Pulse-modulated radio frequency power amplifier system and magnetic resonance system

Abstract

The invention discloses a pulse-modulated radio frequency power amplifier system and a magnetic resonance system. The amplifier system comprises a nonlinear correction unit, a radio frequency pre-amplification and driving amplification unit, a radio frequency power amplification unit, a directional coupler, a pulse synchronous switch unit and a filtering unit. Wherein the non-linear correction unit, the radio frequency pre-amplification and driving amplification unit, the radio frequency power amplification unit and the directional coupler form a closed loop, and the filtering unit is used for filtering signal components lower than a preset working frequency band in the formed closed loop, so that the gain of a low frequency band is reduced, the condition of self-excitation oscillation is destroyed, and the self-excitation performance of the antenna is improved. The self-excitation is avoided, the problem of potential instability of a low-frequency band in a radio frequency power amplifier system is solved, the whole system can work more stably and reliably, meanwhile, pulse modulation of sub-microsecond-level stable time can be achieved, and the problem is solved simply at low cost only by reasonably using the filtering function.

Description

Technical Field

[0001] The embodiments of the present invention relate to the technical field of radio frequency power amplifiers, and in particular to a pulse modulated radio frequency power amplifier system and a magnetic resonance system. Background Art

[0002] For the application of high-power RF amplifiers, such as in magnetic resonance systems, several RF power channels with good consistency are required. Each RF power channel is composed of an RF power amplifier system. The gate control signal is used for processing. The gates of related RF transistors are synchronously turned on or off to achieve pulse modulation in conjunction with the RF signal. The peak power output of the RF power amplifier system after pulse modulation can reach several kilowatts. For this reason, the RF power amplifier system usually needs to perform multi-stage amplification. For example, the RF power amplifier system in the magnetic resonance system usually consists of three stages, so as to achieve milliwatt input to kilowatt output. Each stage of amplification in the system is stable. When the three stages of amplification are combined to form a pulse-modulated RF power amplifier, especially the transistors with kilowatt-level RF output, there are thermal effects and memory effects when working, which may cause instability or potential instability.

[0003] The prior art for high-power RF amplifiers usually uses RC in series with the input gate of the power amplifier, or uses a combination of RC or RLC, and grounds the gate as a stabilization circuit. This stabilization circuit significantly improves the stability of narrowband frequency bands, but has a weaker effect on the stability of broadband frequency bands. In particular, for lower frequency bands, a larger value of capacitor C is required to achieve stability. However, for larger values ​​of capacitor C, due to the limitation of the RC time constant, the stabilization time of the gate usually requires milliseconds or hundreds of milliseconds, which is suitable for conventional slow switching applications, but not suitable for fast switching pulse modulation, and cannot meet the requirements of sub-microsecond transient stabilization time in practical applications. If a smaller capacitor C or RC is used, or the capacitor and resistor RC connected to the ground of the gate are removed, a sub-microsecond transient stabilization time can be achieved, but the lower frequency band is potentially unstable and easily causes self-excitation. Therefore, for RF power amplifier systems, especially in lower frequency bands, improving their stability to avoid self-excitation and enabling them to meet fast switching pulse modulation is a problem that needs to be solved. Summary of the invention

[0004] The embodiments of the present invention provide a pulse modulated radio frequency power amplifier system and a magnetic resonance system to solve the potential instability problem in the low frequency band of the radio frequency power amplifier system and realize pulse modulation with a sub-microsecond stabilization time.

[0005] In a first aspect, an embodiment of the present invention provides a pulse modulated RF power amplifier system, the system comprising: a nonlinear correction unit, a RF preamplifier and driver amplifier unit, a RF power amplifier unit, a directional coupler, a pulse synchronous switch unit and a filter unit; wherein:

[0006] The nonlinear correction unit is used to perform linearity correction according to the system input signal and the coupling signal fed back by the directional coupler, and provide the corrected signal to the RF preamplifier and driver amplifier unit; the RF preamplifier and driver amplifier unit is used to linearly amplify the corrected signal, and provide the obtained initial amplified signal to the RF power amplifier unit; the RF power amplifier unit is used to power amplify the initial amplified signal, and provide the obtained amplified signal to the directional coupler; the directional coupler is used to generate the coupling signal and the system output signal according to the amplified signal; the nonlinear correction unit, the RF preamplifier and driver amplifier unit, the RF power amplifier unit and the directional coupler form a closed loop;

[0007] The pulse synchronous switch unit is used to drive the input gate signal to synchronously provide it to the nonlinear correction unit, the RF preamplifier and driving amplifier unit and the RF power amplifier unit;

[0008] The filtering unit is connected in series in the closed loop and is used to filter out signal components below a preset working frequency band.

[0009] Optionally, the filtering unit includes a first filter and a second filter; the first filter is connected in series between the nonlinear correction unit and the RF preamplifier and driving amplifier unit, and the second filter is connected in series between the RF preamplifier and driving amplifier unit and the RF power amplifier unit.

[0010] Optionally, the filtering unit is a high-pass filter or a band-pass filter.

[0011] Optionally, the system also includes an attenuator and a phase shifter; the attenuator is connected in series between the system input signal and the nonlinear correction unit for gain compensation; the phase shifter is connected in series between the system input signal and the nonlinear correction unit, or between the nonlinear correction unit and the RF preamplifier and driving amplifier unit, for phase compensation.

[0012] Optionally, the RF power amplification unit includes a power divider, a power synthesizer, and a plurality of identical RF power amplifiers; the power divider is used to input the same power to each of the RF power amplifiers based on the initial amplified signal, and the power synthesizer is used to synthesize the power output by each of the RF power amplifiers to obtain the amplified signal;

[0013] The RF power amplifier includes an RF transistor, input matching, output matching, a bias circuit and power decoupling; the input signal of the RF power amplifier is provided to the RF transistor through the input matching, and the output signal of the RF power amplifier is generated through the output matching; the bias circuit is used to provide a gate bias voltage for the RF transistor; the power decoupling is used to provide a power supply voltage for the RF transistor and perform decoupling.

[0014] Optionally, the radio frequency transistor includes a first transistor and a second transistor that are identical and connected to each other at the source; the bias circuit includes an identical first bias circuit and a second bias circuit, both of which include a stabilizing resistor, a stabilizing inductor, a working frequency band decoupling capacitor, a high frequency band decoupling capacitor, a current limiting resistor, a single-pole double-throw switch, a low frequency band decoupling capacitor, and a discharge resistor; wherein the single-pole double-throw switch is selected based on a driving gating signal provided by the pulse synchronous switch unit;

[0015] In the first bias circuit, the output end of the single-pole double-throw switch is connected to the gate of the first transistor through a current limiting resistor, a stabilizing inductor and a stabilizing resistor; one end of the working frequency band decoupling capacitor is connected between the current limiting resistor and the stabilizing inductor, and the other end is grounded; one end of the high frequency band decoupling capacitor is connected between the current limiting resistor and the stabilizing inductor, and the other end is grounded; one input end of the single-pole double-throw switch is connected to the bias voltage, and the other input end is grounded through a discharge resistor; one end of the low frequency band decoupling capacitor is connected between the bias voltage and the single-pole double-throw switch, and the other end is grounded;

[0016] In the second bias circuit, the output end of the single-pole double-throw switch is connected to the gate of the second transistor through a current limiting resistor, a stabilizing inductor and a stabilizing resistor; one end of the working frequency band decoupling capacitor is connected between the current limiting resistor and the stabilizing inductor, and the other end is grounded; one end of the high frequency band decoupling capacitor is connected between the current limiting resistor and the stabilizing inductor, and the other end is grounded; one input end of the single-pole double-throw switch is connected to the bias voltage, and the other input end is grounded through a discharge resistor; one end of the low frequency band decoupling capacitor is connected between the bias voltage and the single-pole double-throw switch, and the other end is grounded.

[0017] Optionally, the input matching includes an input Balun, a first transmission line, a first capacitor, a second transmission line, a third transmission line, a first inductor, a fourth transmission line, a second capacitor, a fifth transmission line, a sixth transmission line, a second inductor and a third capacitor; the input Balun is used to convert the input signal of the RF power amplifier into two equal-power inverting signals through a Balun transformation, wherein one signal is connected to the gate of the first transistor via the first transmission line, the first capacitor, the second transmission line, the third transmission line and the first inductor in sequence, and the other signal is connected to the gate of the second transistor via the fourth transmission line, the second capacitor, the fifth transmission line, the sixth transmission line and the second inductor in sequence; one end of the third capacitor is connected between the second transmission line and the third transmission line, and the other end is connected between the fifth transmission line and the sixth transmission line.

[0018] Optionally, the output matching includes a seventh transmission line, an eighth transmission line, a ninth transmission line, a tenth transmission line, an output Balun and a fourth capacitor; the drain of the first transistor is connected to the output Balun via the seventh transmission line and the eighth transmission line in sequence, and the drain of the second transistor is connected to the output Balun via the ninth transmission line and the tenth transmission line in sequence, and the output Balun is used to generate an output signal of the RF power amplifier based on the two received signals; one end of the fourth capacitor is connected between the seventh transmission line and the eighth transmission line, and the other end is connected between the ninth transmission line and the tenth transmission line.

[0019] Optionally, the power supply decoupling includes a choke inductor and a plurality of decoupling capacitors of different frequency bands; the output end of the output matching is connected to the power supply through the choke inductor; and each decoupling capacitor is connected between the choke inductor and the power supply at one end and grounded at the other end.

[0020] In a second aspect, an embodiment of the present invention further provides a magnetic resonance system, which includes the pulse-modulated radio frequency power amplifier system provided by any embodiment of the present invention.

[0021] The embodiment of the present invention provides a pulse modulated RF power amplifier system, including a nonlinear correction unit, an RF pre-amplifier and drive amplifier unit, an RF power amplifier unit, a directional coupler, a pulse synchronous switch unit and a filter unit. By using the filter unit to filter out signal components below a preset working frequency band in the formed closed loop, the gain of the low frequency band is reduced, thereby destroying the conditions for self-excited oscillation, avoiding the generation of self-excitation, solving the potential instability problem of the low frequency band in the RF power amplifier system, and making the entire system work more stably and reliably. At the same time, pulse modulation with a sub-microsecond stabilization time can also be achieved, and the problem can be solved in a low-cost and simple manner only by using the filtering function reasonably. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A schematic diagram of the structure of a pulse modulated radio frequency power amplifier system provided in Embodiment 1 of the present invention;

[0023] Figure 2 A schematic diagram of the structure of another pulse modulated radio frequency power amplifier system provided in Embodiment 1 of the present invention;

[0024] Figure 3 A schematic diagram of the structure of another pulse modulated radio frequency power amplifier system provided in the first embodiment of the present invention;

[0025] Figure 4 A schematic diagram of the structure of a single-channel radio frequency power amplifier provided in Embodiment 1 of the present invention;

[0026] Figure 5 Field-circuit collaborative simulation of the PCB layout corresponding to the final-stage single-channel RF power amplifier provided in the first embodiment of the present invention;

[0027] Figure 6 for Figure 5 Corresponding simulation results;

[0028] Figure 7 This is an output test spectrum diagram of the pulse modulated RF power amplifier system provided in the first embodiment of the present invention;

[0029] Figure 8 A waveform of the time domain power envelope acquisition of a single pulse on a spectrum analyzer under occasional instability provided in the first embodiment of the present invention;

[0030] Fig. 9 The waveform collected by the oscilloscope at another time under occasional instability provided in the first embodiment of the present invention;

[0031] Fig.10 The stability coefficient simulation result of the final-stage single-channel RF power amplifier provided in the first embodiment of the present invention;

[0032] Fig.11 A schematic diagram of the structure of a filter unit provided in Embodiment 1 of the present invention;

[0033] Fig.12 The simulation result of the filter unit circuit provided in the first embodiment of the present invention;

[0034] Fig.13 The gain closed-loop test result without a filter provided in the first embodiment of the present invention;

[0035] Fig.14 The gain closed-loop test result with a filter is provided in the first embodiment of the present invention. DETAILED DESCRIPTION

[0036] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for ease of description, only parts related to the present invention, rather than all structures, are shown in the accompanying drawings.

[0037] In addition, the terms "first", "second", etc. may be used in this article to describe various directions, actions, steps or elements, etc., but these directions, actions, steps or elements are not limited by these terms. These terms are only used to distinguish a first direction, action, step or element from another direction, action, step or element. For example, without departing from the scope of the embodiment of the present invention, the first filter can be referred to as the second filter, and similarly, the second filter can be referred to as the first filter. Both the first filter and the second filter are filters, but they are not the same filter. The terms "first", "second", etc. cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the embodiment of the present invention, the meaning of "multiple" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0038] Embodiment 1

[0039] Figure 1 This is a schematic diagram of the structure of a pulse modulated RF power amplifier system provided in Embodiment 1 of the present invention. This embodiment can be applied to the design and production of high-power RF power amplifiers required in the fields of magnetic resonance systems, wireless communications, radars, etc. Figure 1As shown, the system includes: a nonlinear correction unit 10, a radio frequency preamplifier and driver amplifier unit 40, a radio frequency power amplifier unit 60, a directional coupler 70, a pulse synchronous switch unit 80 and a filter unit 30; wherein the nonlinear correction unit 10 is used to perform linearity correction according to the system input signal and the coupled signal fed back by the directional coupler 70, and provide the corrected signal to the radio frequency preamplifier and driver amplifier unit 40; the radio frequency preamplifier and driver amplifier unit 40 is used to linearly amplify the corrected signal, and provide the obtained initial amplified signal to the radio frequency power amplifier unit 60; the radio frequency power amplifier unit 60 is used to power the initial amplified signal amplifies, and provides the obtained amplified signal to the directional coupler 70; the directional coupler 70 is used to generate the coupling signal and the system output signal according to the amplified signal; the nonlinear correction unit 10, the RF preamplifier and driving amplifier unit 40, the RF power amplifier unit 60 and the directional coupler 70 form a closed loop; the pulse synchronous switch unit 80 is used to drive the input gate signal to synchronously provide it to the nonlinear correction unit 10, the RF preamplifier and driving amplifier unit 40 and the RF power amplifier unit 60; the filtering unit 30 is connected in series in the closed loop to filter out signal components below the preset working frequency band.

[0040] Specifically, the pulse modulation signal generated by the signal source can be directly input as the system input signal RF_IN to the nonlinear correction unit 10, and the nonlinear correction unit 10 can realize the correction of the gain and phase of the whole system, so that the system output signal RF_OUT outputted by the latter stage maintains a linear relationship with the system input signal RF_IN to meet the linearization requirements of the high-power RF amplifier. The nonlinear correction unit 10 can use the system input signal as a standard signal, and use the coupled signal RF_Couple fed back by the directional coupler 70 as the output sampling signal to perform linearity correction, and obtain the corrected signal RF_IN2 to provide to the RF preamplifier and driver amplifier unit 40. The RF transistor input power of the final power amplifier stage needs to be met by RF preamplifier and RF driver amplification, and the corrected signal RF_IN2 can be linearly amplified by the RF preamplifier and driver amplifier unit 40 to obtain the initial amplified signal RF_DRV to provide to the RF power amplifier unit 60. The RF power amplifier unit 60 can further increase the output power through several identical final-stage RF power amplifiers, in conjunction with a power divider and a power synthesizer, so that the initial amplified signal RF_DRV can be power amplified to obtain an amplified signal RF_PA to be provided to the directional coupler 70. The directional coupler 70 can generate a coupling signal RF_Couple based on the amplified signal RF_PA and feed it back to the nonlinear correction unit 10, and generate a system output signal RF_OUT for output. Then the nonlinear correction unit 10, the RF pre-amplifier and drive amplifier unit 40, the RF power amplifier unit 60 and the directional coupler 70 can form a closed loop. For the RF transistors of the final power amplifier stage, a load-pulling method is usually used for matching to achieve high power and high efficiency, and the system output signal RF_OUT can be connected to the load.

[0041] The designated gating signal UNBLANK can be driven by the pulse synchronization switch unit 80 to synchronously provide the driving gating signal UNBLANK_DRV to the nonlinear correction unit 10, the RF preamplifier and driving amplifier unit 40 and the RF power amplifier unit 60, so that the system input signal RF_IN maintains time correlation within the gating signal UNBLANK to achieve RF pulse modulation.

[0042] The filter unit 30 is connected in series in the closed loop formed above, and can be placed at any position in the closed loop according to actual conditions. Figure 1 In the figure, the example of connecting the filter unit 30 in series between the nonlinear correction unit 10 and the RF pre-amplifier and driving amplifier unit 40 is used for illustration. Further, there may be multiple filter units 30, which are connected in series at different positions in the closed loop to further improve the stabilization effect.

[0043] Two conditions need to be met to generate oscillations, namely amplitude balance and phase balance. The amplitude balance condition is that |AF| is equal to 1, and |AF| is the sum of the closed-loop forward and feedback gains. The phase balance condition is that φA+φF is equal to (2n+1)*π, where n is a positive number starting from 0, A is the total gain of the amplifier, F is the feedback gain of the amplifier, φA is the phase of the cascade gain, and φF is the phase of the feedback gain. As long as one of the conditions is not met, no oscillation will occur. The stability can be improved by destroying the amplitude balance condition of the self-excited oscillation, so that the cascade gain A remains unchanged in the preset working frequency band, and the cascade gain A is reduced in the low frequency band in the potential unstable range, thereby destroying the conditions for self-excited oscillation and avoiding the generation of self-excitation. Specifically, the signal components below the preset working frequency band can be filtered out by the filtering unit 30 to achieve the gain of the preset working frequency band unchanged, while the gain of the low frequency band is reduced.

[0044] Among them, optional, such as Figure 2 As shown, the filter unit 30 includes a first filter 310 and a second filter 320; the first filter 310 is connected in series between the nonlinear correction unit 10 and the RF preamplifier and driving amplifier unit 40, and the second filter 320 is connected in series between the RF preamplifier and driving amplifier unit 40 and the RF power amplifier unit 60. Further optionally, the filter unit 30 is a high-pass filter or a band-pass filter, wherein a high-pass filter is preferably used to further save costs. Specifically, the corrected signal RF_IN2 output by the nonlinear correction unit 10 can be first filtered by the first filter 310, and then the signal RF_IN4 filtered by the first filter 310 is provided to the RF preamplifier and driving amplifier unit 40, and then the initial amplified signal RF_DRV output by the RF preamplifier and driving amplifier unit 40 can be first filtered by the second filter 320, and then the signal RF_DRV1 filtered by the second filter 320 is provided to the RF power amplifier unit 60. The values ​​of the first filter 310 and the second filter 320 may be the same or different. The packages and models may be different according to the power difference at different positions. Specifically, a π-type structure or a T-type structure may be used to implement a high-pass filter.

[0045] On the basis of the above technical solution, optional, such as Figure 3 As shown, the system further includes an attenuator 90 and a phase shifter 20; the attenuator 90 is connected in series between the system input signal and the nonlinear correction unit 10 for gain compensation; the phase shifter 20 is connected in series between the system input signal and the nonlinear correction unit 10, or between the nonlinear correction unit 10 and the RF preamplifier and driver amplifier unit 40 ( Figure 3 ), which is used for phase compensation.

[0046] Specifically, for high-power RF amplifiers, not only good linearity and stable output are required, but also the consistency of the whole machine composed of the RF power amplifier system. The nonlinear correction technology realizes the linearization of the RF power amplifier, but due to the difference in the nonlinear correction window range between different whole machines and the tolerance of their system components, the gain tolerance between different whole machines is large and the tolerance range of the initial phase is large. Therefore, achieving consistency between different whole machines of the same product of the RF power amplifier system is also a problem that needs to be solved. On the basis of the above technical solution, by setting a phase shifter 20, in the whole machine system test, the tolerance of the initial phase of different whole machines can be fine-tuned. Specifically, leading or lagging phase shifting can be used, and passive π-type structure or T-type structure can be selected, and the insertion loss of the working frequency band is very small. It is also possible to set an attenuator 90 to fine-tune the tolerance of the gain of different whole machines in the whole machine system test. Specifically, a π-type structure or T-type structure composed of resistors can be selected. Therefore, on the basis of nonlinear correction, the gain error and phase error between different hosts of each RF power amplifier system can be reduced through gain compensation of the attenuator 90 and phase compensation of the phase shifter 20, thereby achieving the consistency of gain and phase between the hosts of the pulse modulated RF power amplifier system. Figure 3 Taking the structure as an example, the system input signal RF_IN can be first processed by the attenuator 90, and then the signal RF_IN1 processed by the attenuator 90 is provided to the nonlinear correction unit 10, and then the corrected signal RF_IN2 output by the nonlinear correction unit 10 can be first processed by the phase shifter 20, and then the signal RF_IN3 processed by the phase shifter 20 is provided to the first filter 310.

[0047] On the basis of the above technical solution, optionally, the RF power amplification unit includes a power divider, a power synthesizer and a plurality of identical RF power amplifiers; the power divider is used to input the same power to each of the RF power amplifiers based on the initial amplified signal, and the power synthesizer is used to synthesize the power output by each of the RF power amplifiers to obtain the amplified signal; the RF power amplifier includes an RF transistor, input matching, output matching, a bias circuit and power supply decoupling; the input signal of the RF power amplifier is provided to the RF transistor through the input matching, and the output signal of the RF power amplifier is generated through the output matching; the bias circuit is used to provide a gate bias voltage for the RF transistor; the power supply decoupling is used to provide a power supply voltage for the RF transistor and perform decoupling.

[0048] Further optional, such as Figure 4As shown, in a single-channel RF power amplifier 600, the RF transistor Q1 includes a first transistor and a second transistor that are identical and have their sources connected (grounded); the bias circuit includes a first bias circuit and a second bias circuit that are identical (such as Figure 4 The two bias circuits 604 in the embodiment include a stabilizing resistor, a stabilizing inductor, a working frequency band decoupling capacitor, a high frequency band decoupling capacitor, a current limiting resistor, a single-pole double-throw switch (SPDT), a low frequency band decoupling capacitor and a discharge resistor; wherein the single-pole double-throw switch is selected based on the driving gating signal UNBLANK_DRV provided by the pulse synchronous switch unit 80; in the first bias circuit, the output end of the single-pole double-throw switch U1A is connected to the gate of the first transistor through the current limiting resistor R2A, the stabilizing inductor L2A and the stabilizing resistor R1A; one end of the working frequency band decoupling capacitor C2A is connected between the current limiting resistor R2A and the stabilizing inductor L2A, and the other end is grounded; one end of the high frequency band decoupling capacitor C3A is connected between the current limiting resistor R2A and the stabilizing inductor L2A, and the other end is grounded; one input end of the single-pole double-throw switch U1A is connected to the bias voltage Gate1_Bias, and the other end is connected to the bias voltage Gate1_Bias. One input end is grounded through a discharge resistor R3A; one end of the low-frequency band decoupling capacitor is connected between the bias voltage Gate1_Bias and the single-pole double-throw switch U1A, and the other end is grounded; in the second bias circuit, the output end of the single-pole double-throw switch U1B is connected to the gate of the second transistor through a current limiting resistor R2B, a stabilizing inductor L2B and a stabilizing resistor R1B; one end of the working frequency band decoupling capacitor C2B is connected between the current limiting resistor R2B and the stabilizing inductor L2B, and the other end is grounded; one end of the high-frequency band decoupling capacitor C3B is connected between the current limiting resistor R2B and the stabilizing inductor L2B, and the other end is grounded; one input end of the single-pole double-throw switch U1B is connected to the bias voltage Gate1_Bias, and the other input end is grounded through a discharge resistor R3B; one end of the low-frequency band decoupling capacitor is connected between the bias voltage Gate1_Bias and the single-pole double-throw switch U1B, and the other end is grounded. The current limiting resistor protects the single-pole double-throw switch and prevents the single-pole double-throw switch from being damaged by an instantaneous excessive current. The stabilization time of the rising edge of the transistor gate pulse is related to the corresponding stabilization resistor, current limiting resistor, working frequency band decoupling capacitor and high frequency band decoupling capacitor; the discharge time of the falling edge of the transistor gate pulse is related to the corresponding stabilization resistor, current limiting resistor, discharge resistor, working frequency band decoupling capacitor and high frequency band decoupling capacitor; it is determined by the RC integral time constant composed of the total resistance R and the total capacitance C. The first transistor and the second transistor can be two transistors of the same model in a package, so that the wafer processing batches of the two transistors are the same, the discreteness is minimized, and the consistency is better.

[0049] Among them, the low-frequency decoupling capacitors in the bias circuit can be multiple (such as Figure 4In the embodiment, C4A, C5A, C6A in the first bias circuit, and C4B, C5B, C6B in the second bias circuit) can be used to decouple the frequency band lower than the preset working frequency band to improve the stability of the lower frequency band. Correspondingly, the working frequency band decoupling capacitor is used to decouple the preset working frequency band, and the high frequency band decoupling capacitor is used to decouple the frequency band higher than the preset working frequency band. By optimizing the design of the bias circuit, the value of the stabilizing capacitor can be reduced, and part of the decoupling capacitor is set outside the control switch of the gate, which can improve the transient response time of the transistor gate controlled by the pulse synchronous switch unit 80. The two bias circuit peripheries can be connected to the bias voltage Gate1_Bias together, and the two identical devices can be arranged near the gate of the RF transistor Q1, thereby reducing the influence of the PCB routing on the gate. By using a two-way switch, the driving capability of the gate is improved and the transient response time is improved. By driving the gate control signal UNBLANK_DRV, the gate bias voltage can be provided during the working time, and rapid discharge is performed at other times to ensure that the gate voltage is quickly reduced and the RF transistor Q1 is turned off, thereby achieving the transient response time of the transistor gate voltage, that is, fast rise time, fast fall time, and stabilization time, thereby achieving sub-microsecond stabilization time in transistor pulse modulation.

[0050] Further optional, such as Figure 4 As shown, the input matching 601 includes an input Balun, a first transmission line TLiin1A, a first capacitor C1A, a second transmission line TLiin2A, a third transmission line TLiin3A, a first inductor L1A, a fourth transmission line TLiin1B, a second capacitor C1B, a fifth transmission line Tlin2B, a sixth transmission line TLiin3B, a second inductor L1B and a third capacitor C7 (balance capacitor); the input Balun is used to convert the input signal RF_DRV_A of the RF power amplifier 600 into two equal-power anti-phase signals (with the same amplitude and a phase difference of 180 degrees) through a Balun transformation, one of which passes through the first transmission line TL in1A, the first capacitor C1A, the second transmission line TLiin2A, the third transmission line TLiin3A and the first inductor L1A are connected to the gate of the first transistor, and another signal is connected to the gate of the second transistor through the fourth transmission line TLiin1B, the second capacitor C1B, the fifth transmission line Tlin2B, the sixth transmission line TLiin3B and the second inductor L1B in sequence; one end of the third capacitor C7 is connected between the second transmission line TLiin2A and the third transmission line TLiin3A, and the other end is connected between the fifth transmission line Tlin2B and the sixth transmission line TLiin3B. Among them, the first capacitor C1A and the second capacitor C1B are equal in value and are used for matching and DC isolation, and the first inductor L1A and the second inductor L1B are equal in value.

[0051] Further optional, such as Figure 4 As shown, the output matching 602 includes the seventh transmission line TLout1A, the eighth transmission line TLout2A, the ninth transmission line TLout1B, the tenth transmission line TLout2B, the output Balun and the fourth capacitor C8 (balance capacitor); the drain of the first transistor is connected to the output Balun through the seventh transmission line TLout1A and the eighth transmission line TLout2A in sequence, and the drain of the second transistor is connected to the output Balun through the ninth transmission line TLout1B and the tenth transmission line TLout2B in sequence, and the output Balun is used to generate the output signal RF_PA1 of the RF power amplifier 600 according to the two received signals; one end of the fourth capacitor C8 is connected between the seventh transmission line TLout1A and the eighth transmission line TLout2A, and the other end is connected between the ninth transmission line TLout1B and the tenth transmission line TLout2B. Further, the signal RF_PA1 output by the output Balun can also be used as the final output signal RF_OUT1 of the RF power amplifier 600 after being isolated from DC by capacitor C9.

[0052] Further optional, such as Figure 4 As shown, the power decoupling 603 includes a choke inductor L8 and a plurality of decoupling capacitors of different frequency bands; the output end of the output matching 602 is connected to the power supply VCC_PA through the choke inductor L8; and each decoupling capacitor is connected between the choke inductor L8 and the power supply VCC_PA at one end and grounded at the other end. Figure 4 As shown, the decoupling capacitors may include C14, C12, and C11 from high frequency to low frequency.

[0053] For example, taking a 5T non-magnetic RF power amplifier system as an example, a field effect tube of model MRFX1K80N is used as the final stage RF power amplifier, and 4 identical parts constitute 4-way amplification. The gate drive of the 4-way RF power amplifier unit is connected to different gate voltages to achieve the same static current of the 4-way transistor, with a 1-to-4 power divider at the front stage and a 4-to-1 power synthesizer at the back stage. In order to be suitable for the pulse modulation of magnetic resonance, in the design of the RF power amplifier, according to the field effect tube model of MRFX1K80N, the load pulling of the input and output is performed for matching. The single-pole double-throw switch uses ADG1419, ADG1459, and ADG849 to achieve a pulse edge stabilization time of 40nS-300nS. For the amplitude of each signal, when the RF_IN frequency is 210.67MHz at 0dBm, RF_IN2 and RF_IN4 are near 0dBm, and after the driven and amplified signal, RF_DRV is about 48dBm, and the output RF_OUT is about 69.1dBm, achieving an output of 8KW, and RF_Couple is about 9dB.

[0054] The feasibility of the above solution is verified through simulation design, testing, aging, etc. Specifically, taking the above example as an example, Figure 5 This is the field-circuit co-simulation of the PCB layout corresponding to the final single-channel RF power amplifier. From the layout, we can see the input Balun and output Balun composed of PCB microstrip lines. Figure 6 for Figure 5 The corresponding simulation results show that when the power supply DC voltage is 75V, the output power of the single-channel RF power amplifier is 63.9dBm (2454W). In the open-loop debugging, the attenuator 90 and the phase shifter 20 are replaced with a copper-clad straight-through with a characteristic impedance of 50 ohms in advance, and the reserved first filter 310 and the second filter 320 are replaced with a copper-clad straight-through with a characteristic impedance of 50 ohms in advance. From RF_IN2 to RF_OUT, an open-loop gain test is performed, and the test results are as follows: Figure 7 As shown, it is the result of the open-loop pulse test of the entire system, with an output power of 69.11dBm (8147W). In the closed-loop test of the entire machine, without the first filter 310 and the second filter 320, when the output power is 8KW, the performance is good for a narrow pulse period of 200mS and a pulse width of 1mS. However, it is found that when the wide pulse period is 200mS and the pulse width is 8mS, and it is triggered continuously and quickly for many times, and the water-cooled plate is not added with water cooling, self-excited oscillation occasionally occurs. By sampling and analyzing the RF_Couple signal at the coupling end and filtering the operating frequency 210.78MHz, it is found that the frequency of the modulated self-excited oscillation is in the low frequency band, such as Figure 8 As shown in the figure, it is a waveform of the time domain power envelope acquisition of a single pulse on a spectrum analyzer, such as Fig. 9As shown in the figure, it is the waveform collected by the oscilloscope at another time. In the peak high power, self-excited oscillation occasionally occurs during long pulses. The time domain waveform of self-excited oscillation is analyzed and it mainly appears in the range of 1MHz-30MHz. According to the stability coefficient u of the simulation, it is found in the simulation that the low frequency band 100KHz-40MHz is in the stable area, but the value of the stability coefficient u is low and the stability margin is not large. Fig.10 As shown in the figure. The model for transistor stability simulation is a small signal model. When working at P1db high power, the nonlinearity of the transistor is enhanced, and the stability becomes worse due to the combined effects of thermal effect and memory effect, and occasionally low-frequency self-excitation occurs. Fig.11 As shown, the filter unit 30 uses a π-type structure, and the circuit of the filter unit 30 is simulated. The simulation results are shown in FIG. Fig.12 As shown, it can be seen that below 40MHz, the attenuation is greater than 30dB, and the attenuation is less than 0.1dB in the working frequency band of 210MHz. After the whole machine is closed-loop, a gain closed-loop test is performed to test RF_IN and RF_OUT, and compare the two cases of adding the first filter 310 and the second filter 320 and not adding the first filter 310 and the second filter 320. The results are as follows: Fig.13 and Fig.14 As shown. The working frequency is 210.78MHz, the highest spurious in the low-frequency band without a filter is -26.7dBm, and the spurious in the low-frequency band with a filter is lower than -60dBm, so the gain of the low-frequency band is reduced, the oscillation condition is destroyed, and the occurrence of self-excitation is avoided, so that the entire power amplifier system works more stably and reliably. Afterwards, tests of different pulse times, multiple continuous triggers, aging, and different machines have verified the described effect.

[0055] refer to Figure 3 , through 90 pairs of gain consistency compensation by attenuators and 20 pairs of phase consistency compensation by phase shifters, in the pulse modulation test of 4 complete machines, 2 channels in each complete machine, a total of 8 channels, the carrier is 210.78MHz, 8mS pulse width and 200mS period, each channel achieved an input of 1mW and an output greater than 8KW. Under this pulse condition, the power scan is from -40dBm to 0dBm, the S21 amplitude is 69.5dB and the fluctuation is less than 0.7dB, the S21 phase is -110 degrees and the fluctuation is less than 5 degrees, achieving gain consistency and phase consistency between the complete machines.

[0056] The above example takes the 5T frequency band of magnetic resonance as an example. The pulse modulated RF power amplifier system provided in this embodiment can also be applied to other similar fields, such as 3T, 7T frequency bands, etc., and can be applied to high-power RF amplifiers such as wireless communications and radars.

[0057] The pulse modulated RF power amplifier system provided in the embodiment of the present invention includes a nonlinear correction unit, an RF pre-amplifier and drive amplifier unit, an RF power amplifier unit, a directional coupler, a pulse synchronous switch unit and a filter unit. By using the filter unit to filter out the signal components below the preset working frequency band in the closed loop formed, the gain of the low frequency band is reduced, thereby destroying the conditions for self-excited oscillation, avoiding the generation of self-excitation, solving the potential instability problem of the low frequency band in the RF power amplifier system, so that the entire system can work more stably and reliably, and at the same time, pulse modulation with a sub-microsecond stabilization time can be achieved, and the problem can be solved in a low-cost and simple manner only by using the filtering function reasonably.

[0058] Embodiment 2

[0059] Embodiment 2 of the present invention provides a magnetic resonance system, which includes the pulse-modulated radio frequency power amplifier system provided by any embodiment of the present invention, and has its corresponding beneficial effects.

[0060] Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A pulse modulated radio frequency power amplifier system, characterized in that: include: Nonlinear correction unit, RF pre-amplifier and driver amplifier unit, RF power amplifier unit, directional coupler, pulse synchronous switch unit and filter unit; wherein, The nonlinear correction unit is used to perform linearity correction according to the system input signal and the coupling signal fed back by the directional coupler, and provide the corrected signal to the RF preamplifier and driver amplifier unit; the RF preamplifier and driver amplifier unit is used to linearly amplify the corrected signal, and provide the obtained initial amplified signal to the RF power amplifier unit; the RF power amplifier unit is used to power amplify the initial amplified signal, and provide the obtained amplified signal to the directional coupler; the directional coupler is used to generate the coupling signal and the system output signal according to the amplified signal; the nonlinear correction unit, the RF preamplifier and driver amplifier unit, the RF power amplifier unit and the directional coupler form a closed loop; The pulse synchronous switch unit is used to drive the input gate signal to synchronously provide it to the nonlinear correction unit, the RF preamplifier and driving amplifier unit and the RF power amplifier unit; The filtering unit is connected in series in the closed loop and is used to filter out signal components below a preset working frequency band.

2. The pulse modulated RF power amplifier system according to claim 1, characterized in that: The filtering unit includes a first filter and a second filter; the first filter is connected in series between the nonlinear correction unit and the RF preamplifier and driving amplifier unit, and the second filter is connected in series between the RF preamplifier and driving amplifier unit and the RF power amplifier unit.

3. The pulse modulated RF power amplifier system according to claim 1 or 2, characterized in that: The filtering unit is a high-pass filter or a band-pass filter.

4. The pulse modulated RF power amplifier system according to claim 1, characterized in that: The system also includes an attenuator and a phase shifter; the attenuator is connected in series between the system input signal and the nonlinear correction unit for gain compensation; the phase shifter is connected in series between the system input signal and the nonlinear correction unit, or between the nonlinear correction unit and the RF preamplifier and driving amplifier unit for phase compensation.

5. The pulse modulated RF power amplifier system according to claim 1, characterized in that: The RF power amplification unit includes a power divider, a power synthesizer, and a plurality of identical RF power amplifiers; the power divider is used to input the same power to each of the RF power amplifiers based on the initial amplified signal, and the power synthesizer is used to synthesize the power output by each of the RF power amplifiers to obtain the amplified signal; The RF power amplifier includes an RF transistor, input matching, output matching, a bias circuit and power decoupling; the input signal of the RF power amplifier is provided to the RF transistor through the input matching, and the output signal of the RF power amplifier is generated through the output matching; the bias circuit is used to provide a gate bias voltage for the RF transistor; the power decoupling is used to provide a power supply voltage for the RF transistor and perform decoupling.

6. The pulse modulated RF power amplifier system according to claim 5, characterized in that: The radio frequency transistor includes a first transistor and a second transistor that are identical and connected in source; the bias circuit includes a first bias circuit and a second bias circuit that are identical, both of which include a stabilizing resistor, a stabilizing inductor, a working frequency band decoupling capacitor, a high frequency band decoupling capacitor, a current limiting resistor, a single-pole double-throw switch, a low frequency band decoupling capacitor, and a discharge resistor; wherein the single-pole double-throw switch is gated based on a driving gating signal provided by the pulse synchronous switch unit; In the first bias circuit, the output end of the single-pole double-throw switch is connected to the gate of the first transistor through a current limiting resistor, a stabilizing inductor and a stabilizing resistor; one end of the working frequency band decoupling capacitor is connected between the current limiting resistor and the stabilizing inductor, and the other end is grounded; one end of the high frequency band decoupling capacitor is connected between the current limiting resistor and the stabilizing inductor, and the other end is grounded; one input end of the single-pole double-throw switch is connected to the bias voltage, and the other input end is grounded through a discharge resistor; one end of the low frequency band decoupling capacitor is connected between the bias voltage and the single-pole double-throw switch, and the other end is grounded; In the second bias circuit, the output end of the single-pole double-throw switch is connected to the gate of the second transistor through a current limiting resistor, a stabilizing inductor and a stabilizing resistor; one end of the working frequency band decoupling capacitor is connected between the current limiting resistor and the stabilizing inductor, and the other end is grounded; one end of the high frequency band decoupling capacitor is connected between the current limiting resistor and the stabilizing inductor, and the other end is grounded; one input end of the single-pole double-throw switch is connected to the bias voltage, and the other input end is grounded through a discharge resistor; one end of the low frequency band decoupling capacitor is connected between the bias voltage and the single-pole double-throw switch, and the other end is grounded.

7. The pulse modulated RF power amplifier system according to claim 6, characterized in that: The input matching includes an input Balun, a first transmission line, a first capacitor, a second transmission line, a third transmission line, a first inductor, a fourth transmission line, a second capacitor, a fifth transmission line, a sixth transmission line, a second inductor and a third capacitor; the input Balun is used to convert the input signal of the RF power amplifier into two equal-power inverting signals through a Balun transformation, wherein one signal is connected to the gate of the first transistor through the first transmission line, the first capacitor, the second transmission line, the third transmission line and the first inductor in sequence, and the other signal is connected to the gate of the second transistor through the fourth transmission line, the second capacitor, the fifth transmission line, the sixth transmission line and the second inductor in sequence; one end of the third capacitor is connected between the second transmission line and the third transmission line, and the other end is connected between the fifth transmission line and the sixth transmission line.

8. The pulse modulated RF power amplifier system according to claim 6, characterized in that: The output matching includes a seventh transmission line, an eighth transmission line, a ninth transmission line, a tenth transmission line, an output Balun and a fourth capacitor; the drain of the first transistor is connected to the output Balun via the seventh transmission line and the eighth transmission line in sequence, and the drain of the second transistor is connected to the output Balun via the ninth transmission line and the tenth transmission line in sequence, and the output Balun is used to generate an output signal of the RF power amplifier according to the two received signals; one end of the fourth capacitor is connected between the seventh transmission line and the eighth transmission line, and the other end is connected between the ninth transmission line and the tenth transmission line.

9. The pulse modulated RF power amplifier system according to claim 5, characterized in that: The power supply decoupling includes a choke inductor and a plurality of decoupling capacitors of different frequency bands; the output end of the output matching is connected to the power supply through the choke inductor; one end of each decoupling capacitor is connected between the choke inductor and the power supply, and the other end is grounded.

10. A magnetic resonance system, characterized in that: A pulse modulated radio frequency power amplifier system comprising any one of claims 1-9.

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