CN111510073A - Terahertz broadband third harmonic mixer - Google Patents

Abstract

The invention provides a terahertz broadband third harmonic mixer, comprising a mixer circuit substrate, a metal cavity and a Schottky diode; the mixer circuit substrate is provided with a waveguide-suspended stripline-microstrip Transition structures, ground vias, suspension strips, open line branches, metal floors, IF low-pass filters, IF end microstrip lines, sector probes and microstrip lines, metal cavities include RF end waveguides and local oscillator ends waveguide. The present invention adopts a waveguide-suspended stripline-microstrip transition structure, which reduces the loss caused by mismatch, increases the effective bandwidth, has small size, is easy to process, and has low cost.

Description

A Terahertz Broadband Third Harmonic Mixer

technical field

The invention belongs to the technical field of terahertz odd harmonic mixers, in particular to a terahertz broadband third harmonic mixer.

Background technique

With the increasing demand for ultra-high-speed data transmission and high-resolution imaging, communication, radar and other application systems have higher and higher requirements for working bandwidth, and the development and utilization of terahertz spectrum resources has attracted much attention. In the relevant terahertz application system, the receiver front end must have broadband and low noise figure characteristics to ensure the high sensitivity of the system. However, low-noise amplifiers in the terahertz band are difficult to implement, so a mixer is usually used as the first stage of a terahertz receiver.

Among the fundamental mixers, the local oscillator signal needs to be provided by a terahertz frequency source, which is difficult to manufacture, high cost, and more difficult to implement than the mixer, while the harmonic mixer can reduce the required frequency of the local oscillator signal. It is 1/N of the fundamental frequency (N is the harmonic order), which reduces the difficulty of realizing the local oscillator signal and enhances the isolation between the local oscillator and the RF port. Therefore, the front end of the terahertz receiver usually adopts a harmonic mixer. At present, the research on harmonic mixers in the terahertz band mostly focuses on even-order harmonic mixers, and there are few reports on odd-order harmonic mixers, especially third-order harmonic mixers. Due to the special structural requirements of the odd harmonic mixer, the balanced diode pair is usually located in the RF end or the local oscillator end waveguide. And the design scheme is difficult to apply to the terahertz frequency band. At present, the terahertz frequency band usually adopts waveguide gradient, fin line transition and other methods for impedance matching, but there are usually problems such as large size, insufficient bandwidth, and excessive loss.

SUMMARY OF THE INVENTION

In view of the deficiencies in the prior art, the present invention provides a terahertz broadband third harmonic mixer with small size, while reducing loss and increasing effective bandwidth.

The present invention achieves the above technical purpose through the following technical means.

A terahertz broadband third harmonic mixer, comprising a mixer circuit substrate, a metal cavity and a Schottky diode; the metal cavity includes a radio frequency end waveguide and a local oscillator end waveguide;

The mixer circuit substrate is provided with a waveguide-suspended stripline-microstrip transition structure, an open line branch, an intermediate frequency low-pass filter and a sector probe;

The waveguide-suspended stripline-microstrip transition structure includes a triangular window, a suspended stripline and a microstrip, the triangular window is located on the back of the mixer circuit substrate, and the vertex position is the same as that of the mixer circuit substrate. Front-side Schottky diodes are center-aligned;

The Schottky diode is provided with three solder pins D1, D2 and D3, and the middle solder pin D3 is fixed on the pad at the suspension strip line;

The suspension strip line is connected with the inverted T-shaped microstrip line, and the microstrip line is connected to the open line branch in the horizontal direction near the suspension strip line;

The microstrip line is connected to a fan-shaped probe near the end of the local oscillation end waveguide in the horizontal direction, and the fan-shaped probe is located inside the local oscillation end waveguide;

The microstrip line is connected with an intermediate frequency low-pass filter in the vertical direction, and the intermediate frequency low-pass filter is composed of two identical CMRC units;

The waveguide-suspended stripline-microstrip transition structure, the Schottky diode, and the suspended stripline are all located inside the waveguide at the radio frequency end.

Preferably, a plurality of grounding through holes are machined on the outer edge of the triangular window toward the direction of the local oscillator end waveguide.

Preferably, the soldering legs D1 and D2 are fixed on the pads at the ground vias on both sides.

Preferably, the mixer circuit substrate is further provided with a metal ground; on the front side of the mixer substrate, the metal ground covers the ground through hole and its vicinity; on the reverse side of the mixer circuit substrate, the metal ground Cover the position other than the triangular opening and the fan-shaped probe; the grounding through hole is connected to the metal floor on the front and back of the mixer circuit substrate.

Preferably, the open line branches include four open lines, which are connected to two sides of the microstrip line opposite each other.

Preferably, the length of the open line is one twelfth of the wavelength of the local oscillator signal.

Preferably, the vertical end of the microstrip line is connected to an intermediate frequency end microstrip line, and the impedance of the intermediate frequency end microstrip line is 50Ω.

Preferably, the intermediate frequency low-pass filter consists of two identical CMRC units.

Preferably, the short-circuit surface distance from the Schottky diode to the RF end waveguide is λ _{RF, TE10} /4, where λ _{RF, TE10} are the wavelength of the RF signal.

Preferably, the size of the RF end waveguide meets the WR06 standard, and the size of the local oscillator end waveguide meets the WR19 standard.

The present invention adopts the above-mentioned technical scheme, and has the following advantages compared with the prior art:

(1) The third harmonic mixer of the present invention adopts a waveguide-suspended stripline-microstrip transition structure, and realizes two functions at the same time: the broadband transition between the waveguide and the suspended stripline structure is performed in the radio frequency band, and the The frequency band performs broadband transition of suspended stripline and microstrip line structure; reduces the loss caused by mismatch, increases the effective bandwidth, small size, easy processing, low cost, and the total length is only one wavelength of the radio frequency signal.

(2) The third harmonic mixer of the present invention adopts a waveguide-suspended stripline-microstrip transition structure, and the ground through holes on both sides are close to the ground pins of the Schottky diode, providing good RF and DC grounding , to avoid parasitic effects and energy loss introduced by the redundant grounding structure, and further reduce the frequency conversion loss of the mixer.

(3) The present invention makes full use of the series-connected double Schottky junctions in a single Schottky diode chip, and uses a single chip to form a balanced diode pair structure, reducing the diode pair and the waveguide-suspended stripline-microstrip transition structure. The relative position deviation is conducive to ensuring the consistency of batch assembly.

Description of drawings

1 is a schematic structural diagram of a terahertz broadband third harmonic mixer according to the present invention;

Fig. 2 is a schematic diagram of the waveguide-suspended stripline-microstrip transition structure according to the present invention, Fig. 2(a) is a front view of the waveguide-suspended stripline-microstrip transition structure of the present invention, and Fig. 2(b) is the present invention The reverse view of the waveguide-suspended stripline-microstrip transition structure;

Fig. 3 is the schematic diagram of the electric field direction change when the radio frequency signal and the local oscillator signal are transmitted in the present invention; Fig. 3 (a) is the electric field direction change schematic diagram when the radio frequency signal is transmitted from the radio frequency end waveguide to the Schottky diode in the present invention, Fig. 3 ( b) is a schematic diagram of the change of the electric field direction when the local oscillator signal is transmitted from the microstrip line to the Schottky diode in the present invention;

4 is a schematic diagram of a simulation curve of frequency conversion loss of the present invention;

Among them: 1-RF end waveguide, 2-Waveguide-suspended stripline-microstrip transition structure, 3-Schottky diode, 4-ground via, 5-suspended stripline, 6-open line branch, 7- Metal floor, 8-IF low-pass filter, 9-IF end microstrip line, 10-Sector probe, 11- LO end waveguide, 12-Microstrip line.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

As shown in Figure 1, the terahertz broadband third harmonic mixer of the present invention is composed of a mixer circuit substrate, a metal cavity on which the circuit substrate is placed, and a Schottky diode 3. The mixer circuit substrate is provided with There are waveguide-suspended stripline-microstrip transition structure 2, grounding vias 4, suspension strips 5, open line branches 6, metal paving 7, IF low-pass filter 8, IF end microstrip line 9, sector probe The needle 10 and the microstrip line 12, the metal cavity where the circuit substrate is placed, includes the RF end waveguide 1 and the local oscillator end waveguide 11.

As shown in Figures 2(a) and (b), the waveguide-suspended stripline-microstrip transition structure 2 includes a section of triangular fenestration, a suspended stripline 5 and a microstrip line 12, wherein the triangular fenestration is located in the mixer On the back of the circuit substrate, the vertex position is aligned with the center of the Schottky diode 3 on the front side of the mixer circuit substrate; the outer edge of the triangular window is machined toward the local oscillator end waveguide 11 with a number of grounding through holes 4; the metal floor 7 is used for mixing The front side of the mixer substrate covers the ground through hole 4 and its adjacent positions, and the reverse side of the mixer circuit substrate covers the position other than the triangular opening and the fan-shaped probe 10; Floor 7; in this embodiment, the metal floor 7 is preferably made of copper.

As shown in Figure 2(a), the Schottky diode 3 has three solder pins D1, D2 and D3. The middle solder pin D3 is the common solder pin of the double Schottky junction, corresponding to the 5 pads of the suspension strip line. Pin D3 is glued to the pad at 5 of the suspension strip line by conductive glue; the solder pins D1 and D2 at both ends of the diode correspond to the 4 pads of the ground vias on both sides respectively, and the solder pins D1 and D2 are glued with conductive glue On the pads at the ground vias 4 on both sides. The dual Schottky junctions are in phase series with respect to the RF signal, and in anti-phase parallel with respect to the local oscillator signal and the IF signal, so one Schottky junction is driven in phase by the RF and local oscillator signals, and the other Schottky junction is driven by the RF and IF signals. The local oscillator signal is driven in reverse phase to form a balanced diode pair structure and output an intermediate frequency signal. The grounding via 4 is adjacent to the solder pins D1 and D2 at both ends of the Schottky diode 3 to form a good RF and DC grounding and avoid adding redundant branches.

The suspended strip line 5 is connected with the inverted T-shaped microstrip line 12, and the microstrip line 12 is connected to the open line branch 6 near the suspended strip line 5 in the horizontal direction. As shown in FIG. 1, the open line branch 6 includes four open lines, They are connected on both sides of the microstrip line 12 opposite to each other. When the RF signal is transmitted from the RF end waveguide 1 to the Schottky diode 3, a part of it will continue to transmit and leak to the LO and IF ports, and a part of the leaked RF signal can be reflected back to the Schottky diode 3 to avoid energy loss and improve the Port isolation. The length of each open line is one quarter of the wavelength of the RF signal, that is, one-twelfth of the wavelength of the local oscillator signal (λ _{LO, TEM} /12), which has little effect on the local oscillator signal. By adjusting the line width and spacing, Assist other circuit structures (waveguide-suspended stripline-microstrip transition structure 2, suspension strip 5, IF low-pass filter 8, IF end microstrip line 9 and sector probe 10) to perform impedance matching in the local oscillator frequency band. The frequency of the local oscillator signal is located in the cut-off frequency band of the waveguide 1 at the RF end, so the isolation from the local oscillator signal to the RF end can be guaranteed.

The end of the microstrip line 12 close to the local oscillator end waveguide 11 in the horizontal direction is connected to the fan-shaped probe 10. The fan-shaped probe 10 is located inside the local oscillator end waveguide 11, and the mixer circuit substrate containing the fan-shaped probe 10 is connected to the The local oscillator end waveguide 11 is fixed. The local oscillator signal is input from the local oscillator end waveguide 11 , is transferred from the sector probe 10 to the microstrip line 12 , and transmitted to the Schottky diode 3 .

The microstrip line 12 is sequentially connected with an intermediate frequency low-pass filter 8 and an intermediate frequency end microstrip line 9 in the vertical direction, and the intermediate frequency end microstrip line 9 is located at the vertical end of the microstrip line 12 . The intermediate frequency low-pass filter 8 consists of two identical CMRC (Compact Microstrip Resonant) units, which are cascaded to enhance the suppression of the local oscillator signal and the radio frequency signal.

The waveguide-suspended stripline-microstrip transition structure 2, the Schottky diode 3, and the suspended stripline 5 are all located inside the waveguide 1 at the RF end, and the waveguide-suspended stripline-microstrip transition structure is formed by conductive adhesive. 2. The three-part mixer circuit substrate of the Schottky diode 3 and the suspended strip line 5 is fixed to the waveguide 1 at the radio frequency end. In this embodiment, the short-circuit distance between the Schottky diode 3 and the waveguide 1 at the RF end is λ _{RF, TE10} /4 (where λ _{RF, TE10} are the wavelengths of the RF signal), so as to ensure that the diode is located at the position with the maximum electric field strength of the RF signal, reducing the loss of small RF signals.

Preferably, the size of the RF end waveguide 1 in this embodiment meets the WR06 standard, the size of the local oscillator end waveguide 11 meets the WR19 standard, and the impedance of the IF end microstrip line 9 is 50Ω.

As shown in Figure 3(a), the RF signal is transmitted from the standard waveguide 1 at the RF end to the waveguide-suspended stripline-microstrip transition structure 2, and finally reaches the Schottky diode 3. During this process, the direction of the electric field of the RF signal remain unchanged; as shown in Figure 3(b), the local oscillator signal is transmitted from the microstrip line 12 to the Schottky diode 3, and in this process, the quasi-TEM mode (transverse electromagnetic mode) is always maintained. During the transmission process of the RF signal and the local oscillator signal, the mode change and energy waste are avoided, the frequency conversion loss is reduced, and the working bandwidth is effectively improved. The RF signal and the local oscillator signal are loaded on the Schottky diode 3, and the Schottky diode 3 is used for nonlinear mixing. Through impedance matching and filtering, the required intermediate frequency signal is extracted at the intermediate frequency port.

The overall simulation model of the mixer is constructed, and the specific implementation steps include: (1) Considering the parasitic effect introduced by the diode package size, establish a three-dimensional electromagnetic model of the Schottky diode 3 in three-dimensional electronic simulation software (such as HFSS); (2) In the three-dimensional electronic simulation software, the three-dimensional electromagnetic model of the passive circuit such as the fan-shaped probe 10, the intermediate frequency low-pass filter 8, the open circuit branch 6, etc. is established, and the preliminary simulation is carried out, wherein the mixer circuit substrate material is set to Rogers5880, thickness is 0.127mm; (3) the three-dimensional electromagnetic model of the Schottky diode 3 and the three-dimensional electromagnetic model of the passive circuit are used to establish a complete three-dimensional electromagnetic model of the passive part of the mixer in the three-dimensional electronic simulation software; (4) from the three-dimensional electromagnetic model In the electronic simulation software, the simulation results of the complete three-dimensional electromagnetic model of the passive part of the mixer are exported to obtain the SNP file; (5) the SNP file is imported into the circuit-level simulation software (ADS) to form a multi-port model, which is consistent with the embodiment of SCHOTT. The diode SPICE models of the nonlinear characteristics of the base diode 3 are cascaded to form the overall simulation model of the mixer, and the frequency conversion loss of the mixer is simulated by the harmonic balance method; (6) By adjusting the waveguide-suspended stripline-microstrip transition The size of the structure 2, the optimization of matching branches, and the optimal result of the conversion loss in the broadband range are obtained.

In this embodiment, the total length of the waveguide-suspended stripline-microstrip transition structure 2 is set to 12 mm, the length of each open line is set to 0.32 mm, and the length of the CMRC unit is set to 1.40 mm; according to steps (4)-(6 ) to simulate the overall structure of the mixer, and the simulation results are shown in Figure 4. As can be seen from the figure, when the local oscillator power is 14dBm, in the range of 135-165GHz, the frequency conversion loss of the mixer does not exceed 14.5dB, and in the range of 140-160GHz, the frequency conversion loss of the mixer is 11.5±0.7dB. Compared with the prior art, the terahertz broadband third harmonic mixer of the present invention adopts a waveguide-suspension stripline-microstrip transition structure, which improves the frequency conversion loss and improves the working bandwidth, and is different from the traditional method of changing the size of the waveguide or the fin line. Compared with the transition method, the structure is more compact, and the processing difficulty and tolerance requirements are reduced.

The embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above-mentioned embodiments, and any obvious improvement, replacement or All modifications belong to the protection scope of the present invention.

Claims (10)

1. A terahertz broadband third harmonic mixer is characterized by comprising a mixer circuit substrate, a metal cavity and a Schottky diode (3); the metal cavity comprises a radio frequency end waveguide (1) and a local oscillator end waveguide (11);

the mixer circuit substrate is provided with a waveguide-suspended strip line-microstrip transition structure (2), an open line branch (6), an intermediate frequency low-pass filter (8) and a fan-shaped probe (10);

the waveguide-suspended strip line-microstrip transition structure (2) comprises a triangular window, a suspended strip line (5) and a microstrip line (12), wherein the triangular window is positioned on the back surface of the mixer circuit substrate, and the vertex position of the triangular window is aligned with the center of a Schottky diode (3) on the front surface of the mixer circuit substrate;

the Schottky diode (3) is provided with three welding feet D1, D2 and D3, and the middle welding foot D3 is fixed on a welding pad at the position of the suspension strip line (5);

the suspension strip line (5) is connected with an inverted T-shaped microstrip line (12), and the microstrip line (12) is connected with an open-circuit branch (6) at a position close to the suspension strip line (5) in the horizontal direction;

the tail end of the microstrip line (12) close to the local oscillator end waveguide (11) in the horizontal direction is connected with a fan-shaped probe (10), and the fan-shaped probe (10) is located inside the local oscillator end waveguide (11);

the microstrip line (12) is connected with an intermediate-frequency low-pass filter (8) in the vertical direction, and the intermediate-frequency low-pass filter (8) consists of two stages of same CMRC units;

the waveguide-suspended strip line-microstrip transition structure (2), the Schottky diode (3) and the suspended strip line (5) are all located inside the radio frequency end waveguide (1).

2. The terahertz broadband third harmonic mixer according to claim 1, wherein a plurality of ground through holes (4) are formed in the triangular window at the outer edge towards the direction of the local oscillator end waveguide (11).

3. The terahertz broadband third harmonic mixer according to claim 2, wherein the solder tails D1 and D2 are fixed on the pads at the two-side ground vias (4).

4. The terahertz broadband third harmonic mixer according to claim 3, wherein a metal ground (7) is further provided on the mixer circuit substrate; on the front surface of the mixer substrate, a metal floor (7) covers the grounding through hole (4) and the position nearby the grounding through hole; on the reverse side of the mixer circuit substrate, a metal floor (7) covers the triangular window and the position outside the fan-shaped probe (10); the grounding through hole (4) is communicated with metal flooring (7) on the front side and the back side of the mixer circuit substrate.

5. The terahertz broadband third harmonic mixer according to claim 1, wherein the open-line stub (6) comprises four open-line lines, and the open-line lines are connected to two sides of the microstrip line (12) in a manner of being opposite to each other.

6. The terahertz broadband third harmonic mixer of claim 5, wherein the open line has a length of one twelfth of a wavelength of the local oscillator signal.

7. The terahertz broadband third harmonic mixer according to claim 1, wherein the microstrip line (12) is vertically connected to an intermediate frequency end microstrip line (9), and an impedance of the intermediate frequency end microstrip line (9) is 50 Ω.

8. The terahertz broadband third harmonic mixer according to claim 1, characterized in that the intermediate frequency low pass filter (8) is composed of two identical stages of CMRC units.

9. The terahertz broadband third harmonic mixer according to claim 1, wherein the short-circuit surface distance from the schottky diode (3) to the radio frequency end waveguide (1) is λ_RF，TE10/4, where λ_RF，TE10Is the wavelength of the radio frequency signal。

10. The terahertz broadband third harmonic mixer according to claim 1, wherein the size of the radio frequency end waveguide (1) meets the WR06 standard, and the size of the local oscillator end waveguide (11) meets the WR19 standard.

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