channels in different quadrants can be obtained.

[0059] Select two adjacent channels in a quadrant as receiving channel 1 and receiving channel 2, and then follow the steps below: Figure 3 The position relationship shown in the figure is selected, and two adjacent channels in another adjacent quadrant are selected as transmitting channel 1 and transmitting channel 2. Let transmitting channel 1 transmit, and two adjacent receiving channels 1 and 2 receive in time division, and the mutual coupling phase θ can be obtained. (1，1) and θ (2，1) ; Then let the transmitting channel 21 transmit, and the receiving channels 1 and 2 receive in time division, and the mutual coupling phase θ can be obtained (1，2) and θ (2，2) .

[0060] The third step is to process the measured phase information.

[0061] By processing the measured mutual coupling phase information, the phase difference between adjacent channels can be obtained without the need for additional calibration antennas. (R，T)It can be regarded as the phase change θ at the receiving channel end R 、Phase change at the transmitting channel end θ T and the phase difference θ generated by the coupling path C between the transceiver units C According to the linear relationship of the two-port network model, we can get:

[0062]

[0063] Among them, θ (1，1) is the mutual coupling phase between receiving channel 1 and transmitting channel 1, θ (2，1) is the mutual coupling phase between receiving channel 2 and transmitting channel 1, θ (1，2) is the mutual coupling phase between receiving channel 1 and transmitting channel 2, θ (2，2) is the mutual coupling phase between receiving channel 2 and transmitting channel 2; θ R1 ,θ R2 are the phase changes at receiving channel 1 and receiving channel 2 respectively; θ C1 ,θ C2 are the phase differences caused by the coupling paths C1 and C2 between the transceiver units; the phase changes at receiving channels 1 and 2; θ T1 ,θ T2 They are the phase changes at transmit channel 1 and transmit channel 2 respectively.

[0064] By solving this set of equations, we can get the receiving phase difference between adjacent receiving channel 1 and receiving channel 2:

[0065] θ R1 -θ R2 =[(θ (1，1) -θ (2，2) )-(θ (2，1) -θ (1，2) )] / 2

[0066] Repeating the second and third steps, the receiving phase difference between adjacent channels in each quadrant can be obtained.

[0067] The fourth step is to measure the mutual coupling phase between adjacent transmit and receive channels in adjacent quadrants.

[0068] according to Figure 4 As shown in the position relationship, two adjacent channels in two adjacent quadrants are selected as receiving channel 1 and receiving channel 2, and two adjacent channels in the other two quadrants are selected as transmitting channel 1 and transmitting channel 2. Let transmitting channel 1 transmit, and two adjacent receiving channels 1 and 2 receive in time division, and the mutual coupling phase θ can be obtained. (1，1) and θ (2，1) ; Then let the transmitting channel 21 transmit, and the receiving channels 1 and 2 receive in time division, and the mutual coupling phase θ can be obtained (1，2) and θ(2，2) .

[0069] Step 5: Process the measured phase information according to the method in step 3 to obtain the receiving phase difference θ between two adjacent channels in adjacent quadrants. R1 -θ R2 =[(θ (1，1) -θ (2，2) )-(θ (2，1) -θ (1，2) )] / 2, taking one of the channels as the reference, the receiving phase difference between the channels in the two adjacent quadrants can be obtained.

[0070] By repeating steps 4 to 5, the receiving phase difference between two adjacent channels in each adjacent quadrant can be obtained, thereby obtaining the receiving phase difference of each adjacent channel in the entire array. The phase difference data is used as compensation data to achieve phase compensation of each channel.

[0071] At this point, the self-calibration of each channel of the four-quadrant active phased array antenna system has been completed.

[0072] In summary, this method divides the active phased array antenna into four quadrants. The transmission excitation signal passes through the switch chip, and any quadrant can be selected to be excited. The power amplifier switch chip controls the reception and transmission of each quadrant respectively. By controlling the transmission and reception of different channels in different quadrants, the mutual coupling transmission and reception between channels is measured, and the measured phase information is processed to obtain the phase difference between adjacent channels in each quadrant. Then, by measuring the mutual coupling phase between adjacent transceiver channels in adjacent quadrants, the phase difference of adjacent quadrants is obtained, thereby obtaining the phase difference between adjacent channels in the entire array, and realizing self-calibration of each channel in the phased array antenna system.

[0073] Compared with the prior art, the present invention has the following beneficial effects:

[0074] (1) Compared with the traditional method of adding a calibration antenna, the present invention can realize self-calibration of the phased array antenna system without adding an additional calibration antenna on the antenna array plane, which effectively reduces the volume, cost and design complexity of the phased array antenna.

[0075] (2) Compared with the traditional mutual coupling calibration measurement method, the four-quadrant transmission and reception control replaces the use of multiple TR components to achieve transmission and reception control, which simplifies the test method.

[0076] (3) The power amplifier switch chip is used to control the reception and transmission of each quadrant separately, and a level of switch isolation is added to the transceiver link, which can increase the isolation between the transceiver ports, thereby improving the accuracy of the test and the effectiveness of the calibration.

[0077] The above is only an embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of the claims of the present application.

Claims

1. A self-calibration network for a four-quadrant active phased array antenna system, characterized in that: include: A transmitting excitation module, a two-stage switch chip and an active phased array antenna; the active phased array antenna is divided into four quadrants; The transmitting excitation signal output by the transmitting excitation module passes through a two-stage switch chip, selects any quadrant for excitation, and controls the transmission and reception of the four quadrants.

2. The self-calibration network according to claim 1, characterized in that The quadrant includes a power amplifier switch chip, a radio frequency transceiver port and a radio frequency receiving port; the transmission excitation signal output by the transmission excitation module passes through a two-stage switch chip, selects any quadrant for excitation, passes through a power amplifier switch chip for transmission selection and power amplification, and is output from the radio frequency transceiver port, and then transmitted by the transmission channel selected in the quadrant. The transmission signal will be coupled to the receiving channel selected in another quadrant. The signal is input into another quadrant from the radio frequency transceiver port, passes through a power amplifier switch chip for reception selection, and is finally output from the radio frequency receiving port. By measuring the transmission characteristics from the transmission channel to the receiving channel, the transmission and reception mutual coupling phase between the channels in different quadrants can be obtained.

3. The self-calibration network according to claim 2, characterized in that The two-stage switch chip includes a primary switch chip and two secondary switch chips; the input end of the primary switch chip is connected to the output end of the transmission excitation module, and its output end is respectively connected to the input ends of the two secondary switch chips; the output ends of the two secondary switch chips are respectively connected to the power amplifier switch chips of the two quadrants.

4. A self-calibration method using any one of the self-calibration networks of claims 1-3, characterized in that: include: Based on the adjacent channels in each quadrant, the receiving phase difference between the adjacent channels in each quadrant is measured and obtained; Based on adjacent channels in adjacent quadrants, a receiving phase difference between adjacent channels in adjacent quadrants is measured and obtained; Based on the receiving phase difference between adjacent channels in each quadrant and the receiving phase difference between adjacent channels in adjacent quadrants, the receiving phase difference between adjacent channels in the active phased array antenna is calculated; Phase compensation is performed based on the receiving phase difference between adjacent channels in the active phased array antenna to obtain a self-calibrated active phased array antenna.

5. The method according to claim 4, characterized in that The measuring and obtaining the receiving phase difference between adjacent channels in each quadrant based on adjacent channels in each quadrant includes: Select two adjacent channels in the first target quadrant as a first receiving channel and a second receiving channel; Based on the positions of the first receiving channel and the second receiving channel, two adjacent channels in the second target quadrant are selected as the first transmitting channel and the second transmitting channel; Control the first transmitting channel to transmit a signal, and the first receiving channel and the second receiving channel to receive a signal in a time-sharing manner, and measure and obtain a first mutual coupling phase and a second mutual coupling phase between the first receiving channel and the second receiving channel and the first transmitting channel respectively; Control the second transmitting channel to transmit a signal, and the first receiving channel and the second receiving channel to receive the signal in a time-sharing manner, and measure and obtain a third mutual coupling phase and a fourth mutual coupling phase between the first receiving channel and the second receiving channel and the second transmitting channel respectively; A receiving phase difference between the first receiving channel and the second receiving channel is obtained by calculation based on the first mutual coupling phase, the second mutual coupling phase, the third mutual coupling phase and the fourth mutual coupling phase.

6. The method according to claim 5, characterized in that The mutual coupling phase between the transmitting channel and the receiving channel is equal to the sum of the phase change of the receiving channel, the phase change of the transmitting channel, and the phase difference caused by the coupling path between the transceiver units.

7. The method according to claim 6, characterized in that The receiving phase difference between the first receiving channel and the second receiving channel is equal to the difference between the phase changes of the first receiving channel and the second receiving channel.

8. The method according to claim 7, characterized in that The calculating and obtaining the receiving phase difference between the first receiving channel and the second receiving channel based on the first mutual coupling phase, the second mutual coupling phase, the third mutual coupling phase and the fourth mutual coupling phase comprises: Calculating a difference between the first mutual coupling phase and the fourth mutual coupling phase to obtain a first difference; Calculating a difference between the second mutual coupling phase and the third mutual coupling phase to obtain a second difference; A half of the difference between the first difference and the second difference is calculated to obtain a receiving phase difference between the first receiving channel and the second receiving channel.

9. The method according to claim 4, characterized in that The step of measuring and obtaining the receiving phase difference between adjacent channels in adjacent quadrants based on adjacent channels in adjacent quadrants includes: Selecting two adjacent channels in two adjacent quadrants as a third receiving channel and a fourth receiving channel; Based on the positions of the third receiving channel and the fourth receiving channel, two adjacent channels in the other two adjacent quadrants are selected as the third transmitting channel and the fourth transmitting channel; Control the third transmitting channel to transmit signals, the third receiving channel and the fourth receiving channel to receive signals in time-sharing, and measure and obtain the fifth mutual coupling phase and the sixth mutual coupling phase between the third receiving channel and the fourth receiving channel and the third transmitting channel respectively; Control the fourth transmitting channel to transmit a signal, and the third receiving channel and the fourth receiving channel to receive signals in a time-sharing manner, and measure and obtain a seventh mutual coupling phase and an eighth mutual coupling phase between the third receiving channel and the fourth receiving channel and the fourth transmitting channel respectively; A receiving phase difference between the third receiving channel and the fourth receiving channel is obtained by calculation based on the fifth mutual coupling phase, the sixth mutual coupling phase, the seventh mutual coupling phase and the eighth mutual coupling phase.

10. The method according to claim 9, characterized in that The calculating and obtaining the receiving phase difference between the third receiving channel and the fourth receiving channel based on the fifth mutual coupling phase, the sixth mutual coupling phase, the seventh mutual coupling phase and the eighth mutual coupling phase comprises: Calculating a difference between the fifth mutual coupling phase and the eighth mutual coupling phase to obtain a third difference; Calculating a difference between the sixth mutual coupling phase and the seventh mutual coupling phase to obtain a fourth difference; A half of the difference between the third difference and the fourth difference is calculated to obtain a receiving phase difference between the third receiving channel and the fourth receiving channel.

Images