Digital Tv Transmitter Power Amplifier Design (with Photos)
Digital television terrestrial broadcasting technology uses digital compression technology, the same clarity and sound quality, the user can receive the number of programs increased from 4 to 6 times. The same channel, which can transmit additional data and other information, and interference resistance, and covering the region near-field far-field reception is almost the same, therefore, digital TV has received extensive attention.
Some European and American countries, digital television technology, a 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, localization of digital television transmitters broadcast television industry in China inevitable trend of development.
Digital TV transmitter power amplifier is an important part. Typically, digital TV transmitters in the way of the signal modulated by COFDM output frequency 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, linearity, stability has a higher demand. Requirements on the transmitter power amplifier to work at a higher linear state, the gain stability.
Magnified part of the launch system 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, its frequency band in the 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 design 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 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 the multi-carrier signal;
Bias circuit is simple, without complex temperature compensation with positive, active low-impedance bias circuitry.
LDMOS process technology combines the BPT and GaAs process. 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 role of 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.
