Development of 225MHz～512MHz 50W power amplifier

1 Introduction

Since the invention of solid-state circuits in 1948, solid-state power amplifiers have basically replaced traveling wave tubes in the field of shortwave and ultra-shortwave communications with their advantages of high reliability, long life, high power efficiency and low power supply voltage, becoming the main power amplifier application device. Broadband linear amplifiers with solid-state amplification and broadband power synthesis as technical features are key technologies for communication countermeasures. However, due to device and technical reasons, high-power broadband solid-state power amplifiers have always been imported. Therefore, in order to solve the current technical bottleneck, this paper is based on broadband high-power amplifier modules and uses efficient heat dissipation to independently design a 225MHz~512MHz50W broadband high-power RF amplifier that spans the VHF band and UHF band. The operating temperature range of this amplifier is -40℃～+65℃, the storage range is -55℃～+85℃, and it has protection measures such as overvoltage, overcurrent and standing wave anomaly.

2 Design Process

2.1 Main technical indicators of amplifier components

Operating frequency: 225 MHz～512 MHz;

Output power (P1dB): ≥47 dBm;

Gain: ≥45 dB;

Flatness: ≤±1.5 dB;

Working current: ≤10A;

Operating voltage: +24 V;

Third-order intermodulation: ≤-27 dBc; 2.1.8 Secondary,

Third harmonic: ≤-50 dBc;

Input and output standing wave: ≤1.5.

2.2 Working Principle

The entire amplifier can be functionally divided into three parts: power amplifier, transceiver switch and monitoring part. The main task of the transceiver switch part is to complete the switching of the transceiver channel. The monitoring part mainly completes the real-time monitoring of the power amplifier part and realizes the protection of some unexpected situations such as overvoltage, overcurrent and standing wave abnormality, so as to maximize the protection of the whole machine and conveniently troubleshoot. The technology of these two parts is relatively mature, and this article will not discuss them in detail. This article mainly discusses the design and implementation of the power amplifier part. The power amplifier part mainly performs power amplification of small signals. It is the main part of the power amplifier, through which the electrical performance indicators of the power amplifier, such as gain, output power and power flatness, are realized. The specific schematic block diagram of the power amplifier is shown in Figure 1. Since the working frequency band spans the frequency band and has a bandwidth of nearly 300 MHz, the key to designing the amplifier is how to obtain a better gain flatness q, overcome the characteristic that the power device gain decreases by 6 dB per octave, and obtain a better output standing wave. The author uses ADS software simulation, adopts a company's LDMOS device, selects appropriate input and output matching, takes into account flatness and output power, and finally meets the design requirements.

Figure 1 Amplifier block diagram

2.3 Circuit Design

Due to the high requirements for the second harmonic in the technical indicators, a segmented filtering method was adopted, with 340 MHz as the boundary, 225 MHz to 340 MHz as the low band, and 340 MHz to 512 MHz as the high band. We designed and developed a high-power switch filter that meets the requirements. In order to detect the output standing wave and implement over-standing wave protection, the output end adopts dual directional coupler sampling. At the same time, in order to meet the customer's requirements for providing a receiving channel, a transceiver switch was added. Considering the insertion loss of the filter, directional coupler and transceiver switch after the amplifier, in order to ensure the final output power of 50W, the amplifier output power reaches 80W. The final amplifier part adopts a 90° bridge power synthesis scheme. Its bias circuit adopts a thermistor compensation circuit, and its circuit diagram is shown in Figure 2, which realizes that the static current is stable within ±10% in the range of -40℃ to +65℃, thereby achieving the purpose of improving the linearity of the entire power amplifier.

At present, the design technology of RF LDMOS is very mature, and transistors working at around several hundred MHz can output several hundred watts of power. However, as the output capacity of the device increases, the input/output impedance of the device becomes smaller and smaller, and changes dramatically with frequency, making the design of the matching network very difficult, especially the high-power matching network across the octave. The method of combining multiple low-power transistors to realize a high-power amplifier module can easily solve the design problem of the transistor input/output matching network, and can effectively separate the heat source, reducing the difficulty of system heat dissipation. At the same time, this design method can also reduce the impact caused by the failure of individual devices and improve the reliability of the high-power module.

The design difficulty of this amplifier module is bandwidth and high linear power, so broadband matching technology is the key to successful design. The broadband performance of the transmission line transformer is closely related to the design of the transmission line transformer. Before performing broadband matching of the transistor, the transmission line transformer and the transistor matching circuit should be designed first. The transmission line transformer design uses the transmission line impedance transformer to complete the impedance matching between the high-impedance signal source or load and the low-impedance power transistor input or output, which can maximize the bandwidth potential of the transistor itself. There are two points that must be paid attention to in the design and use of the transmission line transformer: one is the matching relationship between the source impedance, load impedance, and transmission line impedance; the other is that the input and output ends must be used in the prescribed connection and grounding methods. The wide bandwidth of the power amplifier has always been a problem that has troubled designers. The traditional method is to use the transmission line transformer to match the low end of the frequency band through impedance transformation, and at the same time use the low-pass matching part to reduce the impedance of the high end of the frequency band. This technology uses the equivalent inductance of the transmission line transformer and adds a small capacitor to form a π-type matching network, taking into account the entire frequency band, so that the amplifier can achieve broadband and efficient matching within the octave bandwidth.

Figure 2 Thermistor compensation circuit

2.4 Simulation Results

Through simulation calculation, it is relatively easy to achieve broadband operation by using two 60 W LDMOS for push-pull operation. The amplifier part is simulated using ADS software, and the amplifier gain and third-order intermodulation simulation results are shown in Figures 3 and 4:

Figure 3 Amplifier gain simulation results

In Figure 4, each tone is 43.2 dBm, considering that the insertion loss of the amplifier followed by the filter, directional coupler, transceiver switch, etc. is 2 dB, while leaving some margin.

Figure 4 Amplifier third-order intermodulation simulation results

3 Experimental Results

To improve the input standing wave ratio, a 3dB attenuator is added to the input of the amplifier, and the estimated gain of the entire amplifier is 50dB. The output of the amplifier adopts a balanced structure, so the standing wave ratio can be guaranteed. The size of the entire amplifier is: 202 mm×148 mm×29 mm (plus control and directional coupling parts). After debugging, very satisfactory results were obtained. Various indicators are consistent with the simulation results and meet the requirements of the amplifier component. At present, the amplifier has been produced in small batches with stable performance and good consistency. The actual test results of the third-order intermodulation of the amplifier component are shown in Figure 5, and the typical test results are shown in Table 1. The input level and current in the test data in the table are the results measured under the condition that the entire amplifier component outputs 50 W (plus the subsequent band filtering and transceiver switch, the actual output power of the amplifier itself is 80 W). The intermodulation is the result measured under the condition of a two-tone interval of 0.1 MHz and each tone of 41dBm. The results show that the power amplifier designed in this paper has an additional efficiency and linearity of 37% and -27 dBc in the wide frequency range of 225 MHz to 512 MHz. Figure 6 shows the appearance structure of the amplifier.

Figure 5 Amplifier third-order intermodulation measurement results

Figure 6 Amplifier structure

Table 1 UHF band power amplifier component test record

4 Conclusion

This power amplifier consists of an amplifier, a filter, a directional coupling part, and a transceiver switch. Due to the high requirements, the design and development of each part is a challenge. We completed a small batch of amplifiers that meet the design requirements in a short period of time. It has been put into use online and has stable and reliable performance, meeting the requirements.