Directional coupler circuit, circuit board and radio frequency measuring equipment

Abstract

The embodiment of the invention provides a directional coupler circuit, a circuit board and radio frequency measurement equipment, relates to the technical field of electronic circuits, and solves the problem that the radio frequency measurement equipment is difficult to adapt to low-frequency and high-frequency signals in the prior art. According to the directional coupler circuit, the working frequency range of the directional coupler circuit is expanded, a high and low frequency separation mode is adopted, high and low frequencies are separately amplified and converged at a third port, and each port is a port to the ground, so that signal extraction can be facilitated, and the directional coupler circuit can have better passband flatness and high-frequency characteristics.

Description

Technical Field

[0001] The present application relates to the field of electronic technology, and in particular to a directional coupler circuit, a circuit board and a radio frequency measurement device. Background Art

[0002] RF measurement equipment in the field of RF measurement, such as network analyzers and signal source analyzers, all have a core functional unit, namely the directional coupler. The function of the directional coupler is to couple only the reverse wave signal and isolate the forward wave signal, thereby playing the role of measuring the reflected wave energy.

[0003] The two commonly used directional coupler structures in the related art are transmission line coupler and bridge coupler. Among them, the transmission line coupler utilizes the transmission characteristics of signal waves, so its signal bandwidth is related to the coupling line length L, but its effective bandwidth is invalid at points close to DC or close to twice the center frequency. As for the bridge coupler, the bridge coupler can pass DC, but because one of the ports is not a port to the ground, the signal cannot be directly extracted and can only be converted through a differential amplifier or balun (a type of transformer structure). However, the bandwidth of the differential amplifier is generally below 10GHz, and the noise is relatively large, while the low frequency of the balun is generally not lower than 50MHz, and it is also impossible to measure high and low frequencies at the same time. In this regard, the RF measurement equipment in the related art has great limitations and is difficult to adapt to low-frequency and high-frequency signals. Summary of the invention

[0004] The present application provides a directional coupler circuit, a circuit board and a radio frequency measuring device, which solves the problem in the related art that the radio frequency measuring device is difficult to adapt to low-frequency and high-frequency signals. The operating frequency range of the directional coupler circuit of the present solution includes the frequency range from direct current to high frequency, and has better passband flatness and high-frequency characteristics.

[0005] In a first aspect, the present application provides a directional coupler circuit, which is provided with a grounded first port, a second port and a third port, and the directional coupler circuit includes a signal access module, a radio frequency amplification module, a low-frequency amplification module and a signal output module.

[0006] The first access end of the signal access module is connected to the first port through the first resistor, and the first access end of the signal access module is also connected to the second port, the second access end of the signal access module is connected to the first port through the second resistor, and the second access end of the signal access module is also grounded through the third resistor, and the signal access module is used to provide input impedance, and to separate the DC and low-frequency signals and high-frequency signals in the connected signal to be measured;

[0007] The input end of the radio frequency amplification module is connected to the high frequency output end of the signal access module, and the radio frequency amplification module is used to amplify the high frequency signal;

[0008] The first input end of the low-frequency amplification module is connected to the second input end of the signal access module, the second input end of the low-frequency amplification module is connected to the impedance output end of the signal access module, the low-frequency amplification module is used to amplify direct current and low-frequency signals, and the amplification factor of the low-frequency amplification module matches the amplification factor of the radio frequency amplification module;

[0009] The first input end of the signal output module is connected to the output end of the low-frequency amplification module, the second input end of the signal output module is connected to the output end of the radio frequency amplification module, and the output end of the signal output module is connected to the third port.

[0010] In a second aspect, the present application further provides a circuit board, which includes the directional coupler circuit as described in the first aspect.

[0011] In a third aspect, the present application further provides a radio frequency measuring device, which includes the circuit board as described in the second aspect.

[0012] The directional coupler circuit of the present application can expand its operating frequency range by passing direct current, low-frequency and high-frequency signals, and adopts a high-low frequency separation method to amplify the high and low frequencies separately and merge them at the third port. Moreover, each port is a port to the ground, which can help to extract the signal, so that the directional coupler circuit can have better passband flatness and high-frequency characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 A schematic diagram of a transmission line coupler provided for related art;

[0014] Figure 2 A schematic diagram of a bridge coupler provided for related art;

[0015] Figure 3 A principle block diagram of a directional coupler circuit provided in one embodiment of the present application;

[0016] Figure 4 An equivalent circuit diagram of a signal access module provided in one embodiment of the present application;

[0017] Figure 5 A schematic diagram of the circuit structure of a directional coupler circuit provided in one embodiment of the present application. DETAILED DESCRIPTION

[0018] The embodiments of the present application are further described in detail below in conjunction with the accompanying drawings and examples. It is to be understood that the specific embodiments described herein are only used to explain the embodiments of the present application, rather than to limit the embodiments of the present application. It should also be noted that, for ease of description, only the parts related to the embodiments of the present application rather than all structures are shown in the accompanying drawings, and those skilled in the art should be able to think of it after reading the specification of this application that as long as the technical features do not contradict each other, any combination of the technical features can constitute an optional implementation method.

[0019] The terms "first", "second", etc. in the specification and claims of the present application are used to distinguish similar objects, and are not used to describe a specific order or sequence. It should be understood that the data used in this way can be interchangeable when appropriate, so that the embodiments of the present application can be implemented in an order other than those illustrated or described here, and the objects distinguished by "first", "second", etc. are generally a class, and the number of objects is not limited. For example, the first object can be one or more. In addition, "and / or" in the specification and claims represents at least one of the connected objects, and the character " / " generally represents that the objects associated before and after are in an "or" relationship. In the description of the present application, "multiple" means two or more, and "several" means one or more.

[0020] As the core functional unit of RF measurement equipment in the field of RF measurement such as network analyzers and signal source analyzers, the directional coupler only couples the reverse wave signal and isolates the forward wave signal, thereby measuring the reflected wave energy.

[0021] In the related art, a transmission line coupler or a bridge coupler is usually used as a directional coupler. Figure 1 and Figure 2 ,in, Figure 1 A schematic diagram of a transmission line coupler provided for related art, Figure 2 Schematic diagram of a bridge coupler provided for related technology. It is understandable that the transmission line coupler is designed using the wave characteristics of the radio frequency signal. Because the characteristics of the waves on the transmission line determine that the adjacent transmission line can only receive signals at the near signal source end and cannot receive signals at the far signal source end, port P3 is isolated from port P1 and can only couple to the signal transmitted by port P2. The bridge coupler adopts the bridge balance principle. When the resistors R1', R2', R3', and R4' are all equal to the target characteristic impedance, the voltage on the resistor R3', that is, the voltage Vr3 of port P3, is only related to the signal transmitted from port P2, and has nothing to do with the signal transmitted from port P1.

[0022] It can be seen that the transmission line coupler uses the transmission characteristics of signal waves, so its signal bandwidth is related to the length of the coupling line L. For example, if the propagation delay of the signal of this length is t, then its effective bandwidth is effective within the frequency range at one end with a center frequency of 1 / (4t). It is invalid at points close to DC or close to twice the center frequency. The bridge coupler can pass DC, but because its port P3 is not a port to the ground, the signal cannot be directly extracted and can only be converted through a differential amplifier or balun. However, the bandwidth of the differential amplifier is generally below 10GHz and the noise is relatively large. The low frequency of the balun is generally not lower than 50MHz, and it is impossible to measure high and low frequencies at the same time.

[0023] In this regard, the present application provides a directional coupler circuit, which can be applicable to DC signals, so that its operating frequency range can be from DC to high frequency. Figure 3 A schematic diagram of a directional coupler circuit according to an embodiment of the present application is shown in FIG. Figure 3 As shown, the directional coupler circuit is provided with a grounded first port, a second port and a third port, and the directional coupler circuit includes a signal access module 101, a radio frequency amplification module 102, a low frequency amplification module 103 and a signal output module 104.

[0024] The first port, the second port and the third port are all grounded. The first access end of the signal access module 101 is connected to the first port through the first resistor R1, and the first access end of the signal access module 101 is also connected to the second port, the second access end of the signal access module 101 is connected to the first port through the second resistor R2, and the second access end of the signal access module 101 is also grounded through the third resistor R3.

[0025] The input end of the RF amplification module 102 is connected to the high frequency output end of the signal access module 101. The first input end of the low frequency amplification module 103 is connected to the second input end of the signal access module 101, and the second input end of the low frequency amplification module 103 is connected to the impedance output end of the signal access module 101.

[0026] The first input end of the signal output module 104 is connected to the output end of the low frequency amplification module 103 , the second input end of the signal output module 104 is connected to the output end of the radio frequency amplification module 102 , and the output end of the signal output module 104 is connected to the third port.

[0027] It can be understood that the signal access module 101 can provide input impedance and be used to separate the DC and low-frequency signals and high-frequency signals in the connected signal to be measured. Then, the high-frequency signal enters the RF amplification module 102, and the RF amplification module 102 amplifies the high-frequency signal; while the DC and low-frequency signals enter the low-frequency amplification module 103, and the low-frequency amplification module 103 amplifies the DC and low-frequency signals. It is worth noting that the amplification factor of the low-frequency amplification module 103 matches the amplification factor of the RF amplification module 101, that is, the amplification factors of the two are equal, and then the DC and low-frequency signals and high-frequency signals are amplified by the same factor.

[0028] It can be seen that the directional coupler circuit of the present application can expand its operating frequency range through DC, low-frequency and high-frequency signals, and adopts a high-low frequency separation method to amplify the high and low frequencies separately and merge them at the third port, and each port is a port to the ground, which can help to extract the signal, so that the directional coupler circuit can have better passband flatness and high-frequency characteristics.

[0029] In one embodiment, the signal access module includes a first capacitor, a fourth resistor and a first transformer. Specifically, the first end of the first capacitor is connected to the second resistor and the third resistor, and the second end of the first capacitor is connected to the first input end of the first transformer. The first end of the fourth resistor is connected to the first end of the first capacitor, and the second end of the fourth resistor is connected to the second end of the first capacitor, that is, the fourth resistor is connected in parallel to the first capacitor, and the first end of the fourth resistor serves as the second access end of the signal access module, and the second end of the fourth resistor serves as the impedance output end of the signal access module. The resistance value of the fourth resistor matches the equivalent input resistance of the RF amplification module, so that the input impedance of the directional coupler circuit can be matched.

[0030] In addition, the second input end of the first transformer is connected to the first resistor, the first output end of the first transformer is grounded, the second input end of the first transformer serves as the first access end of the signal access module, the second output end of the first transformer serves as the signal output end of the signal access module, and the equivalent inductance of the first transformer is equal to the product of the capacitance of the first capacitor and the square of the resistance of the fourth resistor, so that the signal access module can be further equivalent to a resistor with a fixed resistance (i.e., the fourth resistor).

[0031] It can be understood that after the first transformer is equivalent, the following can be obtained: Figure 4 The equivalent circuit diagram shown is Figure 4An equivalent circuit diagram of a signal access module provided in an embodiment of the present application, wherein the resistor R10 is an equivalent input resistor of the RF amplification module to the ground, which is equivalent to a resistor in parallel with the parasitic inductance of the first transformer. In the figure, the parasitic inductance L2 is connected in series with the first capacitor C1, the fourth resistor R4 is connected in parallel with the first capacitor C1, and the resistor R10 is connected in parallel with the parasitic inductance L2, and the resistance of the fourth resistor R4 is equal to that of the resistor R10, thereby making the input impedance of the circuit equivalent to the resistance of the fourth resistor R4, and making the signal access module equivalent to a fixed resistance relative to the signal to be measured, so as to better match the system and help to measure the signal more accurately.

[0032] In this regard, after the measured signal enters the signal access module, under the action of the first capacitor C1, the high-frequency signal will pass through the first capacitor C1, while the DC and low-frequency signals will enter the low-frequency amplification module through the fourth resistor R4. In addition, under the action of the parasitic inductance L2, the DC and low-frequency signals will pass through the parasitic inductance L2, while the high-frequency signal will enter the RF amplification module through the resistor R10. When the fourth resistor R4 and the tenth resistor R10 are equal, and the equivalent inductance of the first transformer is equal to the product of the capacitance of the first capacitor and the square of the resistance of the fourth resistor, after the measured signal enters the signal access module, it is equivalent to accessing a resistor with a fixed resistance. Among them, the resistance of the fourth resistor R4 and the tenth resistor R10 can be matched to the required equivalent resistance, such as the required equivalent resistance is 50Ω, and accordingly, the resistance of the fourth resistor R4 and the tenth resistor R10 are both 50Ω. Therefore, the signal access module can use the first capacitor and the first transformer to isolate the high-frequency signal outside the low-frequency amplification module and the DC and low-frequency signals outside the RF amplification module, thereby realizing the measurement of DC, low-frequency signals and high-frequency signals, which helps to expand the operating frequency range of the circuit.

[0033] It should be noted that, in one embodiment, the ratio of the input impedance to the output impedance of the first transformer is 1:1, such as using a 1:1 balun as a transformer in the signal access module to transmit the high-frequency signal to the RF amplification module.

[0034] In one embodiment, the low-frequency amplification module includes a fifth resistor, a sixth resistor, a seventh resistor, an eighth resistor, a ninth resistor and an operational amplifier. Specifically, the first end of the fifth resistor is connected to the second input end of the signal access module, the second end of the fifth resistor is connected to the first end of the sixth resistor, the second end of the fifth resistor is connected to the in-phase input end of the operational amplifier, and the second end of the sixth resistor is grounded. The first end of the seventh resistor is connected to the impedance output end of the signal access module, the second end of the seventh resistor is connected to the inverting input end of the operational amplifier, the first end of the eighth resistor is connected to the second end of the seventh resistor, the second end of the eighth resistor is connected to the output end of the operational amplifier, and the first end of the ninth resistor is connected to the output end of the operational amplifier.

[0035] Among them, the first end of the fifth resistor serves as the first input end of the low-frequency amplification module, the first end of the seventh resistor serves as the second input end of the low-frequency amplification module, the second end of the ninth resistor serves as the output end of the low-frequency amplification module, and the resistance value of the ninth resistor matches the equivalent output resistance of the RF amplification module.

[0036] It should be noted that, in one embodiment, the input impedance provided by the signal access module includes a fourth resistor. Accordingly, the fifth resistor and the seventh resistor in the low-frequency amplification module are much larger than the fourth resistor. For example, the ratio of the resistance values ​​of the fifth resistor to the fourth resistor and the ratio of the resistance values ​​of the seventh resistor to the fourth resistor are both greater than or equal to 100, that is, the resistance value of the fifth resistor and the resistance value of the seventh resistor are both 100 times or more of the resistance value of the fourth resistor.

[0037] In one embodiment, the signal output module includes a first inductor and a second capacitor. Specifically, the first end of the first inductor is connected to the output end of the low-frequency amplification module, the first end of the second capacitor is connected to the output end of the radio frequency amplification module, and the second end of the first inductor is connected to the second end of the second capacitor. The first inductor is used to conduct the amplified DC and low-frequency signals, the second capacitor is used to conduct the amplified high-frequency signals, and the reactance ratio corresponding to the first inductor and the second capacitor matches the reactance ratio corresponding to the signal access module.

[0038] It can be understood that, by utilizing the characteristics of the second capacitor and the first inductor, the signal output module can separate the connected signal again, wherein the signal passing through the first inductor retains the DC and low-frequency signal, while the signal passing through the second capacitor retains the high-frequency signal. For this, the output end of the low-frequency amplification module is connected with the first inductor, and the output end of the radio frequency amplification module is connected with the second capacitor, thereby realizing the access and output of the amplified signal. Therefore, through the signal output module, the front-end amplifier circuit can access the amplified DC and low-frequency signals and high-frequency signals, thereby providing the amplified signal to be measured for the subsequent circuit, achieving a better signal-to-noise ratio and voltage standing wave ratio.

[0039] Figure 5 A circuit structure diagram of a directional coupler circuit provided in one embodiment of the present application is shown in the figure. In one embodiment, port P1, port P2 and port P3 in the directional coupler circuit serve as the first port, the second port and the third port respectively, and are all grounded.

[0040] Port P1 is connected to port P2 through a first resistor R1, and port P1 is connected to a grounded third resistor R3 through a second resistor R2. The signal access module in the directional coupler circuit includes a first capacitor C1, a fourth resistor R4, and a first transformer B1. Specifically, a first end of the first capacitor C1 is connected to the second resistor R2 and the third resistor R3, and a second end of the first capacitor C1 is connected to a first input end of the first transformer B1. A first end of the fourth resistor R4 is connected to a first end of the first capacitor C1, and a second end of the fourth resistor R4 is connected to a second end of the first capacitor C1, that is, the fourth resistor R4 is connected in parallel to the first capacitor C1.

[0041] Moreover, the first end of the fourth resistor R4 serves as the second access end of the signal access module, and the second end of the fourth resistor R4 serves as the impedance output end of the signal access module. The resistance value of the fourth resistor R4 matches the equivalent input resistance of the RF amplification module, so that the input impedance of the directional coupler circuit can be matched.

[0042] In addition, the second input end of the first transformer B1 is connected to the first resistor R1, the first output end of the first transformer B1 is grounded, the second input end of the first transformer B1 serves as the first access end of the signal access module, and the second output end of the first transformer B1 serves as the signal output end of the signal access module.

[0043] The low-frequency amplification module includes a fifth resistor R5, a sixth resistor R6, a seventh resistor R7, an eighth resistor R8, a ninth resistor R9 and an operational amplifier U1. Specifically, a first end of the fifth resistor R5 is connected to a first end of the fourth resistor R4, a second end of the fifth resistor R5 is connected to a first end of the sixth resistor R6, a second end of the fifth resistor R5 is connected to a non-inverting input end of the operational amplifier U1, and a second end of the sixth resistor R6 is grounded.

[0044] The first end of the seventh resistor R7 is connected to the second end of the fourth resistor R4, the second end of the seventh resistor R7 is connected to the inverting input terminal of the operational amplifier U1, the first end of the eighth resistor R8 is connected to the second end of the seventh resistor R7, the second end of the eighth resistor R8 is connected to the output terminal of the operational amplifier U1, and the first end of the ninth resistor R9 is connected to the output terminal of the operational amplifier U1.

[0045] In addition, the RF amplification module is marked with a dotted frame in the figure, which equivalently represents the equivalent input resistance (ie, resistance R10) and the equivalent output resistance (ie, resistance R11) of the RF amplification module.

[0046] The signal output module includes a first inductor L1 and a second capacitor C2. Specifically, a first end of the first inductor L1 is connected to a second end of R9, a first end of the second capacitor C2 is connected to a resistor R11, and a second end of the first inductor L1 is connected to a second end of the second capacitor C2.

[0047] Among them, the fifth resistor R5 and the seventh resistor R7 are both much larger than the fourth resistor R4, that is, the resistance of the fifth resistor R5 and the seventh resistor R7 is 100 times or more than the resistance of the fourth resistor R4. The fourth resistor R4 is matched with the resistor R10, and the ninth resistor R9 is matched with the resistor R11. The equivalent inductance of the first transformer is equal to the product of the capacitance of the first capacitor and the square of the resistance of the fourth resistor. The reactance ratio corresponding to the first inductor L1 and the second capacitor C2 is the first reactance ratio, and the value of the first reactance ratio is the product of the resistance of the ninth resistor R9 and the equivalent output resistor (i.e., resistor R11) of the radio frequency amplifier module. Moreover, the equivalent reactance ratio corresponding to the first transformer and the first capacitor is the second reactance ratio, and the second reactance ratio is equal to the first reactance ratio.

[0048] It is understandable that, under the action of the first capacitor C1, the high-frequency signal will pass through the first capacitor C1, while the DC and low-frequency signals will enter the low-frequency amplification module through the fourth resistor R4. In addition, under the action of the parasitic inductance of the first transformer B1, the DC and low-frequency signals will pass through the first transformer B1, while the high-frequency signal will enter the RF amplification module through the resistor R10. The DC and low-frequency signals are further amplified by the differential amplifier circuit formed by the operational amplifier U1 and the corresponding resistors, as well as by the RF amplification module. Similarly, based on the characteristics of the first inductor L1 and the second capacitor C2, the output end of the low-frequency amplification module (i.e., the second end of the ninth resistor R9) is connected to the first inductor L1, and the output end of the RF amplification module (such as one end of the resistor R11 in the figure) is connected to the second capacitor C2, so as to achieve access and output of the amplified signal.

[0049] Therefore, the directional coupler circuit of the present application can replace the resistor between port P2 and port P3 in the bridge coupler by the first transformer, the first capacitor and the fourth resistor, thereby converting the third port from a differential port to a single-ended port to the ground, so that each port is a port to the ground, which can help to extract the signal, and the directional coupler circuit can have better passband flatness and high-frequency characteristics. In addition, the circuit adopts a high-low frequency separation method to amplify the high and low frequencies separately to converge at the third port, avoiding the DC signal from entering the RF amplification module and affecting the operation of the RF amplification module, so that the DC and low-frequency signals can enter the low-frequency amplification module, and then the operating frequency range of the circuit is expanded, so that it can achieve an operating frequency range from DC to high frequency.

[0050] It should be noted that in some embodiments, the RF amplification module can use a RF amplification circuit, such as an operational amplifier in-phase RF amplification circuit, a low-noise amplification circuit, etc. to amplify the high-frequency signal; in addition, the RF amplification module can also use a RF amplifier to amplify the high-frequency signal.

[0051] An embodiment of the present application also provides a circuit board, which includes a directional coupler circuit as provided in the above embodiment, so that the circuit board can separate and amplify DC, low-frequency and high-frequency signals, and connect each port to the ground, which can help to extract the signal, and merge the signal at the third port, so that the operating frequency range of the circuit is expanded, so that it can achieve an operating frequency range from DC to high frequency.

[0052] The embodiment of the present application also provides a radio frequency measuring device, which includes a circuit board as provided in the above embodiment, and the radio frequency measuring device can measure DC, low frequency and high frequency signals, and can also have better passband flatness and high frequency characteristics. Thus, the operating frequency range of the radio frequency measuring device is expanded, so that it can achieve an operating frequency range from DC to high frequency.

[0053] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the process, method, commodity or device including the elements.

[0054] Note that the above are only preferred embodiments of the present application and the technical principles used. Those skilled in the art will understand that the present application 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 application. Therefore, although the present application is described in more detail through the above embodiments, the present application is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present application, and the scope of the present application is determined by the scope of the appended claims.

Claims

1. A directional coupler circuit, characterized in that: The directional coupler circuit is provided with a first port, a second port and a third port which are grounded, and the directional coupler circuit comprises: A signal access module, wherein a first access end of the signal access module is connected to the first port via a first resistor, and the first access end of the signal access module is also connected to the second port, a second access end of the signal access module is connected to the first port via a second resistor, and the second access end of the signal access module is also grounded via a third resistor, and the signal access module is used to provide input impedance, and to separate direct current and low-frequency signals and high-frequency signals in the connected signal to be measured; A radio frequency amplification module, the input end of which is connected to the high frequency output end of the signal access module, and the radio frequency amplification module is used to amplify the high frequency signal; A low-frequency amplification module, wherein a first input end of the low-frequency amplification module is connected to a second input end of the signal access module, and a second input end of the low-frequency amplification module is connected to an impedance output end of the signal access module. The low-frequency amplification module is used to amplify the DC and low-frequency signals, and the amplification factor of the low-frequency amplification module matches the amplification factor of the radio frequency amplification module; A signal output module, wherein a first input end of the signal output module is connected to an output end of the low-frequency amplification module, a second input end of the signal output module is connected to an output end of the radio frequency amplification module, and an output end of the signal output module is connected to the third port.

2. The directional coupler circuit according to claim 1, characterized in that: The signal access module includes a first capacitor, a fourth resistor and a first transformer; The first end of the first capacitor is connected to the second resistor and the third resistor, the second end of the first capacitor is connected to the first input end of the first transformer, the first end of the fourth resistor is connected to the first end of the first capacitor, the second end of the fourth resistor is connected to the second end of the first capacitor, the first end of the fourth resistor serves as the second access end of the signal access module, the second end of the fourth resistor serves as the impedance output end of the signal access module, and the resistance value of the fourth resistor matches the equivalent input resistance of the radio frequency amplification module; The second input end of the first transformer is connected to the first resistor, the first output end of the first transformer is grounded, the second input end of the first transformer serves as the first access end of the signal access module, the second output end of the first transformer serves as the signal output end of the signal access module, and the equivalent inductance of the first transformer is equal to the product of the capacitance of the first capacitor and the square of the resistance of the fourth resistor.

3. The directional coupler circuit according to claim 2, characterized in that: The ratio of the input impedance to the output impedance of the first transformer is 1:

1.

4. The directional coupler circuit according to claim 1, characterized in that: The low-frequency amplification module includes a fifth resistor, a sixth resistor, a seventh resistor, an eighth resistor, a ninth resistor and an operational amplifier; The first end of the fifth resistor is connected to the second input end of the signal access module, the second end of the fifth resistor is connected to the first end of the sixth resistor, the second end of the fifth resistor is connected to the non-inverting input end of the operational amplifier, and the second end of the sixth resistor is grounded; The first end of the seventh resistor is connected to the impedance output end of the signal access module, the second end of the seventh resistor is connected to the inverting input end of the operational amplifier, the first end of the eighth resistor is connected to the second end of the seventh resistor, the second end of the eighth resistor is connected to the output end of the operational amplifier, and the first end of the ninth resistor is connected to the output end of the operational amplifier; Among them, the first end of the fifth resistor serves as the first input end of the low-frequency amplification module, the first end of the seventh resistor serves as the second input end of the low-frequency amplification module, the second end of the ninth resistor serves as the output end of the low-frequency amplification module, and the resistance value of the ninth resistor matches the equivalent output resistance of the RF amplification module.

5. The directional coupler circuit according to claim 4, characterized in that: When the input impedance provided by the signal access module includes a fourth resistor, a resistance ratio of the fifth resistor to the fourth resistor and a resistance ratio of the seventh resistor to the fourth resistor are both greater than or equal to 100.

6. The directional coupler circuit according to claim 1 or 2, characterized in that: The signal output module includes a first inductor and a second capacitor; The first end of the first inductor is connected to the output end of the low-frequency amplification module, the first end of the second capacitor is connected to the output end of the radio frequency amplification module, and the second end of the first inductor is connected to the second end of the second capacitor; Among them, the first inductor is used to conduct the amplified DC and low-frequency signals, the second capacitor is used to conduct the amplified high-frequency signals, and the reactance ratio corresponding to the first inductor and the second capacitor matches the reactance ratio corresponding to the signal access module.

7. The directional coupler circuit according to claim 6, characterized in that: When the output resistance of the low-frequency amplification module is a ninth resistance, the reactance ratio corresponding to the first inductor and the second capacitor is a first reactance ratio, and the value of the first reactance ratio is the product of the ninth resistance and the equivalent output resistance of the RF amplification module.

8. The directional coupler circuit according to claim 7, characterized in that: In the case where the signal access module includes a first transformer and a first capacitor connected in series, the equivalent reactance ratio corresponding to the first transformer and the first capacitor is a second reactance ratio, and the second reactance ratio is equal to the first reactance ratio.

9. A circuit board, characterized in that: The circuit board is a directional coupler circuit as described in any one of claims 1-8.

10. A radio frequency measurement device, characterized in that: The radio frequency measuring device comprises the circuit board as claimed in claim 9.

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