Digital Television Transmission And The Power Amplifier (photos) - Digital Tv, Amplifier,

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Digital television terrestrial broadcasting technology uses digital compression technology, the same clarity and sound quality, the user can receive the number of programs increased 4 ~ 6 times. The same channel, which can transmit additional data and other information, and the anti-interference ability, covering the region near field and far field of the reception is almost the same, therefore, digital television received wide attention.



Some European and American countries, digital television technology, the more in-depth research, has developed the perfect performance of digital television transmitters. Digital TV technology of our relatively late start, still in the experimental stage. To reduce costs, domestic production of digital television transmitters broadcast television industry in China inevitable trend of development.



Digital TV transmitter power amplifier is an important part. Typically, digital television signal transmitter in the output by the COFDM modulated IF analog signals, amplified by the conversion into the part. The modulation includes IFFT (8M) and IFFT (2M) modes, respectively, 6817 and 1705 from the carrier composition. Each carrier frequency spacing between the very close, so easy to fall-modulation signal frequency band, causing intermodulation distortion. Digital television transmitters than conventional types, linear, stability and so has a higher demand. Requirements on the transmitter power amplifier to work at a higher linear state, the gain stability.



Amplification of part of the launch system is divided into motivation and the main amplifier. One incentive part broadband power amplifiers, terrestrial digital television transmission to ensure the normal stability, need to have good stability and reliability of its work frequency band 470MHz ~ 860MHz, working status for the AB class; required gain greater than 10dB, intermodulation inhibition is less than-35dB, the noise power density greater than 130dBc/Hz. In this paper, the latest LDMOSFET devices, and balance the amplification circuit? Design of digital TV transmitter in the driver stage power amplifier, optimized and debug, to meet the system requirements.



1 Power Amplifier 1.1 magnification power amplifier chip selection Motorola LDMOSFET

this paper, the amplification device MRF373 as a power amplifier chip. The chip linear gain and output capability relative to BJT devices have more to upgrade, so that the transmitter greatly improved reliability and maintainability. And the traditional tube amplifiers UHF bipolar compared, LDMOSFET has the following significant advantages:



At high standing wave ratio (VSWR = 10:1) case work;

High gain (typical value 13dB); Saturation curve smoothing is beneficial to both analog and digital RF signal amplification;



Can withstand over a large drive power, especially for DVB-T COFDM modulation in multi-carrier signals;

Bias circuit is simple, without complex temperature compensation is active with low-impedance bias circuit.

LDMOS process technology combines the BPT and GaAs technology. And standard MOS technology is different in the device package on, LDMOS not used beryllium oxide BeO insulating layer, but directly Yingjie in the substrate, thermal conductivity, improved heat resistance of the device, greatly extend the life of the device . Since the negative temperature effect LDMOS tube, its leakage current in the heat automatically when both flow and does not like being bipolar tube temperature effect on the formation of the collector current local hot spots, so easy to damage the pipe. So LDMOS tube greatly enhance the load mismatch and over-stimulate the appetite. Also thanks to the automatic current sharing control LDMOS effect, the input - output characteristic curve in 1dB compression point (the use of large-signal saturation section), the more moderate bent, so the dynamic range was increased, benefit analog and digital TV RF signal amplification. LDMOS small signal amplification in approximately linear time, little or no distortion, very much simplifies the circuit. DC gate current of MOS devices is almost zero, the bias circuit is simple, without complex temperature compensation is active with low-impedance bias circuit.



1.2 select and compare circuit

Small-signal S parameters can be used for A amplifier, signal amplification is required basic limitations of linear region in the transistor. However, related to power amplifiers, due to amplifier operating in nonlinear regions, so the small signal approximation is usually ineffective. At this point must be pursued in the large-signal S parameters of the transistor, or resistance to reasonable design effect.



General, distortion coefficient A minimum working conditions, good linearity. However, in high power applications, due to the efficiency of working conditions A low (50%) does not apply. Class B push-pull amplifier using the circuit form, can be similar to the linear amplifier and the A index.



Push-pull circuit form two independent and free from any internal connections constitutes a single-tube amplifier, through the two balun for power distribution and synthesis of the vector. Barron itself as variable resistance characteristics, thus greatly reducing drag ratio change caused by the difficulties of impedance matching and balun for the even harmonics have a very good inhibition. However, as Barron both sides of the interval is too small, a larger two-way interaction, the application balun amplifier was used, and the circuit's input and output VSWR are relatively poor. In this paper, the form of a balanced amplifier structure shown in Figure 1 below. It works with the balun circuit similar to, but because the application of 3dB bridge, making two-way RF signal isolation between the good, beneficial to the two ports of the match. Relative to the single-tube amplifier structure, its advantages, such as Table 1.
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This article was published on 2011/01/24