A broadband high-power directional coupler covering VLF-VHF frequency band and its implementation method

Abstract

The present invention discloses a broadband high-power directional coupler covering the VLF-VHF frequency band, which mainly solves the problems of increased loss and limited power capacity of existing directional couplers when covering low frequencies. The directional coupler includes a first coaxial transmission line with a ferrite core, two sampling and comparison circuits connected to the two ends of the first coaxial transmission line and symmetrical about the middle of the first coaxial transmission line in circuit structure, and a main power channel and a coupling power output channel connected to the sampling and comparison circuits. Through the above design, the present invention adopts a hybrid circuit composed of a coaxial transmission line and lumped parameter resistors and capacitors, and utilizes the opposite characteristics of the inner and outer conductor currents of the coaxial transmission line to perform the topological design of the corresponding sampling and comparison circuit, compensate for and reduce the parasitic parameters of the resistor, so that the directional coupling circuit can be easily connected to the radio frequency microwave communication and test system to play a monitoring and protection role.

Description

Technical Field

[0001] The present invention relates to a directional coupler, in particular to a broadband high-power directional coupler covering a VLF-VHF frequency band and a realization method thereof. Background Art

[0002] Broadband high-power directional couplers are important components of RF microwave communication and testing systems. They are mainly used to measure the forward and reverse power passing through the system. They are often connected after the high-power transmitter and before the radiating antenna and the device under test. By reading the signals in both the forward and reverse directions, we can obtain information such as the size of the actual transmitted signal in the system, the size of the reflected power, the system load, the impedance matching status of the transmitter and the antenna, and the electrical connection status of the system. This is crucial for real-time monitoring and control of the system and safe operation.

[0003] Directional couplers are divided into two categories: lumped parameters and distributed parameters. Typical circuits are as follows: Figure 1 shown.

[0004] At present, the main way to obtain broadband characteristics is to cascade multiple stages through quarter-wave sections. The odd-even mode analysis method is used in the design to obtain the multi-stage coupling coefficient and the corresponding characteristic impedance. This method is more suitable in the RF microwave band because its physical length is short, such as 1GHz-2GHz, 2GHz-6GHz, and the quarter wavelength is at the centimeter level. When the low end of the operating frequency drops below 100MHz, the limitation of this method is that the volume becomes unrealistic, such as the quarter wavelength of 30MHz is 7.5 meters. So there are two solutions. One is to use a multi-layer printed circuit board to fold the circuit; the other is to abandon the quarter wavelength and use one-eighth or shorter electrical length for comprehensive design. Both methods have their limitations, such as the increased loss of the folding circuit method, limited power capacity, and increased overall circuit thickness. The coupling curve of the comprehensive design method is not flat, the coupling directivity is poor, and a large number of complex peripheral compensation circuits are required, and repeated design iterations are required. Through these two methods, directional couplers working at 1MHz-100MHz and 30MHz-1GHz can be developed, and the power can reach 1kW. However, when the low-end frequency continues to drop to kHz and the power continues to increase, both methods are difficult to meet. Summary of the invention

[0005] The purpose of the present invention is to provide a broadband high-power directional coupler covering the VLF-VHF frequency band and an implementation method thereof, mainly to solve the problems of increased loss and limited power capacity of the existing directional coupler when covering low frequencies.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A broadband high-power directional coupler covering the VLF-VHF frequency band comprises a first coaxial transmission line with a ferrite core, two sampling comparison circuits connected to both ends of the first coaxial transmission line and having a circuit structure symmetrical about the middle of the first coaxial transmission line, and a main power channel and a coupled power output channel connected to the sampling comparison circuits; the sampling comparison circuit comprises a second coaxial transmission line whose inner conductor is connected to the inner conductor of the first coaxial transmission line, a sampling resistor R3 connected between the outer conductors of the first coaxial transmission line and the second coaxial transmission line, and a complementary resistor connected in parallel to both ends of the sampling resistor R3. a compensation capacitor C2, a resistor R2 connected to the common end of the sampling resistor R3, the compensation capacitor C2, and the outer conductor of the first coaxial transmission line, a sampling resistor R1 and a resistor R4 connected to the other end of the resistor R2, and a compensation capacitor C1 connected in parallel to both ends of the sampling resistor R1; wherein the other free end of the sampling resistor R1 is connected to the other end of the inner conductor of the second coaxial transmission line, the other end of the outer conductor of the second coaxial transmission line is grounded, the main power channel is connected to the common end of the sampling resistor R1 and the inner conductor of the second coaxial transmission line, and the coupled power output channel is connected to the other free end of the resistor R4.

[0008] Furthermore, the main power channels in the two sampling and comparing circuits are respectively a main power input channel and a main power output channel, and the corresponding coupling power output channels in the sampling and comparing circuits are respectively a forward coupling power output channel and a reverse coupling power output channel.

[0009] Based on the above directional coupler, the present invention also provides a method for realizing a broadband high-power directional coupler covering the VLF-VHF frequency band, comprising the following steps:

[0010] (S1) determining the two sampling and comparison circuits as a forward sampling and comparison circuit and a reverse sampling and comparison circuit according to the main power input channel selected by the main power input;

[0011] (S2) the sampling resistors in the forward sampling comparison circuit and the reverse sampling comparison circuit respectively sample the voltage from the inner and outer conductors of their respective coaxial transmission lines;

[0012] (S3) By utilizing the inherent characteristic that the current directions of the inner and outer conductors of the coaxial transmission line are opposite, the addition and subtraction signal outputs are obtained in the two coupling power output channels, thereby obtaining the detection values ​​of the forward coupling power and the reverse coupling power.

[0013] Furthermore, the calculation formulas for voltage and current on a coaxial transmission line of any length l are:

[0014]

[0015]

[0016] Wherein, Z0 is the characteristic impedance of the coaxial line, and γ is the transmission characteristic of the coaxial transmission line.

[0017] Furthermore, the voltage obtained by monitoring the port of the forward coupled power output channel is proportional to the sum of the voltages of the sampling resistors in the forward sampling comparison circuit; the voltage obtained by monitoring the port of the reverse coupled power output channel is proportional to the difference between the voltages of the sampling resistors in the reverse sampling comparison circuit.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The present invention adopts a hybrid circuit composed of a coaxial transmission line and a lumped parameter resistor and capacitor, and utilizes the opposite characteristics of the current of the inner and outer conductors of the coaxial transmission line to perform the topological design of the corresponding sampling comparison circuit, compensate and reduce the parasitic parameters of the resistor, so that the typical frequency of the coupler of the present invention is 9kHz~250MHz, the passing power is greater than 3kW, the loss is ≤0.2dB, the coupling flatness is ±0.5dB, and the directivity is ≥20dB. The characteristic impedance in the entire frequency band is 50Ω, which can be easily connected to the existing radio frequency system to monitor the transmission power exceeding 3kW. And the directional coupler is composed of a coaxial transmission line and a forward and reverse sampling circuit. The structure of the forward and reverse sampling circuit is exactly the same, except that the signal flow direction in the circuit is different, and the addition and subtraction of the signal are used to obtain the forward and reverse coupling signal outputs. Vice versa, if the input and output signals are reversed, the opposite forward and reverse coupling model output can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the structure of a typical circuit of a directional coupler in the prior art.

[0021] Figure 2 It is a schematic diagram of the structure of the present invention.

[0022] Figure 3 It is a schematic diagram of the structure of voltage and current transmission of the coaxial transmission line in the present invention.

[0023] Figure 4 This is the forward power coupling diagram of the P3 port in the directional coupler.

[0024] Figure 5 This is the reverse power coupling diagram of the P4 port in the directional coupler.

[0025] Figure 6 This is the simulation result diagram of 9kHz~250MHz directional coupler. DETAILED DESCRIPTION

[0026] The present invention is further described below in conjunction with the accompanying drawings and embodiments. The embodiments of the present invention include but are not limited to the following embodiments.

[0027] Example

[0028] like Figure 2 As shown, the present invention discloses a broadband high-power directional coupler covering the VLF-VHF frequency band, including a first coaxial transmission line b with a ferrite core C, two sampling comparison circuits connected to both ends of the first coaxial transmission line and symmetrical about the middle of the first coaxial transmission line, the two sampling circuits are constructed exactly the same, but the signal flows in different directions in the circuits, and the addition and subtraction of the signals are used to obtain the output of the coupled signals in different positive and negative directions. Vice versa, if the input and output signals are reversed, the opposite positive and negative coupled signal outputs can be obtained.

[0029] The sampling and comparison circuit includes a second coaxial transmission line whose inner conductor is connected to the inner conductor of the first coaxial transmission line, a sampling resistor R3 connected between the outer conductors of the first coaxial transmission line and the second coaxial transmission line, a compensation capacitor C2 connected in parallel to both ends of the sampling resistor R3, a resistor R2 connected to the common end of the sampling resistor R3, the compensation capacitor C2, and the outer conductor of the first coaxial transmission line, a sampling resistor R1 and a resistor R4 connected to the other end of the resistor R2, and a compensation capacitor C1 connected in parallel to both ends of the sampling resistor R1; wherein the other free end of the sampling resistor R1 is connected to the other end of the inner conductor of the second coaxial transmission line, the other end of the outer conductor of the second coaxial transmission line is grounded, the main power channel is connected to the common end of the sampling resistor R1 and the inner conductor of the second coaxial transmission line, and the coupled power output channel is connected to the other free end of the resistor R4. Figure 2 In the figure, P1 and P2 are the main power channels, and P3 and P4 are the forward and reverse coupled power outputs. If P1 is power input, then P2 is power output, P3 is the forward power coupling port, and P4 is the reverse power coupling port; if P2 is power input, then P1 is power output, P4 is the forward power coupling port, and P3 is the reverse power coupling port.

[0030] The basic principle of this circuit utilizes the characteristic that the currents in the inner conductor and outer conductor of the coaxial transmission line are equal in magnitude and opposite in direction, as shown in 0.

[0031] The voltage in a coaxial transmission line refers to the voltage difference between the two conductors. The current flows in the two conductors separately. The formulas for the voltage and current on a coaxial line of arbitrary length l are shown below.

[0032]

[0033]

[0034] Where Z0 is the characteristic impedance of the coaxial line, and γ is the transmission characteristic of the coaxial line. Therefore, when the size of the coaxial line is determined, the voltage and current at each point on the line can be calculated. If the voltage to ground at any point on the inner conductor is If the current is I(l), then the voltage on the outer conductor to ground is The current is -I(l).

[0035] The sampling resistors R1, R3, R5, and R7 respectively sample the voltage from the inner and outer conductors of the input and output ends, and utilize the inherent characteristic that the current directions of the inner and outer conductors of the coaxial line are opposite to each other to obtain addition and subtraction signal outputs at the P3 and P4 ports, thereby obtaining the detection values ​​of the forward power and reverse power.

[0036] like Figure 4 The figure shows the A part of the directional coupler. When the signal is input from the P1 port, the power transmission direction on the coaxial core is to the right. Through circuit analysis, the current direction on R1 and R2 is as shown in the figure. At this time, the voltage monitored at the P3 port is proportional to U R1 +U R2 The greater the input power of the P1 port, the greater the detection voltage.

[0037] like Figure 5 Shown is the B section of the directional coupler. Figure 4 At the same time, the power transmission direction on the coaxial core is to the right. Through circuit analysis, the current direction on R6 and R7 is as shown in the figure. At this time, the voltage monitored by the P4 port is equal to U R7 -U R6 The greater the input power of the P1 port, the greater the detection voltage.

[0038] By adjusting the resistance value in the circuit, the voltage at the P3 port can be much greater than the voltage at the P4 port, and both are proportional to the input power at the P1 port. Therefore, P3 can indicate a forward coupled signal. On the contrary, if the signal is input from P2, it can be regarded as a reflected signal. The voltage monitored at the P4 port is proportional to U R7 +U R6 The difference between P3 and P4 indicates the directionality.

[0039] In this embodiment, according to the coupling ratio, the resistance of the sampling resistors R1 and R7 is relatively high, and the power and voltage tolerance requirements are also relatively high. The resistance of the sampling resistors R3 and R5 is very small, but the power passing through is relatively large, and the parasitic parameters are required to be small.

[0040] The compensation capacitors C1, C2, C3, and C4 in the figure are used to compensate for the parasitic inductance and capacitance of the resistor itself. Since the time constants of the RC circuits in the directional coupler vary greatly, the sampling signals cannot be accurately aligned in certain frequency bands, and the addition and subtraction values ​​will be different, affecting the accuracy of the coupling value and the quality of the directionality. Since the parasitic parameters of different resistors vary greatly, the same resistors will also differ due to discreteness, so capacitors need to be added to compensate and adjust the time constants of the forward and reverse sampling circuits so that the forward and reverse signals are superimposed at the correct time and completely offset or strengthened to obtain higher directionality. Match the positive and reverse signals and accurately offset them to improve the coupling and directionality characteristics. The time constant of the RC circuit is:

[0041] τ = RC;

[0042] Among them, τ represents the time required for the voltage across the capacitor to rise to 0.63 times the maximum value, and R and C are the resistance and capacitance in the RC circuit respectively.

[0043] For example, the sampling resistor R1 has a large resistance value, which will introduce a large parasitic capacitance; while the sampling resistor R3 has a small resistance value and a small parasitic capacitance. The difference in their time constants τ can reach 10 5 Therefore, it is necessary to adjust the time constants of the two circuits to reduce the difference.

[0044] Therefore, capacitors C1, C2, C3, and C4 are added to adjust and compensate for the parasitic capacitances of R1, R3, R5, and R7, respectively, so that the time constants of these RC circuits do not affect the coupling accuracy and directivity.

[0045] like Figure 6 From the simulation results in , we can see that the loss of the directional coupler is ≤0.2dB, the coupling flatness is ≤±0.5dB, and the directivity is ≥20dB. The selected coaxial cable and voltage divider resistors meet the high-power working requirements of 3kW. In actual use, when the power required is higher or it works in a high-temperature environment, a heat sink and forced air cooling device can be added.

[0046] Through the above design, the present invention adopts a hybrid circuit composed of a coaxial transmission line and a lumped parameter resistor and capacitor, and utilizes the opposite characteristics of the current of the inner and outer conductors of the coaxial transmission line to perform the topological design of the corresponding sampling comparison circuit, compensate and reduce the parasitic parameters of the resistor, so that the directional coupling circuit can work simultaneously in the 9kHz~100MHz frequency band, pass power greater than 3kW, loss ≤0.2dB, coupling flatness ±0.5dB, and directivity ≥20dB. Each port is matched with a 50Ω system, and can be easily connected to the radio frequency microwave communication and test system to play a monitoring and protection role. Therefore, it has a high use value and promotion value.

[0047] The above embodiment is only one of the preferred implementation modes of the present invention and should not be used to limit the protection scope of the present invention. Any changes or modifications that are made to the main design concept and spirit of the present invention and have no substantive significance, and the technical problems they solve are still consistent with the present invention, should be included in the protection scope of the present invention.

Claims

1. A broadband high-power directional coupler covering the VLF-VHF frequency band, characterized in that: The invention comprises a first coaxial transmission line with a ferrite core, two sampling comparison circuits connected to both ends of the first coaxial transmission line and having a circuit structure symmetrical about the middle of the first coaxial transmission line, and a main power channel and a coupled power output channel connected to the sampling comparison circuits; the sampling comparison circuit comprises a second coaxial transmission line having an inner conductor connected to the inner conductor of the first coaxial transmission line, a sampling resistor R3 connected between the outer conductors of the first coaxial transmission line and the second coaxial transmission line, a compensation capacitor C2 connected in parallel to both ends of the sampling resistor R3, a resistor R2 connected to a common end of the sampling resistor R3, the compensation capacitor C2 and the outer conductor of the first coaxial transmission line, a sampling resistor R1 and a resistor R4 connected to the other end of the resistor R2, and a compensation capacitor C1 connected in parallel to both ends of the sampling resistor R1; wherein the other free end of the sampling resistor R1 is connected to the other end of the inner conductor of the second coaxial transmission line, the other end of the outer conductor of the second coaxial transmission line is grounded, the main power channel is connected to the common end of the sampling resistor R1 and the inner conductor of the second coaxial transmission line, and the coupled power output channel is connected to the other free end of the resistor R4.

2. A broadband high-power directional coupler covering the VLF-VHF frequency band according to claim 1, characterized in that: The main power channels in the two sampling and comparing circuits are respectively a main power input channel and a main power output channel, and the corresponding coupling power output channels in the sampling and comparing circuits are respectively a forward coupling power output channel and a reverse coupling power output channel.

3. The method for realizing a broadband high-power directional coupler covering the VLF-VHF frequency band as claimed in claim 1 or 2, characterized in that: The steps include: (S1) determining the two sampling and comparison circuits as a forward sampling and comparison circuit and a reverse sampling and comparison circuit according to the main power input channel selected by the main power input; (S2) the sampling resistors in the forward sampling comparison circuit and the reverse sampling comparison circuit respectively sample the voltage from the inner and outer conductors of their respective coaxial transmission lines; (S3) By utilizing the inherent characteristic that the current directions of the inner and outer conductors of the coaxial transmission line are opposite, the addition and subtraction signal outputs are obtained in the two coupling power output channels, thereby obtaining the detection values ​​of the forward coupling power and the reverse coupling power.

4. The method for realizing a broadband high-power directional coupler covering the VLF-VHF frequency band according to claim 3, characterized in that: The calculation formula for voltage and current on a coaxial transmission line of arbitrary length l is: Wherein, Z0 is the characteristic impedance of the coaxial line, and γ is the transmission characteristic of the coaxial transmission line.

5. The method for realizing a broadband high-power directional coupler covering the VLF-VHF frequency band according to claim 3, characterized in that: The voltage obtained by monitoring the port of the forward coupled power output channel is proportional to the sum of the voltages of the sampling resistors in the forward sampling comparison circuit; the voltage obtained by monitoring the port of the reverse coupled power output channel is proportional to the difference between the voltages of the sampling resistors in the reverse sampling comparison circuit.

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