Design of a millimeter-wave power amplifier for V-band close-range detection

The power amplifier is an indispensable key component of the millimeter wave frequency band transmitter. The output power determines the range and anti-interference ability of the entire system. In the millimeter wave system, as the frequency increases, the output power of a single MMIC chip can no longer meet the actual use requirements, especially in the non-atmospheric window frequency band, because the transmission of electromagnetic waves in this frequency band is severely attenuated by the absorption of oxygen molecules and water vapor molecules. It is generally used in military confidentiality work and short-range radar detection and communication systems. The output power of the corresponding device is also small. Therefore, the power synthesis method is often used to combine multiple amplifier units together to achieve a larger power output.

　　

The amplifier operates in the V band and is used in a missile-borne short-range detection system, making full use of the attenuation characteristics of the non-atmospheric window band to achieve confidentiality and anti-interference.

　　

1 Power Amplifier Design

　　

1.1 Technical index requirements

　　

According to the basic requirements of the system, the main technical indicators of the amplifier are: working bandwidth 2 GHz; output power ≥ 200 mW; gain 20~25 dBm; input and output port WR15.

　　

1.2 Selection of power devices

　　

In order to meet the requirements of technical indicators, a three-terminal FET power amplifier with a wide operating frequency band is selected. The FMM5715X of Eudyna Devices is selected as the power synthesis unit. The FMM5715X is a multi-tube synthesis power monolithic chip with a port impedance matching of 50 Ω. The operating frequency is 57 to 64 GHz; the operating temperature range is -45 to +85℃, and the storage temperature range is -55 to +125℃; the maximum allowable input power is 3 dBm; it can work with a single power supply. Under the condition of DC bias 3 V/150 mA, the typical performance P1 dB at 60 GHz is 16 dBm, the saturated power is 17 dBm, and the small signal gain is 17 dB. The characteristic parameters are shown in Figure 1.

 

　　

 

1.3 Synthetic Network Design

　　

1.3.1 Overall plan of synthetic network

　　

In the V band, the output power of a single tube is far from meeting the power output requirements. Even if a multi-tube MMIC power device is used, a single device cannot meet the technical indicators. Therefore, the use of multi-device power synthesis technology is an inevitable choice to complete this project. The current more mature power synthesis technology is to use a two-way bridge with better port standing wave, and realize multi-way synthesis by multi-stage cascade. The designed amplifier uses a two-way binary multi-level power synthesis technology based on a waveguide low-loss transmission line structure. The synthesis network consists of two parts, a power driving stage and a power amplification synthesis stage. Each part includes a 3-level binary network, which is composed of a waveguide branch line bridge and a waveguide-microstrip transition. The synthesis network block diagram is shown in Figure 2.

　　

 

When 8-way power distribution is used, the network loss of each level is calculated as 0.3 dB and the path loss is calculated as 0.5 dB. If all the power devices are to work in saturation during synthesis, the input power of FMM5715X should be >2 dBm. After taking the above losses into account, the power converted to the input end of the power distribution network is 12.4 dBm. Obviously, a single-way FMM5715X driver stage is sufficient to meet this requirement.

　　

When all power devices in the synthesis network are in saturation working state, for single-stage loss of 0.3 dB, 3-stage power synthesis, the synthesis efficiency caused by loss is 80%; if the maximum amplitude and phase imbalance between the synthesis branches are 3 dB and 30° respectively, the corresponding synthesis efficiency is 90%; for 8-way power synthesis, the total synthesis efficiency is

 

　

 

When the device is saturated, the 8-way combined output is 17+7.07=24.07 dBm or 255 mW, which meets the technical requirements. The loss of each part of the circuit is 4 8 dB, and the small signal gain of the entire combined amplifier is about 29.2 dB.
1.3.2 Two-way power distribution/combination network

　　

In the research of millimeter-wave solid-state integrated power synthesis technology, there is a two-way waveguide microstrip integrated power distribution/synthesis network, as shown in Figure 3. The structure consists of two face-to-face microstrip probes inserted through the waveguide E surface to achieve in-phase broadband power distribution/synthesis, while completing the transition between waveguide and microstrip. The two microstrip lines are in a face-to-face position. When the solid-state power device is integrated, it can provide a good heat dissipation channel to ensure the reliable operation and performance of the device. In order to obtain sufficient installation space for solid-state power devices, the size of the waveguide part of the synthesis network is appropriately increased. In order to meet the standard waveguide port conditions, the appropriate waveguide impedance transformation section is selected according to the working bandwidth requirements. Electromagnetic field analysis shows that in the range of 55 to 60 GHz, the loss of the structure is <2 dB, which is equivalent to a single waveguide microstrip transition structure; due to the structural symmetry, the two microstrip ports have good amplitude and phase balance characteristics.

 

　　

 

1.3.3 V-band 3 dB waveguide branch bridge

　　

Power synthesis of more than two levels needs to be based on a waveguide bridge. The low loss, wide bandwidth, good port standing wave and branch isolation of the bridge are the prerequisites for stable and reliable power synthesis technology. Since the relative bandwidth required by this project is relatively narrow, the index requirements can be met by using three branch nodes. In the structure, considering the feasibility of bridge processing, the cross-sectional size of the bridge waveguide is appropriately increased, and the length of the coupling hole of the waveguide branch node is reduced and the width is increased, so that all processing dimensions are >1 mm, reducing the difficulty of mechanical processing, increasing the corresponding error capacity, and reducing processing costs. This structural improvement reduces the size of the connection discontinuity between the branch bridge and the waveguide-microstrip three-port network in the synthesis network.

　　

Figure 4 shows the 3 dB waveguide branch bridge model and the electromagnetic field analysis results. The electromagnetic field analysis results show that the amplitude imbalance of the bridge is <0.5 dB in the frequency range of 55 to 60 GHz, the isolation between the two output ports is >15 dB, and the return loss is <-16 dB; in the entire frequency band, the phase difference between the two output ports is a constant 90°.

 

　　

 

Figure 5 shows a microstrip integrated two-stage four-way power synthesis/distribution network of the millimeter-wave waveguide structure realized by the above two bridge cascades. This structure uses a branch waveguide as input, and uses this branch waveguide to realize the first-stage 2-equal division of power. Then two slots are opened on both sides of the branch waveguide at positions symmetrical about the center, and the energy is coupled out using a microstrip probe. The input energy can be divided into four equal parts, and the distribution network can also be used as a synthesis network. The electromagnetic field simulation results show that: within the required frequency band, the imbalance of the four-way output port is <0.5 dB, and the input port return loss is <-15 dB; within the entire frequency band, ports 2 and 3 (or ports 4 and 5) have ideal in-phase characteristics, and the phase difference between the two output ports 2 and 4 or 5 (or 3 and 4 or 5) is a constant 90°.

 

　　

 

2. Physical object and test

　　

The actual V-band power amplifier is shown in Figure 6. The performance of the power amplifier was tested in the laboratory. The input of the power amplifier was 0 dBm, and the actual power output was shown in Figure 7. Within the working frequency band, the output power fluctuated within the band <0.5 dB.

 

　　

 

3 Conclusion

　　

For the application of missile-mounted short-range detection system, a peak output power of several hundred mW is generally required. The power synthesis method is used to realize the design of a 250 mW V-band power amplifier. The actual design results show that the size and power consumption of the power amplifier meet the overall requirements of the system, and it has a good heat dissipation effect, which provides technical guarantee for the realization of the overall plan of the V-band detection system and its engineering application.