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Digital TV terrestrial broadcasting technology uses digital compression technology to increase the number of programs that users can receive by 4 to 6 times in the same resolution and sound quality. In the same channel, additional data and other information can be transmitted at the same time, and the anti-interference ability is strong, and the receiving effects of the near field and the far field in the coverage area are almost the same. Therefore, digital television has received extensive attention.Some countries in Europe and the United States have made in-depth research on digital TV technology, and have developed a digital TV signal transmitter with perfect performance. The research on digital TV technology in China started relatively late and is still in the experimental stage. In order to reduce costs, the localization of digital TV transmitters is an inevitable trend in the development of China's radio and television industry.
Power amplifiers are an important part of digital TV transmitters. Normally, the signal in the digital television transmitter is modulated by the COFDM method and then output an intermediate frequency analog signal, which is sent to the amplifying portion by up-conversion. The modulation method includes two modes, IFFT (8M) and IFFT (2M), which are composed of 6817 and 1705 carriers, respectively. The frequency spacing between each carrier is very close, so the intermodulation signal can easily fall within the frequency band, causing crosstalk. Digital TV transmitters have higher requirements in terms of linearity and stability than traditional types. The power amplifier in the transmitter must be operated in a higher linear state and the gain is stable.
The amplified portion of the transmitting system is divided into an excitation and main amplification circuit. The excitation part is a broadband power amplifier. In order to ensure the normal stability of terrestrial digital TV transmission, it needs to have good stability and reliability. Its working frequency range is 470MHz~860MHz, the working state is AB class; the required gain is greater than 10dB, the intermodulation suppression Less than -35dB, the noise power density is greater than 130dBc/Hz. This article uses the latest LDMOS FET devices, and balanced amplifier circuit structure? Design the driver stage power amplifier in the digital TV transmitter, optimized and debugged to meet system requirements.
1 power amplifier design
1.1 Amplifier chip selection of power amplifier
In this paper, Motorola LDMOS FET device MRF373 is used as the amplifier chip of the power amplifier. The chip has a large improvement in linearity, gain and output capability compared to BJT devices, which greatly improves the reliability and maintainability of the transmitter. Compared with traditional decimeter-wave bipolar power amplifier tubes, LDMOS FETs have the following significant advantages:
Can work at high standing wave ratio (VSWR = 10:1);
High gain (typically 13dB);
The saturation curve is smooth, which is beneficial to analog and digital TV RF signal amplification;
Can withstand large overdrive power, especially suitable for COFDM modulated multi-carrier signals in DVB-T;
The bias circuit is simple and does not require complex active low-impedance bias circuits with positive temperature compensation.
The LDMOS manufacturing process combines BPT and gallium arsenide processes. Unlike the standard MOS process, LDMOS does not use the BeO yttrium oxide isolation layer on the device package, but is directly bonded to the substrate. The thermal conductivity is improved, the device's high temperature resistance is improved, and the device lifetime is greatly extended. . Due to the negative temperature effect of the LDMOS tube, the leakage current automatically flows while being heated, and does not form a hot spot locally at the collector current as the positive temperature effect of the bipolar tube, so that the tube is not easily damaged. Therefore, the LDMOS tube greatly enhances the load mismatch and over-excitation tolerance. Also due to the automatic current sharing action of the LDMOS tube, its input-output characteristic curve is relatively gentle at 1dB compression point (saturation section of large signal application), so the dynamic range is widened, which is beneficial to analog and digital TV RF signal amplification. LDMOS is approximately linear in small signal amplification, with almost no intermodulation distortion, which greatly simplifies the correction circuit. The MOS device's DC gate current is almost zero, and the bias circuit is simple, eliminating the need for complex active low-impedance bias circuits with positive temperature compensation.
1.2 Circuit Structure Selection and Comparison
The small-signal S-parameter can be used in the design of a class A amplifier, which means that the amplification of the signal is basically limited to the linear region of the transistor. However, when it comes to high power amplifiers, small signals are usually nearly ineffective because the amplifier operates in a non-linear region. At this point, the large signal S-parameter or impedance of the transistor must be obtained to obtain a reasonable design effect.
Generally speaking, the Class A working state has the smallest distortion coefficient and good linearity. However, in the case of high power applications, it is not suitable due to the low efficiency (50%) of Class A operating conditions. Using the circuit form of a class A and B push-pull amplifier, a linear index similar to that of a class A amplifier can be obtained.
The push-pull circuit form consists of two independent single-tube amplifiers without any internal connections, with vector distribution and synthesis of power through two baluns. Because the balun itself has the characteristics of varistor, the impedance matching caused by the varistor ratio is greatly reduced, and the balun has a good suppression effect on even harmonics. However, since the spacing between the two sides of the balun is too small, the two paths have a large mutual influence, so the amplifier applying the balun structure has poor stability, and the input and output standing waves of the circuit are relatively poor. This article uses the form of a balanced amplifier, the structure shown in Figure 1. Its working principle is similar to that of the balun structure, but due to the application of the 3dB bridge, the isolation between the two RF signals is better, which is beneficial to the matching of the two ports. The advantages are shown in Table 1 with respect to the single-tube amplifier structure.
1.3 Matching network design
Since the MRF373 does not provide an internal match, a matching network is built in the amplifying circuit. The amplifying circuit in the digital television reflection system operates at 470 MHz to 860 MHz, and impedance matching is required in a wide frequency range. The basic idea of ​​the design of the wideband amplifier matching circuit is to use a reactive matching network for multi-stage impedance transformation between the input and output of the amplifier and the stage. The network only plays a matching role, no additional power loss, can guarantee the maximum transmission coefficient, balance the device characteristics, and can meet the system
Where VDD is the operating voltage, VDS(SAT) is the knee voltage, and P is the output power.
According to the above formula, the RL of the MRF373 is approximately 6 Ω.
The amplifying circuit herein uses a method in which a separating element and a distributed parameter element are used in combination. Since inductors have higher heat losses than capacitors, inductors are often avoided in such circuits and replaced with high impedance transmission lines. A hybrid type of matching network typically includes several serially connected transmission lines and parallel capacitances in spaced configurations. The input matching part of the amplifier adopts four-section impedance transformation, and the output matching adopts a hybrid circuit of five-section impedance transformation. The input and output matching network topology diagram is shown in Figure 2 and Figure 3.
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2 circuit optimization and simulation results
Since the digital television transmission system requires that the amplifying circuit must operate in a linear amplification state, it can be analyzed by a small signal S-parameter method. With the small signal S-parameter file provided by the device manufacturer, the small signal S-parameter simulation of the whole circuit can be performed by ADS to obtain small signal gain, port matching, isolation and stability factor K. Table 2 shows the S parameters of MRF373 at (Vce = 26V, Ic = 500 mA).
Circuit simulation with ADS does not meet the design requirements, and circuit optimization is required on this basis. When only the small-signal S-parameter is used as a model to design the power amplifier, the steps of circuit optimization are generally as follows: firstly, as far as possible, aiming at the matching of RL (load resistance of the maximum output power), optimizing and determining the value of the output matching circuit component; Optimize the component values ​​of the input matching circuit to improve the gain and input matching circuitry. It should be noted that before optimization, it is necessary to obtain the output circuit model as complete as possible, and then optimize it at the operating frequency to achieve the best match with RL. Figure 4 shows the simulation results of the amplifier circuit, and Figure 5 shows the final optimization results of the circuit.
3 test results
After a lot of experiments and repeated debugging, the measured results are shown in Figure 6. The driver stage amplifier operates in a linear state. As can be seen from the gain curve of Fig. 6, the gain in the entire frequency band is flat, about 12 dB, which is roughly the same as the simulation result. The return loss is less than 15 dB and the in-band standing wave ratio is less than 1.3. When the input power is 2 watts, the output power is 25W measured by the power meter, the signal amplitude is stable, and the cross-talk suppression is less than -35 dB. The indicators meet the system requirements, and are close to the indicators of amplifiers in similar digital TV transmitters abroad, and the cost is greatly reduced, which lays a foundation for the domestic development of digital TV transmitters in the future.
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