Directional coupler and radio frequency power amplifier system based on directional coupler output

Abstract

The present invention relates to a directional coupler and a radio frequency power amplifier system based on the output of the directional coupler. By adopting a multi-port directional coupler for high-power and low-power coupling to replace the existing two high-power directional couplers, not only the cost is reduced, but also the insertion loss of the high-power radio frequency channel is reduced. The multi-port directional coupler is matched with a control conditioning board, a splitter, a multi-channel radio frequency power amplifier, and a combiner to form a radio frequency power amplifier system, which is connected to a spectrometer. On the one hand, the standard signal output by the spectrometer and the feedback signal of the directional coupler are used to correct the nonlinearity caused by the power amplifier, so as to realize non-magnetic linear radio frequency power amplification and abnormal protection, so as to obtain linearized radio frequency power suitable for a magnetic resonance imaging system. On the other hand, the spectrometer can be used to monitor the output power of the multi-port directional coupler and the antenna matching, so as to double protect the radio frequency power amplifier system.

Description

Technical Field

[0001] The present invention relates to the field of radio frequency power amplifiers for medical magnetic resonance imaging, and in particular to a directional coupler and a radio frequency power amplifier system based on the output of the directional coupler. Background Art

[0002] In magnetic resonance imaging systems, the RF power amplifier is a commonly used device. The RF power output is usually between 5KW and 35KW (kilowatts), which is called a high-power amplifier. The RF power amplifier is generally connected to the RF coil to amplify the input RF signal and output it to the RF coil. The output of the RF power amplifier is required to be linear, that is, the RF power amplification capability is consistent for RF signal powers of different sizes, with the same amplitude gain and the same output phase. Therefore, it is necessary to add a control part to adjust the amplitude gain and output phase of the RF power amplifier to achieve constant gain and constant phase, which is called nonlinear correction.

[0003] For conventional nonlinear correction, a dual directional coupler is added from the output end. The coupling channel is used to sample the output signal, which is attenuated greatly by the attenuator to reduce the power of the signal and enter the control part to achieve amplitude and phase control. The isolation port of the dual directional coupler monitors the matching of the RF power to the coil and is used to detect the voltage standing wave ratio of the output port to protect against abnormal situations.

[0004] "Design of a Non-Magnetized 5T Magnetic Resonance RF Power Amplifier", Journal of Spectroscopy, June 2022, Vol. 39, No. 2, P163-173, with an output power of 2KW using a single dual directional coupler, reducing the coupling port to improve directivity. The output of the coupling port is tens of watts, but a larger attenuator (40dB) is required to reduce the output power, thereby achieving a few milliwatts of power to the control processing board for nonlinear correction, and another port is used to monitor the output and antenna matching. The attenuator of the coupling channel not only attenuates the signal power, but also causes a reduction in the signal-to-noise ratio, especially when the signal-to-noise ratio is poor at low power input, affecting the calibration of the modulated signal. The output of this method requires the addition of another high-power directional coupler, and the outputs of the coupling and isolation ends are given to the spectrometer to monitor the output power and antenna matching, which increases the cost.

[0005] The output of the RF signal generated by the spectrometer is related to the number of bits of the DAC. Within the working bandwidth, theoretically, the signal-to-noise ratio (SNR) can be calculated using the following formula:

[0006] SNR=6.02N+1.76dB

[0007] Where: N is the number of bits of the DAC. For a 12-bit DAC, the maximum signal-to-noise ratio is 74dB.

[0008] The usual broadband high-speed DAC is 12 bits, and the signal-to-noise ratio of the output signal is 74dB, which is very good. However, after the signal is modulated, the energy of the main carrier part accounts for the majority, and the signal-to-noise ratio is high. The other sideband signals are basically lower than the carrier signal, and some can reach -35dBc. The signal output by the spectrometer usually requires a 40dB dynamic range. For example, there is a sideband signal with an amplitude close to the frequency band that is -35dBc lower than the main peak signal. In this way, the signal-to-noise ratio of the main signal is 74dB, and the signal-to-noise ratio of the sideband signal is reduced to 39dB. After the directional coupler and the 40dB fixed attenuator, the signal-to-noise ratio of the main signal is reduced to 34dB, and the signal-to-noise ratio of the nearby sideband signal is reduced to -1db, which is basically submerged in the noise, which has a great impact on the processing of the modulated signal. This is because the attenuator is connected behind the directional coupler, and the signal amplitude, phase, modulation and other information need to be extracted from it, and there are requirements for the signal-to-noise ratio, and the attenuation cannot be too large. If only the peak amplitude is extracted, the signal-to-noise ratio requirement is relatively low, which is why power detection is often connected using an attenuator.

[0009] The spectrometer monitors the output of the RF power amplifier, and it is necessary to add a high-power dual directional coupler, which is connected to the antenna port and the output port of the first directional coupler. The coupling end and isolation end of the dual directional coupler are connected to the attenuator to reduce the power and send it to the spectrometer for monitoring. The addition of a second high-power directional coupler increases the complexity and cost.

[0010] The signal control and conditioning board uses the signal fed back by the directional coupler for nonlinear calibration. When doing theoretical analysis, it is usually simplified and considered to be an ideal signal, ignoring the signal-to-noise ratio. In fact, there are certain requirements for the signal-to-noise ratio of the input signal. When the signal-to-noise ratio is poor, especially when a weak signal is input, the noise superimposed on the control signal can easily cause a large fluctuation. As a result, when the RF power amplifier is within the linear range and the input power is low, the noise fluctuation interferes with the amplitude and phase.

[0011] The output of the dual directional coupler coupling channel is connected to a larger attenuator, which not only attenuates the signal power but also reduces the signal-to-noise ratio. In addition, the use of two high-power dual directional couplers increases the cost.

[0012] How to use a high-power, low-cost directional coupler to achieve multi-port output, where the main coupling port achieves greater attenuation and has a better signal-to-noise ratio is a problem that needs to be solved. Summary of the invention

[0013] In view of the above-mentioned prior art, the purpose of the present invention is to propose a multi-port directional coupler, improve the existing 4-port directional coupler, add port 5 and port 6, so that it can realize the function of the second high-power directional coupler in the existing directional coupler that requires 2 high powers. Further, by cascading the multi-port directional coupler and the low-power directional coupler to form a dual directional coupler, replacing the existing 2 high-power directional couplers, not only the cost is reduced, but also the insertion loss of the high-power radio frequency channel is reduced. The aforementioned multi-port dual directional coupler is matched with a control conditioning board, a splitter, a multi-channel radio frequency power amplifier, and a combiner to form a radio frequency power amplifier system, and is connected to a spectrometer. On the one hand, a nonlinear correction is performed by referring to the standard signal output by the spectrometer and the feedback signal of the power amplifier to realize non-magnetic linear radio frequency power amplification, so as to obtain a linearized radio frequency power suitable for a magnetic resonance imaging system. On the other hand, the spectrometer can be used to monitor the output power of the multi-port directional coupler and the antenna matching monitoring to realize the second protection function and protect the radio frequency power amplifier system. In order to achieve the above-mentioned purpose, the technical scheme of the present invention is as follows.

[0014] In the first aspect, the present invention proposes a multi-port directional coupler. The multi-port directional coupler uses a PCB microstrip circuit and has 6 ports, namely port 1, port 2, port 3A, port 4, port 5 and port 6; wherein: port 1 is the input of the multi-port directional coupler, port 2 is the output of the multi-port directional coupler; port 3A is the main coupling port with a given directivity value; port 4 is the main isolation port with a given directivity value; port 5 is a slave coupling port for detecting the transmission power of port 1; port 6 is a slave isolation port for detecting the reflected power of port 2; ports 5 and 6 are mirror-symmetric with ports 3A and 4.

[0015] The above technical solution proposes a 6-port directional coupler, which is a multi-port directional coupler with a given characteristic value. Through a mirror setting, a symmetrical slave coupling port 5 and a slave isolation port 6 are formed, which can replace the functions of the coupling port and the isolation port of a high-power directional coupler.

[0016] As an improvement of the above technical solution, the multi-port directional coupler has a three-layer PCB, the third layer is the bottom PCB as a reference ground, port 1 to port 2 are on the top PCB; port 5 and port 6, port 3A and port 4 are on the second layer PCB to improve the insulation of the port 1 to port 2 channel to prevent sparking during high power output.

[0017] As an implementation of the above technical solution, the port 3A of the multi-port directional coupler is connected to the slave directional coupler after connecting the first attenuator to improve the coupling degree; the input power peak of the slave directional coupler is less than 100 watts, and the coupled port is used as the output port 3B. In this implementation, when the multi-port directional coupler is working, the power peak can be greater than or equal to 1 kilowatt, and the coupling end of the high-power directional coupler and the low-power directional coupler are connected in two stages, and the coupled signal channel achieves a large attenuation of the key signal channel, which not only meets the input power requirement of the control part, but also improves the signal-to-noise ratio of the input signal, especially the low-power sideband input of the modulated signal is more improved.

[0018] In other implementations of the above technical solution, port 4, port 5 or port 6 can be connected to a slave directional coupler after connecting a second attenuator to improve the signal-to-noise ratio, wherein the input power peak of the slave directional coupler is less than 100 watts, and the coupled port is used as the output port.

[0019] In a second aspect, the present invention proposes a radio frequency power amplifier system based on directional coupler output, the system comprising a control conditioning board, a pre-power amplifier, a driving amplifier, a splitter, a radio frequency transistor amplifier, a combiner and a dual directional coupler; the dual directional coupler comprises a multi-port directional coupler and a slave directional coupler, the input power peak of the slave directional coupler is less than 100 watts; the multi-port directional coupler uses a PCB microstrip circuit and is a main directional coupler, having 6 ports, namely port 1, port 2, port 3A, port 4, port 5 and port 6; wherein: port 1 is the input of the multi-port directional coupler, port 2 is the output of the multi-port directional coupler, and port 2 is connected to the antenna port; port 3A is a main coupling port with a given directivity value, at port 3A, an attenuator is formed by adding a resistor for fine-tuning, and then a slave directional coupler is connected; port 4 is a main isolation port with a given directivity value, monitoring the matching of the radio frequency output and the antenna; port 5 is a slave coupling port, outputting the attenuated power relative to port 1; port 6 is a slave isolation port, outputting the attenuated power relative to port 2; the signals output from port 5 and port 6 are sent to the For the spectrometer, in one embodiment, the attenuator used for attenuation is a π-type or T-type attenuator composed of chip resistors, or a coaxial attenuator; Port 5 and Port 6 are mirror-symmetrical with Port 3A and Port 4; in the directional coupler, Port 3A1 is the input port, and the coupled port is used as the output coupled port 3B, which is connected to the control conditioning board to form a closed-loop correction of the radio frequency link, and the remaining two ports are connected to matching loads; in the system, the feedback signal of port 3B is used to perform nonlinear correction on the standard signal RF_IN1 generated by the spectrometer to form a signal RF_IN 2; send the signal RF_IN2 to the pre-amplifier for preliminary RF power amplification to form a signal RF_IN3; send the signal RF_IN3 to the driver amplifier for further RF power amplification to form a signal RF_IN4; send the signal RF_IN4 to the power divider for power distribution and then to the RF transistor power amplifier, and then to the power combiner to synthesize the signal RF_C1, output the signal RF_C1 as the total power to the multi-port directional coupler to generate the signal RF_C2, and send the signal RF_C2 to the RF antenna of the magnetic resonance.

[0020] In the above technical solution, the multi-port directional coupler cooperates with the RF power amplifier, combiner and control conditioning part to realize a non-magnetic linearized RF power amplifier. Connecting the spectrometer realizes RF output power monitoring and antenna matching monitoring. Since the coupling end of the high-power directional coupler and the two-cascade coupling signal channel of the low-power directional coupler are used in the multi-port directional coupler, the cost is reduced. Since the above-mentioned multi-port directional coupler realizes a large attenuation of the key signal channel, it meets the input power requirement of the control part and improves the signal-to-noise ratio of the input signal, especially the low-power sideband input of the modulated signal is more improved. Compared with two high-power directional couplers, the insertion loss of the RF channel signal of one high-power directional coupler is smaller.

[0021] As an improvement of the above technical solution, the multi-port directional coupler has a three-layer PCB, the third layer is the bottom layer as a reference ground, port 1 to port 2 are on the top layer PCB, port 5 and port 6, port 3A and port 4 are on the second layer PCB, so as to improve the insulation of the channel from port 1 to port 2.

[0022] In the above technical solution, the power divider performs equal power and same phase power distribution, and 2 to the power of N constitutes M channels, each channel is sent to the RF transistor power amplifier, and the M RF transistor power amplifiers enter the corresponding power combiner, and the power combiner performs equal power and same phase power synthesis.

[0023] In the above technical scheme, the RF transistor power amplifier receives a channel signal distributed by the power divider, and sends the channel signal to the balun composed of the PCB microstrip line to form a signal with equal power and a phase difference of 180 degrees; the input of the transistor amplifier is matched according to the impedance of the optimal input power to achieve balanced power amplification, and then the balanced matching output is implemented according to the impedance of the optimal output power, and it is consistent with the impedance of the output balun. Finally, it is converted into a single-ended signal output by the output balun and sent to the corresponding power combiner.

[0024] In a third aspect, the present invention provides a method for linearizing radio frequency power for a magnetic resonance imaging system, the method comprising the following steps:

[0025] The multi-port directional coupler and the slave directional coupler are cascaded in two stages, wherein the input power peak of the slave directional coupler is less than 100 watts; the multi-port directional coupler uses a PCB microstrip circuit and has 6 ports, namely port 1, port 2, port 3A, port 4, port 5 and port 6; port 1 is the input of the multi-port directional coupler, port 2 is the output of the multi-port directional coupler, and port 2 is connected to the antenna port; port 3A is the main coupling port with a given directivity value, and an attenuator is formed by adding a resistor at port 3A for fine-tuning, and then the slave directional coupler is connected; port 4 is the main isolation port with a given directivity value, outputting the attenuated power relative to port 1 and the reflected power of port 2, monitoring the matching of the RF output and the antenna, and performing the first abnormal protection; port 5 is the slave coupling port, outputting the attenuated power relative to port 1; port 6 is the slave isolation port, outputting the attenuated power relative to port 1 and the reflected power of port 2; the signals output from ports 5 and 6 are sent to the spectrometer after attenuation; ports 5 and 6 are mirror-symmetrical with ports 3A and 4;

[0026] In the slave directional coupler, port 3A1 is the input port, and the coupled port is used as the output coupled port 3B, which is connected to the control conditioning board to form a closed-loop correction of the RF link, and the other two ports are connected to matching loads;

[0027] The signal output from port 3B is used as the feedback signal of the multi-port directional coupler to perform nonlinear correction on the standard signal RF_IN1 generated by the spectrometer. One implementation method of the nonlinear calibration is to use a linear calibration method using analog negative feedback. The signal formed after calibration is RF_IN2.

[0028] Send the signal RF_IN2 to the pre-power amplifier for preliminary RF power amplification to form a signal RF_IN3;

[0029] The signal RF_IN3 is sent to the driving amplifier for further amplification of the RF power to form a signal RF_IN4;

[0030] The signal RF_IN4 enters the power divider for power distribution and then is sent to the RF transistor power amplifier, and then enters the power combiner to synthesize the signal RF_C1, and the signal RF_C1 is output as the total power to the directional coupler to generate the signal RF_C2, and the signal RF_C2 is sent to the RF antenna of the magnetic resonance. The closed-loop correction of the signal output from the directional coupler port 3B and the RF_IN1 signal ensures the linearity of the final output RF_C2. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0032] Figure 1 , one Schematic diagram of a 6-port directional coupler in an embodiment;

[0033] Figure 2 , one A schematic diagram of a radio frequency power amplifier system based on a 6-port directional coupler in an embodiment;

[0034] Figure 3 , one A schematic diagram of a transistor pair balanced amplifier structure of a power amplifier 1 in an embodiment;

[0035] Figure 4 , one Schematic diagram of an open-loop test of an RF power amplifier system based on a directional coupler with an output of 8KW;

[0036] Figure 5 , one A schematic diagram of a main directional coupler PCB board in an embodiment;

[0037] Figure 6 , one A schematic diagram of a simulation structure of a slave directional coupler in an embodiment;

[0038] Figure 7 , one A simulation of the coupling degree from the directional coupler 3A1 to 3B in an embodiment;

[0039] Figure 8 , isolation simulation from port 1 to port 4 of the main directional coupler;

[0040] Fig. 9 , coupling simulation from port 1 to port 3B of the main directional coupler;

[0041] Fig.10 , insertion loss from port 1 to port 2 of the main directional coupler. DETAILED DESCRIPTION

[0042] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments.

[0043] In one embodiment, a 5T non-magnetic RF power amplifier, with a frequency of 210.78MHz, an output P1dB peak power of 8KW (69dBm), a saturation power of about 8.4KW (69.25dBm), a test duty cycle of 8%, 8 millisecond (mS) pulse emission, and a period of 1 second (S). Figure 1 A multi-port dual directional coupler with a given directivity value is shown, using a PCB microstrip circuit. The coupler achieves a large attenuation of the key signal channel through the coupling end of the high-power directional coupler and the two cascades of the low-power directional coupler, which not only meets the input power requirements of the control part, but also improves the signal-to-noise ratio of the input signal, especially the low-power sideband input of the modulated signal.

[0044] Among them, the high-power directional coupler is a multi-port directional coupler, which serves as the main directional coupler, and the low-power directional coupler is the slave directional coupler. It should be noted that in some embodiments, a power peak greater than or equal to 1KW (kilowatt) is regarded as high power, in some embodiments, a power peak between 5KW-35KW is regarded as high power, and in other embodiments, high power is defined as a peak of several kilowatts, such as 10KW or 20KW. In some embodiments, low power is defined as less than tens of watts, such as less than 10W, and in some embodiments, a power peak less than 100W is regarded as low power.

[0045] In directional coupler design, coupling and isolation are:

[0046] Directivity = Isolation - Coupling

[0047] The two influence each other, and the desired directivity here is better than 15dB to 20dB.

[0048] The expected coupling and isolation values ​​are higher, but due to the mutual influence of PCB processing technology and multiple ports, only compromises can be considered in engineering.

[0049] For high-power directional couplers, meeting power requirements and low insertion loss are given priority, followed by coupling, which is required to be 25dB to 30dB, and isolation, which is 45dB to 50dB.

[0050] For low power directional couplers, the coupling degree is required to be 25dB to 30dB.

[0051] In this way, the multi-port directional coupler designed, after the master and slave directional couplers are cascaded through the 3dB attenuator composed of resistors, the coupling degree requirement ranges from port 1 to port 3B from 55dB to 60dB, and its signal-to-noise ratio is only reduced by 3dB, which meets the signal-to-noise ratio requirement of the control conditioning board signal input; for other special needs, if the attenuator is not needed, the resistance value is selected to be 0 ohm to form a direct pass, and the attenuation value is 0dB. Figure 1 In the figure, port 1 is the input of the multi-port directional coupler, and port 2 is the output of the multi-port directional coupler. Ports 1 and 2 have low insertion loss in the working frequency band, with the loss less than 0.1dB, and the peak power of the transmitted pulse is 10KW; port 3A and port 4 have given directivity values; port 3A is the main coupling port with given coupling degree, and port 4 is the main isolation port, which outputs the attenuated power relative to port 1 and the reflected power of port 2, and is used to monitor the matching of the RF output and the antenna. Through the mirror setting, another symmetrical slave coupling port 5 and slave isolation port 6 are formed. Port 5 is a slave coupling port, which outputs the attenuated power relative to port 1; port 6 is a slave isolation port, which outputs the attenuated power relative to port 1 and the reflected power of port 2. At the main coupling port 3A, a π-type or T-type 3dB attenuator composed of resistors is added, and the attenuator is used for fine-tuning. A π-type attenuator is used here, with 300 ohms on both sides, 18 ohms in the middle, and a 3W chip power resistor. It is then connected to the input terminal 3A1 of the second-stage low-power directional coupler. The second-stage directional coupler only has a coupling port used as output coupling 3B, and the other two ports are matched.

[0052] For the RF power amplifier system of 5T magnetic resonance, except for the control and conditioning board, the rest is integrated on a PCB board. The entire PCB board has 6 layers, consisting of a multi-port directional coupler and the entire power amplifier system, of which the directional coupler uses 3 layers. Specifically, the main parameters of the multi-port directional coupler are: the center frequency is 210.78MHz, Roger4350B board, 3-layer PCB, 3-layer PCB thickness is 1.6mm, the 3rd layer is the bottom layer as the reference ground, and the thickness from the first layer to the second layer is 0.29mm. The line width from port 1 to port 2 is 3.5mm, its characteristic impedance is 50 ohms, and the length is 50mm. On the top PCB, the 8.4KW test is normal. The line width of the coupling section of port 3A and port 4 is 2.8mm, the length is 27.2mm, and the edge distance between the positive projection to the top layer and port 1 and port 2 is 0.8mm. Ports 5 and 6, ports 3A and port 4, are on the second layer, and the internal edge distance is 5.1mm. Port 5 and port 6, port 3A and port 4 are on the second layer PCB to improve insulation and prevent arcing at high power.

[0053] refer to Figure 8 , Fig. 9 , Fig.10, the simulation parameters of the main directional coupler, the insertion loss from port 1 to port 2 is 0.04dB, the coupling from port 1 to port 3A is 25.8dB, and the isolation from port 1 to port 4 is 50.4dB. Figure 7 , the coupling degree from directional coupler port 3A1 to 3B is 23.8dB. Thus, after the 3dB resistor attenuator and the main directional coupler are connected, the coupling degree from port 1 to port 3B is (23.8dB+3dB+25.8dB), which is equal to 52.6dB, which does not meet the required 55dB to 60dB. The coupling degree of the directional coupler needs to be optimized, with the goal of 30dB coupling degree, and then the PCB board verification is done.

[0054] Here, according to the required port 3A, the second-stage auxiliary directional coupler is connected after the first attenuator is connected. From port 1 to the coupled port 3B, after passing through the main directional coupler 3A, the directional coupler is output to the 3B port to achieve higher attenuation, thereby reducing the use of attenuators with higher values, achieving an RF signal output of about 0dBm, and ensuring an excellent output signal-to-noise ratio. From 3A to 3B, the coupling degree is improved, the attenuation of the resistor is reduced, and the signal-to-noise ratio is improved. The attenuation value of the first attenuator is relatively low, such as 3db, and the interface part requires a better signal-to-noise ratio. For other ports 4, port 5 or port 6, if there are special requirements for the signal-to-noise ratio, the same method can be used to improve the signal-to-noise ratio by connecting the second attenuator and then connecting the slave directional coupler. If not required, the attenuation value of the attenuator connected later is relatively large, such as between 20dB and 40dB. Here, the output of these three ports is used for power detection through the attenuator, and the signal-to-noise ratio requirement is not high. According to the power requirements of the output connection port, such as 0dBm, an attenuator composed of resistors is usually used for connection or other external attenuators.

[0055] Port 3B is used to connect the control and conditioning board, output feedback signals to perform nonlinear calibration on the standard signal output by the spectrometer; it is also used to detect the input power of port 1. Port 2 is connected to the antenna port as a high-power output port. Port 4 is the main isolation port, outputting the attenuated power relative to port 1 and the reflected power of port 2, inputting the control and conditioning board, monitoring the output, and performing the first protection when the antenna is mismatched (for example, the voltage standing wave ratio is 3), shutting down the input power of the power amplifier. For example: Port 5 is attenuated by about 30dB relative to port 1, and based on the symmetry of the structure, port 6 is attenuated by about 30dB relative to port 2. Port 1 inputs 69dBm pulse peak power, port 5 is attenuated to 39dBm, and then connected to a π-type attenuator composed of a 3W chip resistor, attenuating by about 24dB for fine-tuning, followed by a 1W attenuator with a fixed 15dB, attenuating from 39dBm to about 0dBm, and sent to the spectrometer to monitor the RF transmission power. The power of port 6 is the power of the isolation attenuation of port 1 plus the power of the reflected coupling of port 2. For example, if the peak power of the input pulse of port 1 is 69dBm and the isolation of port 6 is 50db, the coupled power of port 6 from port 1 is 19dBm (69dbm-50dB), about 79mW; for example, if port 2 is mismatched and half-reflected, half of the energy is reflected, which is 69dbm-3db, equal to 66dBm, and the energy coupled from port 6 to port 2 is 66dBm-30dB, equal to 36dBm, about 3981mW. At this time, the total energy of port 6 is 39.1dBm (4060mW), which is about 0.1dBm after 39dB attenuation and is sent to the spectrometer for matching status monitoring.

[0056] Port 5 is used to detect the transmission power of port 1, and port 6 is used to detect the reflected power of port 2, and is connected to a spectrometer. This replaces the function of the second high-power directional coupler, and is usually connected to a spectrometer for output power detection and matching status monitoring. When an abnormality occurs (such as the voltage standing wave ratio is 5), the spectrometer stops outputting power and performs the second level of protection.

[0057] The RF power amplifier system based on multi-port directional coupler is used for magnetic resonance to provide RF signal for magnetic resonance. Figure 2 As shown. The spectrometer is used to monitor the output of the RF power amplifier and the antenna matching. The standard signal RF_IN1 generated by the spectrometer is sent to the control and conditioning board, and combined with the feedback signal of the directional coupler, nonlinear calibration is performed to form the signal RF_IN2. The signal RF_IN2 is sent to the pre-power amplifier for preliminary RF power amplification to become the signal RF_IN3, which is then sent to the driver amplifier for further RF power amplification, and then sent to the power divider to achieve linear amplification.

[0058] Among them, the power divider performs equal power and same phase power distribution, and there are M channels composed of 2 to the power of N, and the power amplified by a single power amplifier in each channel is the same. Each channel is sent to the RF transistor power amplifier, and the M RF transistor power amplifiers enter the corresponding power combiner. The power combiner performs equal power and same phase power synthesis, and the synthesized signal RF_C1 is output as the total power, and the total power is M times the power amplified by a single power amplifier. Finally, it is sent to the 6-port directional coupler and sent to the RF antenna of the magnetic resonance by the signal RF_C2. By changing the number of channels M, the total power to be increased can be changed in a linear manner.

[0059] For RF transistor power amplifiers, the M paths are the same, and the first path is used here for illustration. Figure 3 , signal RF_PA_IN1 is sent to the Balun composed of PCB microstrip line to form a signal with equal power and 180 degrees difference. The input of the transistor amplifier is balanced input matched, and matched according to the impedance of the optimal input power to ensure the transmission of maximum power and in a stable working state without self-excited oscillation; the signal after balanced input matching is input into the balanced power amplifier, and then output through balanced output matching. The output of the RF transistor amplifier is balanced matched according to the impedance of the optimal power, and the output power is consistent with the impedance of the output Balun. Finally, the Balun is output and converted into a single-ended signal RF_OUT1 output and sent to the power combiner. In this embodiment, the single-channel power amplifier uses a RF high-power amplifier of MRFX1K80N field effect tube. When the central operating frequency of the RF pulse signal is 210.78MHz, the working DC voltage is 75V, the period is 100mS, the pulse width is 200uS, and the input peak power of the RF pulse signal is 14W. The saturated output peak power of the RF pulse signal is 2400W. In this embodiment, four identical power amplifier channels are used to form an 8KW output. The splitter is connected in series by two stages to form four output ports, which are connected to four power amplifier input channels. The combiner is connected in series by two stages to form four input ports, which are connected to four power amplifier output channels.

[0060] For the control conditioning board, the nonlinear calibration of the RF power amplifier system is realized according to the signal detected by the output of the directional coupler port 3B and the signal input by the spectrometer; the reflection of port 2 is detected by port 4, and after attenuation, the input signal conditioning board performs power detection and calculates the matching of the antenna. When there is no reflection and ideal matching at port 2, there is no power coupling between port 2 and port 4, and only coupling between port 1 and port 4 exists. The coupling coefficient is equal to the isolation, about 50dB, and the output power of port 4 is 69dBm-50dB, which is equal to 19dBm; when the output of port 2 is mismatched and its voltage standing wave ratio (VSWR) is 3, the output power of port 2 is reflected by nearly 25%, and the coupling between port 4 and port 2 is 30dB. Port 2 is 8KW×25%, which is equal to 2KW (63dBm), and port 4 is 63dBm-30dB, which is equal to 33dBm. Therefore, the output of port 4 changes from 19dBm without reflection to 33dBm caused by reflection, and is sent to the signal detection board through a fixed attenuator for linear power detection to determine the matching of the output of port 2. When the VSWR is equal to 3, the output of port 4 is 33dBm, which is considered to be a serious mismatch. The signal processing board shuts down the output of the RF signal to protect the RF power amplifier system. This is the first level of protection provided by the power amplifier system.

[0061] In the above implementation, by reducing a high-power directional coupler, the insertion loss and cost of the total RF output power are reduced; the present invention controls the signal of the conditioning board at the coupling end input, improves the signal-to-noise ratio of the input signal, and thus improves the linearity of the entire RF power amplifier.

[0062] In another embodiment, the RF power amplifier system based on the directional coupler is used for a 5T non-magnetic RF power amplifier, whose center frequency is 210.78MHz and the P1dB output open-loop power is 8KW. Figure 4 The transistor pair used is NXP's MRFX1K80H, and the operating voltage is 75Vdc. The directional coupler uses Rogers4350B as the PCB substrate, see Figure 5 .

[0063] The test of the main directional coupler, using the data tested by the vector network analyzer, shows that the insertion loss from port 1 to port 2 is 0.06dB, the coupling degree of port 3A to port 1 is 32dB, the isolation degree of port 4 to port 1 is 43dB, the coupling degree of port 5 to port 1 is 32dB, and the isolation degree of port 6 to port 1 is 43dB; there are differences between the test data and the simulation. It is necessary to adjust according to the PCB processing technology to increase the isolation to 50dB.

[0064] The coupling degree from the directional coupler port 3B to the port 3A1 is 25 dB.

[0065] In the main directional coupler, the directivity of port 4 for port 3A is -(43-32) equal to 9dB when input from port 1, so it does not reach the given target of 15dB and needs further improvement. The difference in the tested results of the main directional coupler is analyzed. Due to the standard thickness parameter (0.254mm) processed by the factory and the thickness of the top and second layers of the PCB design (0.29mm), the difference in thickness causes the change in the results, resulting in the failure to reach the predetermined target of directivity. Therefore, based on the standard thickness as a fixed value, other parameters are further fine-tuned to improve isolation, thereby achieving an improvement in directivity to the predetermined range of 15dB to 20dB.

[0066] The coupling degree is confirmed by simulation from the directional coupler. For the simulation structure diagram, see Figure 6 , simulation results Figure 7 The coupling degree from port 3A1 to port 3B is 23.8dB. The next step is to optimize the isolation to 30db, and then process the slave directional coupler for test verification. Then, a 3dB attenuator composed of resistors is used to connect the master directional coupler and the slave directional coupler, and the isolation degree from port 1 to port 3B of the connected 6-port directional coupler is tested to meet the coupling degree of 55dB to 60dB, and finally integrated together.

[0067] The cascaded directional coupler designed in this embodiment extracts a coupling signal with a high signal-to-noise ratio by using a nonlinear calibration method of analog negative feedback, and realizes linear correction of the amplitude and phase of the output signal of the output port of the directional coupler. It should be noted that using a similar directional coupler structure, extracting a coupling signal with a high signal-to-noise ratio, and performing nonlinear calibration, no matter which nonlinear calibration method is used, all fall within the scope of protection of the present invention.

[0068] Although the embodiments of the present invention are described above in conjunction with the accompanying drawings, the present invention is not limited to the above specific embodiments and application fields, and the above specific embodiments are only illustrative and instructive, rather than restrictive. A person of ordinary skill in the art can also make many forms under the guidance of this specification and without departing from the scope of protection of the claims of the present invention, all of which belong to the protection of the present invention.

Claims

1. A multi-port directional coupler, Features: The multi-port directional coupler uses a PCB microstrip circuit and has 6 ports, namely port 1, port 2, port 3A, port 4, port 5 and port 6; wherein: Port 1 is the multi-port directional coupler input, and port 2 is the multi-port directional coupler output; Port 3A is a main coupling port with a given directivity value, and the coupling degree is improved by connecting a slave directional coupler after connecting a first attenuator, and the input power peak of the slave directional coupler is less than 100 watts, and the coupling port is used as an output port; Port 4 is the main isolation port with a given directivity value, used to detect the reflected power of port 2; Port 5 is the slave coupling port, used to detect the transmission power of port 1; Port 6 is a slave isolation port, used to detect the reflected power of port 2; Port 5 and Port 6 are mirror images of Port 3A and Port 4.

2. The multi-port directional coupler according to claim 1, Features: The multi-port directional coupler has a three-layer PCB, the third layer is the bottom PCB as a reference ground, port 1 to port 2 are on the top PCB; port 5, port 6, port 3A and port 4 are arranged on the second layer PCB to improve the insulation of the channel from port 1 to port 2.

3. The multi-port directional coupler according to claim 1, Features: Port 4, port 5 or port 6 can be connected to a slave directional coupler after being connected to a second attenuator to improve the signal-to-noise ratio. The input power peak of the slave directional coupler is less than 100 watts, and the coupled port is used as the output port.

4. RF power amplifier system based on directional coupler output, Features: The radio frequency power amplifier system includes a control conditioning board, a pre-power amplifier, a driver amplifier, a splitter, a radio frequency transistor amplifier, a combiner and a dual directional coupler; The dual directional coupler comprises a multi-port directional coupler and a slave directional coupler, wherein the input power peak value of the slave directional coupler is less than 100 watts; The multi-port directional coupler uses a PCB microstrip circuit and has 6 ports, namely port 1, port 2, port 3A, port 4, port 5 and port 6; among which: Port 1 is the input of the multi-port directional coupler, port 2 is the output of the multi-port directional coupler, and port 2 is connected to the antenna port; Port 3A is the main coupling port with a given directivity value. At port 3A, an attenuator is formed by adding a resistor for fine-tuning, and then the slave directional coupler is connected; Port 4 is the main isolation port with a given directivity value, used to detect the reflected power of port 2 and monitor the matching of the antenna; Port 5 is the slave coupling port, used to detect the input power of port 1; Port 6 is a slave isolation port, used to detect the reflected power of port 2 and monitor the matching of the antenna; The signals output from ports 5 and 6 are sent to the spectrometer after being attenuated; Port 5 and Port 6 are mirror images of Port 3A and Port 4; In the slave directional coupler, port 3A1 is the input port, and the coupled port is used as the output coupled port 3B, which is connected to the control conditioning board to form a closed-loop correction of the RF link, and the other two ports are connected to matching loads; In the system, the feedback signal of port 3B is used to perform nonlinear correction on the standard signal RF_IN1 generated by the spectrometer to form a signal RF_IN2; Send the signal RF_IN2 to the pre-power amplifier for preliminary RF power amplification to form a signal RF_IN3; The signal RF_IN3 is sent to the driving amplifier for further amplification of the RF power to form a signal RF_IN4; The signal RF_IN4 is sent to the power divider for power distribution and then to the RF transistor power amplifier, and then enters the power combiner to synthesize the signal RF_C1. The signal RF_C1 is output as the total power to the directional coupler to generate the signal RF_C2, and the signal RF_C2 is sent to the RF antenna of the magnetic resonance.

5. The radio frequency power amplifier system according to claim 4, Features: The multi-port directional coupler has a three-layer PCB, the third layer is the bottom PCB as a reference ground, port 1 to port 2 are on the top PCB; port 5, port 6, port 3A and port 4 are arranged on the second layer PCB to improve the insulation of the channel from port 1 to port 2.

6. The radio frequency power amplifier system according to claim 4, Features: The power divider performs equal power and same phase power distribution. There are M channels composed of 2 to the power of N. Each channel is sent to the RF transistor power amplifier. The M RF transistor power amplifiers enter the corresponding power combiner, and the power combiner performs equal power and same phase power synthesis.

7. The radio frequency power amplifier system according to claim 4, Features: The RF transistor power amplifier receives a channel signal distributed by the power divider, and sends the channel signal to the balun composed of the PCB microstrip line to form a signal with equal power and 180 degrees difference in phase; The input of the transistor amplifier is matched to the impedance of the optimal input power to achieve balanced power amplification, and then the output is balanced and matched to the impedance of the optimal output power, and it is consistent with the impedance of the output balun. Finally, the output balun converts it into a single-ended signal output and sends it to the corresponding power combiner.

8. A method for linearizing radio frequency power for a magnetic resonance imaging system, It is characterized in that The method comprises the following steps: The multi-port directional coupler and the slave directional coupler are cascaded in two stages, wherein the input power peak of the slave directional coupler is less than 100 watts; the multi-port directional coupler uses a PCB microstrip circuit and has 6 ports, namely port 1, port 2, port 3A, port 4, port 5 and port 6; port 1 is the input of the multi-port directional coupler, port 2 is the output of the multi-port directional coupler, and port 2 is connected to the antenna port; port 3A is the main coupling port with a given directivity value, and an attenuator is formed by adding a resistor at port 3A for fine-tuning, and then the slave directional coupler is connected; port 4 is the main isolation port with a given directivity value, outputting the attenuated power relative to port 1 and the reflected power of port 2 to monitor the matching of the antenna; port 5 is the slave coupling port, outputting the attenuated power relative to port 1; port 6 is the slave isolation port, outputting the attenuated power relative to port 1 and the reflected power of port 2 to monitor the matching of the antenna; the signals output from ports 5 and 6 are sent to the spectrometer after attenuation; ports 5 and 6 are mirror-symmetrical with ports 3A and 4; In the slave directional coupler, port 3A1 is the input port, and the coupled port is used as the output coupled port 3B, which is connected to the control conditioning board to form a closed-loop correction of the RF link, and the other two ports are connected to matching loads; The signal output from port 3B is used as the feedback signal of the multi-port directional coupler to perform nonlinear correction on the standard signal RF_IN1 generated by the spectrometer to form a signal RF_IN2; Send the signal RF_IN2 to the pre-power amplifier for preliminary RF power amplification to form a signal RF_IN3; The signal RF_IN3 is sent to the driving amplifier for further amplification of the RF power to form a signal RF_IN4; The signal RF_IN4 enters the power divider for power distribution and then is sent to the RF transistor power amplifier, and then enters the power combiner to synthesize the signal RF_C1. The signal RF_C1 is output as the total power to the directional coupler to generate the signal RF_C2, and the signal RF_C2 is sent to the RF antenna of the magnetic resonance.

9. The method according to claim 8, Features: The nonlinear correction adopts a linear calibration method of simulated negative feedback.

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