Microwave power amplifiers are mainly divided into vacuum and solid-state forms. Power amplifiers based on vacuum devices have played an important role in the development history of military equipment. Because of their advantages in power and efficiency, they are still widely used in radar, communication, electronic countermeasures and other fields. Later, with the advent of GaAs transistors, solid-state devices began to replace vacuum tubes in the low frequency band. Especially with the application of new materials such as Gan and SiC, the competitiveness of solid-state devices has been greatly improved. This paper will introduce and analyze the development of the two devices and the product of their competition and integration - microwave power module (MPM), so as to fully understand the international advanced level and promote the development of domestic technology.

Vacuum amplifiers：

Compared with solid-state devices, the main advantages of vacuum devices are high working frequency, wide frequency band, high power and high efficiency. The main disadvantages are large volume and mass. They have their own advantages in the field of vacuum traveling wave tube and magnetron device. Among them, the main advantage of TWT is frequency bandwidth, the main advantage of klystron is high power, and the main advantage of magnetron is high efficiency. TWT is the most widely used, so this paper mainly takes TWT as an example to introduce vacuum devices.

1.1 History development

The development of vacuum electronic devices can be traced back to World War II. In 1963, TWTA technology made substantial progress in design reform, improved the power and efficiency of RF output, and the package was more compact. In 1973, the first TWT amplifier in Europe was successfully developed. However, by the mid-1970s, semiconductor devices have sprung up, and the investment in vacuum devices has been greatly reduced. Its development has encountered great difficulties. Until the beginning of the 21st century, the American armed forces ad hoc committee discussed the history, current situation and development of power devices in detail, and pointed out the balanced investment strategy between vacuum devices and solid-state devices. In 2015, DARPA launched invest and havoc programs respectively to support the development of vacuum power devices and the growing needs of military systems, especially millimeter wave and THz traveling wave tubes. At present, vacuum devices have made great progress and are widely used in radar, communication, electronic warfare and other systems.

1.2 Research and application status

With the continuous progress of technology, at this stage, TWT mainly presents the following characteristics. First, it has the characteristics of high frequency, broadband and high efficiency, which can effectively reduce the volume, weight, power consumption and heat consumption of the system. It has stronger adaptability on satellite borne, missile borne and airborne platforms, so it has outstanding advantages in military applications. The second is the high temperature resistance, which makes the power and phase of TWT fluctuate slightly with the change of temperature, and greatly reduces the environmental control requirements of the system. Third, it has the characteristics of anti strong electromagnetic interference and attack, which makes it show a solid survivability in the confrontation of high-power microwave weapons and microwave bombs. Fourth, the service life has been greatly improved. Statistical research shows that the service life of high-power TWT is generally greater than 5000h, and the service life of small and medium-power products is greater than 10000h, reaching the whole life cycle of weapons. Figure 1 shows the statistics of mean time to first failure (MTTF) of products before 2000. It can be seen that the stability of vacuum devices in various systems has been improved, and the MTTF of Space TWT has reached the order of millions of hours, showing high reliability.

Public reports show that vacuum devices are widely used in American combat platforms, which are the main power devices of electronic warfare, radar and communication in active service. The newly developed high-frequency band, miniaturized traveling wave tube and power module further promote the continuous emergence of high-performance equipment. Typical applications include vehicle mounted air defense and antimissile system, ground-based remote early warning and intelligence system, airborne fire control system, UAV Communication system, electronic warfare system, space and satellite communication system, etc. The following introduces several important technologies of traveling wave tube currently being studied and applied.

1.2.1 Active array technology of traveling wave tube

In recent years, foreign countries have mainly developed a series of miniaturized traveling wave tubes in higher frequency band, with frequency bands covering x, Ku, K, Ka, 140 GHz, etc., and constantly made breakthroughs in new technologies. After nearly 10 years of efforts in China, the volume of TWT has been reduced by one order of magnitude on the premise of maintaining high power and high efficiency, which has laid a good foundation for active array technology.

The active array of TWT can be divided into two types: unit amplification type and subarray amplification type. Compared with passive phased array, its single TWT has low power requirements and relatively high reliability and service life. At the same time, each channel is relatively independent, and the failure of one channel will not affect other channels, so the reliability of the system is high. Moreover, the whole radiation array can work independently in multiple areas to realize the ability of multi-objective and multi task of the system. Compared with solid-state active phased array, it has longer action distance and greater power, and the supporting cooling vehicle and power vehicle are relatively short and powerful, with high system mobility and strong battlefield survivability. Because of its all metal and ceramic sealing structure, it has stronger survivability in the face of high-power microwave weapons. At the same array power, the number of units required will be 1 order of magnitude less, so the cost will be greatly reduced. Compared with monopulse radar, its operating range, resolution, multi-target, multi mission, life and mission reliability will be better. At present, the research on Ku band sparse array low grid lobe technology based on traveling wave tube is being carried out in China, in order to achieve − 20 dB grid lobe under the condition of array element spacing of 30 mm.

In addition, the research progress of miniaturized high-power circulator matched with TWT active array is rapid. New array technologies such as aperiodic array composed of unequal size units, aperiodic ring array with radial equal spacing and irregular array of subarrays can well solve the grid lobe problem caused by large unit spacing, which jointly ensure the promotion of active array of TWT.

1.2.2 Millimeter wave and THz TWT

The development of 5g mobile communication technology puts forward the demand for millimeter wave power amplifier from Ka to W band. In the future, 5g needs broadband access to an area, but where optical fiber cannot be used, millimeter wave band can only be selected. THz wave has great application potential in the field of radar detection because of its high frequency, wide bandwidth and narrow beam. However, with the increase of frequency, the processing technology requirements of devices are higher and higher. In recent years, the comprehensive introduction of micro mechanical (MEMS) micromachining technology has improved the traditional technology, making the working frequency of vacuum devices enter the millimeter wave and THz frequency band, and the maximum of existing devices has reached 1 THz. In recent years, short millimeter wave TWT has become more and more mature, and has initially formed a series of related products.

1.3 Development trend

1.3.1 higher frequency band

There is no doubt that the high operating frequency band is the absolute advantage of TWTA. In the high frequency band, the output power and efficiency of solid-state power amplifier (SSPA) are far lower than TWTA, so high frequency is the inevitable development trend of TWTA. MEMS micromachining technology promotes the research of millimeter wave and THz band. As the demand for space saturated Ka / Ka Band TV and other multimedia applications has gradually increased, the research of Ka / Ka Band TV has become a hot spot, and the demand for space saturated Ka / Ka Band TV has gradually increased. THz band communication has extremely high transmission rate. With the progress of waveguide technology, TWTA has great application potential in outer space exploration.

1.3.2 Higher efficiency

Since its application, the efficiency of traveling wave tubes in various bands has been continuously improved. At present, Ku band 88125h manufactured by L3 company has an efficiency of 73%, which is the highest value publicly reported at present. At present, the power efficiency has been very high, generally better than 90%. Further improving the efficiency will be a research and development challenge. Therefore, it mainly depends on improving the efficiency of TWT to increase the total efficiency. Optimizing the spiral pitch distribution of TWT is an effective method to improve the efficiency.

1.3.3 Miniaturized traveling wave tube

TWTA miniaturization technology has been significantly improved in the past few decades, and the development of TWT active array and other technologies promotes the miniaturization of TWT. In addition, a potential change of TWTA is to increase the use of mini TWT. Mini TWT is a small version of traditional TWT and the basis of microwave power module. Although it can not achieve high RF output power, it not only reduces the volume, but also improves the efficiency, especially in satellite communication and other fields.

Solid state amplifiers

Solid state devices, that is, semiconductor electronic devices. Similar to TWTA, SSPA usually needs to be equipped with integrated power supply. The difference is that SSPA uses field effect transistor as the main device of RF power amplification, with low working voltage and easier implementation. In order to realize the output power of SSPA, many single transistors need to be placed in parallel to achieve high output power. Solid state devices have the advantages of small volume, low noise and good stability. The disadvantages are low application frequency band, small monomer output power and low efficiency.