Terahertz wave band planar third harmonic mixer based on branch line coupler

Abstract

A terahertz wave band planar third harmonic mixer based on a branch line coupler relates to the technical field of terahertz communication and radar, and comprises a branch line coupler, an intermediate frequency low-pass filter, a radio frequency local oscillator dual-frequency band-pass filter, two Schottky diode single tubes, an open-circuit line, a T-junction and two sections of matching circuits. By adopting a pure plane structure, the terahertz filter has the advantages of being simple in circuit structure, easy to integrate, low in frequency conversion loss and the like, and can be applied to the fields of miniaturized and high-integration terahertz communication, radar and the like. A single balance structure and a branch line coupler are used as a 90-degree hybrid network, a radio frequency signal and a local oscillator signal are respectively input into two Schottky diodes at a 90-degree phase difference for frequency mixing, and an intermediate frequency signal after frequency mixing is output after passing through structures such as an intermediate frequency low-pass filter, so that improvement of radio frequency echo and local oscillator echo is facilitated, and the performance of the system is improved. When the device and other devices are integrated in one assembly in a gold wire bonding mode, the frequency conversion performance is hardly influenced.

Description

Technical Field

[0001] The invention relates to a terahertz band planar third harmonic mixer based on a branch line coupler, and belongs to the technical field of miniaturized and highly integrated terahertz communication and radar. Background Art

[0002] At present, the development of high-performance terahertz mixers is one of the key research directions of terahertz technology. As the mixer frequency increases, the mixer's requirements for the frequency and power of the local oscillator signal are also getting higher and higher. However, a high-frequency local oscillator (LO) signal with sufficient power is not easy to achieve. In these systems, it is common to use a harmonic mixer to reduce the local oscillator frequency. The harmonic mixer requires a local oscillator signal that is only 1 / N of the RF frequency (where N is the harmonic order), and its complexity is greatly reduced compared to those transmitters using fundamental mixers.

[0003] As terahertz application systems develop towards miniaturization and high integration, devices are also continuously improved towards monolithicization. The structure of waveguide combined with suspended microstrip line is not conducive to system integration. How to improve the structure of terahertz harmonic mixer to meet the requirements of miniaturized and highly integrated terahertz systems is a technical problem that technicians in this field urgently need to solve. Chinese patent CN 118352763 A discloses a terahertz band planar third harmonic mixer suitable for miniaturized and highly integrated terahertz systems, but the radio frequency and echo of the harmonic mixer using a 180° hybrid network are generally poor. In the terahertz frequency band, it is generally necessary to package it into a module and use it with a terahertz waveguide isolator of the corresponding frequency band. Otherwise, the frequency conversion performance will be deteriorated during integration, greatly increasing the cost and integration difficulty. Summary of the invention

[0004] The object of the present invention is to provide a terahertz band planar third harmonic mixer based on a branch line coupler to improve radio frequency echo and local oscillator echo and solve the defects affecting frequency conversion performance mentioned in the background technology.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions: a terahertz band planar third harmonic mixer based on a branch line coupler, comprising a branch line coupler, a radio frequency local oscillator dual-frequency bandpass filter connected to the branch line coupler, a first matching circuit, an open line, a first diode, a second diode, a second matching circuit, an intermediate frequency low-pass filter and a T-junction; the branch line coupler comprises a plurality of main lines and branch lines, and the branch lines are used to offset the amplitude imbalance caused by high-frequency loss; the branch line coupler has a plurality of contacts, which are respectively connected to a radio frequency port, a local oscillator port and a nonlinear element; the output end of the nonlinear element is connected in sequence through a second matching circuit, an intermediate frequency low-pass filter and a T-junction to achieve signal mixing, filtering and output.

[0006] Preferably, the branch line coupler includes an input 50 ohm microstrip line, a first main line, a second main line, a third main line, a fourth main line, a first branch line, a second branch line, a third branch line, and an output 50 ohm microstrip line. The first branch line, the second branch line, and the third branch line contain three sections of microstrip lines, which are used to offset the amplitude imbalance caused by increased loss at high frequencies.

[0007] More preferably, the branch line coupler has an a contact, a b contact, a c contact and a d contact, the a contact is connected to the RF port, the b contact is connected to the anode of the first diode via the RF local oscillator dual-frequency bandpass filter, the first matching circuit and the open line in sequence, the c contact is connected to the cathode of the second diode via the RF local oscillator dual-frequency bandpass filter, the first matching circuit and the open line in sequence, and the d contact is connected to the local oscillator port; the cathode of the first diode and the anode of the second diode are connected to the second matching circuit, the intermediate frequency low-pass filter in sequence and connected to a T-junction to form a circuit, and the combined end of the T-junction is connected to the intermediate frequency port.

[0008] More preferably, the first diode and the second diode are both Schottky diodes.

[0009] More preferably, the branch length and the spacing between branches of the branch line coupler are both approximately one-fourth of the wavelength of the local oscillator signal, and the RF signal amplitudes of the branch line coupler from the a contact to the b contact and the c contact are equal, and the phase difference is 90°; the local oscillator signal amplitudes of the branch line coupler from the d contact to the b contact and the c contact are equal, and the phase difference is 90°.

[0010] More preferably, the RF local oscillator dual-frequency bandpass filter utilizes the transmission line quarter-wavelength transformation principle and adopts a quarter-wavelength short-circuit bandpass filter to pass the local oscillator signal f LO ; and because the RF frequency is about three times the LO frequency, the LO quarter-wavelength short-circuit is about the RF f RF The three-quarter wavelength short-circuit line can also be used as a bandpass filter for RF signals; in addition, the RF local oscillator dual-band bandpass filter can be used for the intermediate frequency f IF And the DC circuit provides actual grounding; where: f RF Represents the frequency of the RF signal, f LO Indicates the local oscillator signal frequency, f IF Indicates the local oscillator signal frequency.

[0011] More preferably, the length of the open line is one quarter of the wavelength corresponding to the second harmonic frequency of the local oscillator frequency, so as to provide equivalent grounding for the second harmonic of the local oscillator frequency.

[0012] More preferably, the intermediate frequency low-pass filter adopts a compact microstrip resonant unit structure for passing the intermediate frequency signal and suppressing the radio frequency signal fRF and spurious signals.

[0013] More preferably, the first matching circuit and the second matching circuit both adopt a three-segment microstrip structure, the microstrip lines at both ends are 50 ohms, the line length is adjustable, and the line length and line width of the microstrip line in the middle segment are both adjustable.

[0014] The terahertz band planar third harmonic mixer based on the branch line coupler can be applied to terahertz communication or radar systems.

[0015] The present invention adopts a pure planar structure and utilizes a 90° single-balanced mixer structure. Through a branch line coupler, the RF signal and the local oscillator signal are respectively input into two Schottky diodes with a 90° phase difference for mixing. The intermediate frequency signal after mixing is output after passing through an intermediate frequency low-pass filter and other structures. Compared with the prior art, the present invention is conducive to improving the RF echo and the local oscillator echo. When it is integrated into a component with other devices by gold wire bonding, the frequency conversion performance is almost unaffected. In addition, the present invention also maintains the advantages of simple structure, excellent frequency conversion performance and low return loss, and can be used in miniaturized, highly integrated terahertz communication and radar transceiver systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a structural schematic diagram of the present invention;

[0017] Figure 2 is a structural schematic diagram of a branch line coupler;

[0018] Figure 3 This is a simulation result diagram of the balance of the RF signal from the branch line coupler from the a point to the b point and the c point in the embodiment;

[0019] Figure 4 It is a simulation result diagram of the balance of the local oscillator signal of the branch line coupler at the d contact input to the b contact and the c contact in the embodiment;

[0020] Figure 5 This is a simulation result diagram of the RF local oscillator dual-frequency bandpass filter in the embodiment;

[0021] Figure 6 4 is a diagram showing the simulation results of the intermediate frequency low-pass filter in the embodiment;

[0022] Figure 7 This is a diagram showing the simulation results of the down-conversion loss of a 220 GHz planar third harmonic mixer in the embodiment;

[0023] Figure 8 It is a down-conversion spectrum diagram of a 220 GHz planar third harmonic mixer in an embodiment;

[0024] Fig. 9This is a diagram showing the simulation result of the down-conversion return loss of a 220 GHz planar third harmonic mixer in the embodiment;

[0025] Fig.10 Schematic diagram of the difference in radio frequency and local oscillator echo between the present invention and the existing terahertz band planar third harmonic mixer in the embodiment.

[0026] In the figure:

[0027] 1——Branch line coupler 2——RF local oscillator dual-frequency bandpass filter

[0028] 3——First matching circuit 4——Open line 5——First diode

[0029] 6——Second diode 7——Second matching circuit 8——Intermediate frequency low-pass filter

[0030] 9——T-junction 101——50 ohm microstrip line input 102——first main line

[0031] 103——Second main line 104——Third main line 105——Fourth main line

[0032] 106——first branch line 107——second branch line 108——third branch line

[0033] 109——Output 50 ohm microstrip line. DETAILED DESCRIPTION

[0034] In order to facilitate the understanding of those skilled in the art, the present invention is further described below in conjunction with embodiments and drawings. The contents mentioned in the implementation modes are not intended to limit the present invention.

[0035] like Figure 1 As shown, a terahertz band planar third harmonic mixer based on a branch line coupler includes a branch line coupler 1, a radio frequency local oscillator dual-frequency bandpass filter 2, a first matching circuit 3, an open line 4, a first diode 5, a second diode 6, a second matching circuit 7, an intermediate frequency low-pass filter 8 and a T-junction 9.

[0036] Among them, the branch line coupler 1 includes an input 50 ohm microstrip line 101, a first main line 102, a second main line 103, a third main line 104, a fourth main line 105, a first branch line 106, a second branch line 107, a third branch line 108, and an output 50 ohm microstrip line 109. The first branch line 106, the second branch line 107, and the third branch line 108 contain three sections of microstrip lines, which are used to offset the amplitude imbalance caused by the increased loss at high frequencies.

[0037] The branch line coupler 1 has a contact a, a contact b, a contact c and a contact d, the contact a is connected to the radio frequency port, the contact b is connected to the anode of the first diode 5 through the radio frequency local oscillator dual-frequency bandpass filter 2, the first matching circuit 3 and the open line 4 in sequence, the contact c is connected to the cathode of the second diode 6 through the radio frequency local oscillator dual-frequency bandpass filter 2, the first matching circuit 3 and the open line 4 in sequence, and the contact d is connected to the local oscillator port; the cathode of the first diode 5 and the anode of the second diode 6 are connected to the second matching circuit 7 and the intermediate frequency low-pass filter 8 in sequence and combined with the T-junction 9, and the combined end of the T-junction 9 is connected to the intermediate frequency port.

[0038] The cathode of the first diode 5 and the anode of the second diode 6 are connected to the second matching circuit 7 and the intermediate frequency low-pass filter 8 in sequence, and are combined into one path through a T-junction 9, and the combined end of the T-junction 9 is connected to the intermediate frequency port.

[0039] In this embodiment, the amplitude of the local oscillator signal from the branch line coupler 1 from the d point to the b point and the c point is equal, and the phase difference is 90°. The branch line coupler 1 is a four-port microwave device with a phase difference of 90° between the two output ports, also known as a branch line bridge, which uses a multi-branch branch line coupler to expand the working bandwidth. The microstrip branch line coupler is related to the quarter-wavelength transmission line, and the quarter-wavelength is only valid near the center frequency, so when the branch length and the spacing between the branches are both about λ LO / 4, the branch line coupler can be used in the local oscillator frequency band. The local oscillator signal enters from the d contact, passes through the input 50 ohm microstrip line 101, passes through the third main line 104 and the fourth main line 105, and is output from the output 50 ohm microstrip line 109 to the c contact; the other path is the first branch line 106, the first main line 102, the second main line 103 or the third main line 104, the second branch line 107, the second main line 103 or the third main line 104, the fourth main line 105, the third branch line 108, and then outputs from the output 50 ohm microstrip line 109 to the b contact. No matter which path, it passes through λ more than the signal reaching the c contact. LO / 4, so for the local oscillator signal, the signal input from contact d and output from contact b lags 90° in phase compared to the signal output from contact c.

[0040] Furthermore, the RF signal amplitude from the branch line coupler to the b and c points is equal, and the phase difference is 90°. RF Approximately the local oscillator frequency f LO Three times the wavelength of the local oscillator λ LO / 4 is approximately three-quarters of the radio frequency wavelength 3λ RF / 4. The RF signal enters from the a contact, passes through the input 50 ohm microstrip line 101, passes through the first main line 102, the second main line 103, and is output from the output 50 ohm microstrip line 109 to the b contact; the other path is the first branch line 106, the third main line 104, the fourth main line 105 or the first main line 102, the second branch line 107, the fourth main line 105 or the first main line 102, the second main line 103, the third branch line 108, and then outputs from the output 50 ohm microstrip line 109 to the c contact. No matter which path, it passes 3λ more than the signal reaching the b contact. RF / 4, so for the local oscillator signal, the phase of the signal input from contact a and output from contact c lags 270° compared to the signal output from contact b, that is, the phase of the signal output from contact b lags 90° compared to the signal output from contact c.

[0041] The first diode 5 and the second diode 6 are both Schottky diodes. The terahertz band planar third harmonic mixer based on the branch line coupler is arranged on a quartz substrate, and the Schottky diode is bonded to the substrate with a conductive adhesive in a hybrid integration manner.

[0042] RF local oscillator dual-frequency bandpass filter 2 uses the transmission line quarter-wavelength transformation principle and adopts a quarter-wavelength short-circuit bandpass filter to pass the local oscillator signal f LO ; and because the RF frequency is about three times the LO frequency, the LO quarter-wavelength short-circuit is about the RF f RF The three-quarter wavelength short-circuit line can also be used as a bandpass filter for RF signals; in addition, the RF local oscillator dual-band bandpass filter can be used for the intermediate frequency f IF And the DC circuit provides actual grounding; where: f RF Represents the frequency of the RF signal, f LO Indicates the local oscillator signal frequency, f IF Indicates the local oscillator signal frequency.

[0043] The length of the open line 4 in this embodiment is one quarter of the wavelength corresponding to the second harmonic frequency of the local oscillator frequency, and is used to provide an equivalent grounding for the second harmonic of the local oscillator frequency. The length of the open line 4 is one quarter of the wavelength corresponding to the second harmonic frequency. According to the quarter-wavelength conversion principle, the open line 4 has an equivalent grounding effect on the second harmonic component 2f of the local oscillator. LO and the combined spurious f RF -f LO The signal is equivalent to a short circuit, so it is 2f LO and f RF -f LO A loop is provided.

[0044] In order to suppress the RF signal and stray signals, the intermediate frequency low-pass filter 8 adopts a compact microstrip resonant unit structure to pass the intermediate frequency signal and suppress the RF signal fRF and spurious signals.

[0045] The RF signal amplitude from the branch line coupler to the b and c contacts is equal, and the phase difference is 90°. The LO signal amplitude from the d contact to the b and c contacts is equal, and the phase difference is 90°. In this way, the branch line coupler structure forms a 90° hybrid network for both RF and LO, which can realize third harmonic mixing.

[0046] In order to fully utilize the energy of the local oscillator and RF signal and reduce the deterioration of the third harmonic mixing and conversion loss caused by other harmonic components, it is necessary to provide a loop for the main harmonic components. The main frequency components of the third harmonic down-conversion are f RF 、f LO , 2f LO 、f RF ±2f LO 、f RF ±3f LO etc., where f RF +3f LO As the intermediate frequency component is required for mixing, it is necessary to provide a loop for other frequency components and match them so that the energy can be used for third harmonic mixing to the maximum extent.

[0047] Specifically, the intermediate frequency low-pass filter 8 adopts a compact microstrip resonant unit structure, which can pass the intermediate frequency signal and suppress the radio frequency signal f RF and spurious signals, such as 2f LO etc.; the first matching circuit 3 and the second matching circuit 7 both adopt a three-segment microstrip structure, the microstrip lines at both ends are 50 ohms, the line length can be adjusted, and the line length and line width of the microstrip line in the middle section can be adjusted to achieve impedance matching.

[0048] This embodiment takes a 220GHz planar third harmonic mixer as an example to specifically introduce the design of each unit circuit therein, which is also effective for the design of mixers in other frequency bands. In this embodiment, the radio frequency frequency of the planar third harmonic mixer is 215-225GHz, the intermediate frequency range is 10.2-20.2GHz, and the local oscillator frequency is 204.8 / 3=68.27GHz.

[0049] The branch line coupler 1 is a four-port network with a 90° phase difference between two ports. The balance of the branch line coupler 1 mainly includes amplitude imbalance and phase imbalance. Figure 3The simulation result diagram of the balance of the branch line coupler 1 from the a contact to the b contact and the c contact in the range of 215 to 225 GHz is given. The amplitude imbalance of the branch line coupler 1 is expressed as the difference in the S parameter amplitude from the a contact to the c contact and the d contact, and the phase imbalance of the branch line coupler 1 is expressed as the difference in the S parameter phase from the a contact to the c contact and the d contact. It can be seen from the simulation results that at the RF frequency of 215 to 225 GHz, the amplitude imbalance of the branch line coupler 1 is less than 0.8 dB, and the phase imbalance is 90°±2°. The branch line coupler 1 has good amplitude and phase balance in the RF frequency range.

[0050] Figure 4 The figure is a simulation result of the balance of the branch line coupler 1 from the d-contact to the b-contact and the c-contact at 68.27GHz. The amplitude imbalance of the branch line coupler 1 is expressed as the difference between the S parameter amplitudes from the d-contact to the c-contact and the d-contact, and the phase imbalance of the branch line coupler 1 is expressed as the difference between the S parameter phases from the d-contact to the c-contact and the d-contact. It can be seen from the simulation results that at the local oscillator frequency of 68.27GHz, the amplitude imbalance of the branch line coupler 1 is 0.06dB and the phase imbalance is 89.8°. The branch line coupler 1 has good amplitude and phase balance at the local oscillator frequency.

[0051] In this embodiment, the RF local oscillator dual-frequency bandpass filter 2 adopts a quarter-wavelength short-circuit bandpass filter, which can pass the local oscillator signal f LO ; and because the RF frequency is about three times the LO frequency, the LO quarter-wavelength short-circuit is about the RF f RF The three-quarter wavelength short-circuit line can also be used as a bandpass filter for RF signals. One port of the RF local oscillator dual-band bandpass filter 2 is port 1, and the other port is port 2. Then dB(S(1,1)) represents the return loss of port 1, and dB(S(2,1)) represents the transmission loss from port 1 to port 2. Since the structure has reciprocity, the return loss of port 1 is equal to the return loss of port 2, and the transmission loss from port 1 to port 2 is equal to the transmission loss from port 2 to port 1. The simulation results are shown in Figure 2. Figure 5 As shown in the simulation results, it can be seen that the bandpass filter can pass the 215-225 GHz RF signal and the 68.27 GHz local oscillator signal, and the 136.6 GHz 2f LO In addition, the RF local oscillator dual-band pass filter can be used for the intermediate frequency f IF and DC return path to provide actual grounding.

[0052] The intermediate frequency low-pass filter 8 adopts a CMRC (compact microstrip resonant cell) structure, which has the advantages of compact structure, wide stop band and good out-of-band suppression. One port of the intermediate frequency low-pass filter 8 is port 1, and the other port is port 2. Then dB(S(1,1)) represents the return loss of port 1, and dB(S(2,1)) represents the transmission loss from port 1 to port 2. Since the structure has reciprocity, the return loss of port 1 is equal to the return loss of port 2, and the transmission loss from port 1 to port 2 is equal to the transmission loss from port 2 to port 1. The simulation results are shown in Figure 6 As shown in the simulation results, it can be seen that the low-pass filter can pass the intermediate frequency signal of 10.2~20.2GHz; the stop band range is 62~250GHz, and it can block the RF signal of 215~225GHz, the local oscillator signal of 68.27GHz and the 2f LO 、(f RF -f LO ) and other spurious signals are suppressed by more than 20dB, providing equivalent grounding for these signals.

[0053] In addition, when designing the mixer, it is also necessary to accurately model the first diode 5 and the second diode 6. It will be more accurate to combine the three-dimensional electromagnetic field model with the diode SPICE parameters.

[0054] Finally, the S parameters obtained by simulating the three-dimensional electromagnetic field model of the branch line coupler 1, the RF local oscillator dual-band pass filter 2, the open line 4, the first diode 5, the second diode 6, the intermediate frequency low-pass filter 8 and the T-junction 9 are imported into the circuit simulation software. The conversion loss of the mixer is optimized by adjusting the line width and length of the first matching circuit 3 and the second matching circuit 7, thereby obtaining all the final circuit parameters. Figure 7 The simulation results of the down-conversion conversion loss of the mixer are given. It can be seen from the simulation results that when the local oscillator power is 11dBm, the down-conversion conversion loss of the planar third harmonic mixer is 14.9-15.9dB, which has good frequency conversion performance.

[0055] Figure 8 The down-conversion spectrum of the mixer is given. From the simulation results, it can be seen that when the frequency is below 50GHz, there are almost no large spurious signals near the intermediate frequency, and the local oscillator signal is far away from the intermediate frequency signal, which can be easily filtered out by an external planar low-pass filter to obtain a relatively pure output spectrum.

[0056] Fig. 9is the return loss of the RF and local oscillator of the mixer. It can be seen from the simulation results that the return loss of the RF and local oscillator both reach -20dB, which is conducive to integrating the mixer into the 220GHz RF receiving component to avoid performance degradation caused by poor return.

[0057] In addition, the terahertz band planar third harmonic mixer based on the branch line coupler provided in the present embodiment adopts a single balanced structure, and the spurious responses related to the local oscillator or the radio frequency even harmonics generated by the two mixers cancel each other out, so the mixer has the characteristic of suppressing certain spurious responses related to the local oscillator or the radio frequency even harmonics. This characteristic is the main reason for using a balanced mixer instead of a single diode mixer. Compared with the 180-degree single balanced mixer disclosed in patent CN 118352763A mentioned in the background technology, the 90-degree single balanced mixer of the present invention has great advantages in radio frequency echo and local oscillator echo. This is because the 90° third harmonic mixer containing a branch line coupler is conducive to improving the radio frequency echo and the local oscillator echo, and when it is integrated into a component with other devices by gold wire bonding, the frequency conversion performance is almost unaffected. Fig.10 As shown in the figure, compared with the 180-degree single-balanced mixer, the difference in RF and LO echo is very obvious.

[0058] The mixer monolithic chip with a purely planar structure of the present invention can be connected to other devices by means of gold wire bonding, and does not require an isolator for a radio frequency or local oscillator port, which can greatly increase the density of devices per unit volume. It is suitable for miniaturized, highly integrated terahertz systems, and has a significant improvement in fields such as astronomy and electronic countermeasures that have strict requirements on volume and weight.

[0059] In order to make it easier for ordinary technicians in the field to understand the improvements of the present invention over the prior art, some drawings and descriptions of the present invention have been simplified, and the above-mentioned embodiments are preferred implementation schemes of the present invention. In addition, the present invention can also be implemented in other ways. Any obvious replacement without departing from the concept of the present technical solution is within the protection scope of the present invention.

Claims

1. A terahertz band planar third harmonic mixer based on a branch line coupler, characterized in that: It comprises a branch line coupler (1), a radio frequency local oscillator dual-frequency bandpass filter (2) connected to the branch line coupler (1), a first matching circuit (3), an open line (4), a first diode (5), a second diode (6), a second matching circuit (7), an intermediate frequency low-pass filter (8) and a T-junction (9); The branch line coupler (1) comprises a plurality of main lines and branch lines, wherein the branch lines are used to offset the amplitude imbalance caused by high-frequency loss; the branch line coupler (1) has a plurality of contacts, which are respectively connected to a radio frequency port, a local oscillator port and a nonlinear element; the output end of the nonlinear element is connected in sequence through a second matching circuit (7), an intermediate frequency low-pass filter (8) and a T-junction (9) to achieve signal mixing, filtering and output.

2. The terahertz band planar third harmonic mixer based on branch line coupler according to claim 1, characterized in that: The branch line coupler (1) comprises an input 50 ohm microstrip line (101), a first main line (102), a second main line (103), a third main line (104), a fourth main line (105), a first branch line (106), a second branch line (107), a third branch line (108), and an output 50 ohm microstrip line (109); the first branch line (106), the second branch line (107), and the third branch line (108) contain three sections of microstrip lines, which are used to offset amplitude imbalance caused by increased loss at high frequencies.

3. The terahertz band planar third harmonic mixer based on branch line coupler according to claim 1, characterized in that: The branch line coupler (1) comprises an a contact, a b contact, a c contact and a d contact, wherein the a contact is connected to a radio frequency port, the b contact is connected to the anode of a first diode (5) via a radio frequency local oscillator dual-frequency bandpass filter (2), a first matching circuit (3) and an open line (4) in sequence, the c contact is connected to the cathode of a second diode (6) via a radio frequency local oscillator dual-frequency bandpass filter (2), a first matching circuit (3) and an open line (4) in sequence, and the d contact is connected to the local oscillator port; the cathode of the first diode (5) and the anode of the second diode (6) are connected to a second matching circuit (7), an intermediate frequency low-pass filter (8) and a T-junction (9) in sequence to form a circuit, and the combined end of the T-junction (9) is connected to the intermediate frequency port.

4. The terahertz band planar third harmonic mixer based on branch line coupler according to claim 1, characterized in that: The first diode (5) and the second diode (6) are both Schottky diodes.

5. The terahertz band planar third harmonic mixer based on branch line coupler according to claim 1, characterized in that: The branch length and the spacing between the branches of the branch line coupler (1) are both approximately one-quarter of the wavelength of the local oscillator signal. The amplitude of the radio frequency signal from the a connection point to the b connection point and the c connection point of the branch line coupler (1) is equal, and the phase difference is 90°; the amplitude of the local oscillator signal from the d connection point to the b connection point and the c connection of the branch line coupler (1) is equal, and the phase difference is 90°.

6. The terahertz band planar third harmonic mixer based on branch line coupler according to claim 1, characterized in that: The radio frequency local oscillator dual-frequency bandpass filter (2) adopts a quarter-wavelength short-circuit bandpass filter.

7. The terahertz band planar third harmonic mixer based on branch line coupler according to claim 1, characterized in that: The length of the open line (4) is one quarter of the wavelength corresponding to the second harmonic frequency of the local oscillator frequency, and is used to provide equivalent grounding for the second harmonic of the local oscillator frequency.

8. The terahertz band planar third harmonic mixer based on branch line coupler according to claim 1, characterized in that: The intermediate frequency low-pass filter (8) adopts a compact microstrip resonant unit structure and is used to pass the intermediate frequency signal and suppress the radio frequency signal and stray signals.

9. The terahertz band planar third harmonic mixer based on branch line coupler according to claim 1, characterized in that: The first matching circuit (3) and the second matching circuit (7) both adopt a three-segment microstrip structure, the microstrip lines at both ends are 50 ohms, the line length is adjustable, and the line length and line width of the microstrip line in the middle segment are both adjustable.

10. An application of the terahertz band planar third harmonic mixer based on branch line coupler according to claim 1, characterized in that: Used in terahertz communications or radar systems.

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