Keywords: TV transmitter; PA; composition and design
1 Introduction
China’s major television stations in the analog transmitter is still the basic stage, the digital transmitter to the development work has just started. In the digital transmitter in general, digital television signals modulated by COFDM mode output frequency analog signals, amplified by the conversion into the part. The modulation, including IF-FT (8K) and IFFT (2K) modes, respectively 6817 and 1705 carriers, each carrier frequency spacing between the very close, so very easy to fall-modulation signal band, causing intermodulation distortion, linear amplifier will seriously affect poor digital transmitter of the key indicators MER (Modulation Error Rate) and the Shoulder (with shoulder). Therefore, the requirement to enlarge part of the linear state should be working on, to affect the signal quality. This content is produced according to the problem, study the basic television transmitter power amplifier module and the design of the corresponding program.
2 TV transmitter power amplifier overview
2.1 The basic requirements for TV transmitter power amplifier
Television broadcasting on the most basic requirement is that the transmitter power amplifier power gain should be high enough. Power amplifier is a broadband amplifier, can cover the entire UHF band, the launch of the channel changes without the need for by-channel adjustment, power amplifier modules at all levels of the most advanced solid-state amplification of imports. Basic requirements are: amplifiers with high linearity, wide dynamic range that the digital modulation signal peak in the dynamic range of the transmitter when there is still a good linearity; even if the defect in a state of the module, the transmitter can still broadcast; transmitter frequency should be high enough precision and frequency stability, low phase noise, to ensure that signals are transmitted at the lowest possible bit error rate and SNR.
2.2 TV Transmitter Power Amplifier Specifications
DVB_T terrestrial digital television transmitters in the conventional power amplifier of the specific indicator is as follows:
Frequency range: 470MHz to 860MHz; amplification by: AB linear amplification; input impedance: 50Ω; Output impedance: 50Ω; return loss: ≥ 15dB; input power: 1W (nominal); output power: 1000W (maximum); power input : 120VDC ± 15%; Power consumption: 1800W (maximum); RF input connector: N Head (Yam head); RF output interface: DIN7/16 head; Protection: RF reflected power is too large (≥ 50W), over current, overheating ; Cooling: Forced air cooling; ambient temperature: -5 ℃ to 45 ℃; Altitude: Up to 3000m; Weight: about 15kg.
2.3 Composition of TV transmitter power amplifier
DVB_T terrestrial digital TV transmitter in the main 500 (W) and 1000 (W), of which 500 (W) with two power amplifier modules, 1000 (w) has four power amplifier modules. Power amplifier generally includes the following nine modules: RF control module; 100W power amplifier module (RF drive level); four-way power divider; 270W power amplifier module (radio frequency amplifier stage); four-way power combiner; power supply soft start; 3.5kw DC / DC switching power supply; control and display module; cooling system.
3, TV transmitter power amplifier design
3.1 RF Control Module
RF amplifier input signal is then sent to the module, after processing, then amplified, and then sent to the 100W power amplifier module (RF drive level). Specific process, according to chronological order as follows:
(1) gain control circuit. Adjust amplifier gain adjustment on the front panel potentiometer, the control module through the power amplifier to produce gain control command. The command sent to the module. Adjust the gain of the RF signal. Adjustment range: +1-3 dB.
(2) phase adjustment circuit. Conditioning amplifier phase adjustment on the front panel potentiometer, the control module through the power amplifier to generate phase adjustment command. The command sent to the module, through the MB coupled device to adjust the RF signal phase. Adjustment range: +35-35 degrees.
(3) amplifier. The 3dB coupler RF signal after divided into 2-way, from MRF6522-10 Zoom, and then by 3dB coupler synthesis. These include: input RF signal power measurement, radio frequency drive signal power measurement and pre-bias control.
3.2 270W (final stage) amplifier module
As the digital signal is a wideband signal transmitter, power amplifier module has therefore asked a very good linearity and gain evenly. Class AB power amplifier module consists of two BLF861 LDMOS, bias circuits and matching networks, etc., and use the 3dB directional coupler points as second-class power divider and power combiner. 270W function module circuit diagram shown in Figure 1:
3.3 Control Module
The role of control modules: acquisition sampling signal from the RF module, for processing, production control amplifier source of the command module; the amplifier’s status through the display board, showing the front panel of the power amplifier; to accept commands from the controller, while the work of the state to the control amplifier to go.
Four-way splitter and four 3.4 Synthesizer
Four-way splitter and four synthesizers are realized using microstrip power distribution or power combining. 100W Power Amplifier Module for Quad splitter to the RF amplified output signal, assigned to four 270W input power amplifier module, power amplifier module as the excitation signal. Taking into consideration that each side of the isolation between China and Japan as well as the absorption of reflected power, installation of the balance resistor. In order to reduce the reflected power, designed multi-stage 1 / 4 of the impedance converting circuit.
Four synthesizer to 4 270W Power Amplifier Module RF output signal after amplification, were synthesized. In order to reduce the reflected power, also designed the impedance converting circuit. Four synthesizer input connector through L29, sent to the power combiner.
3.5 Power Supply System
Switching power supply system, will come from the power supply +140 VDC, through the switch transformation method, into a power amplifier module required +32 VDC. Meanwhile, the current and voltage input and output sample signals to the control panel, under the control board’s order, to provide overcurrent protection and overvoltage protection. Switching power supply system block diagram shown in Figure 2:
4 Conclusion
With the further promotion of digital television and popular demand for terrestrial digital television transmitters will be increasing. The current most of the domestic digital TV transmitter also uses imported parts, the transmitter power amplifier is the core component, the localization of the power amplifier will greatly reduce costs. This is the main power in the terrestrial digital television transmitters R & D projects, the completion of the TV transmitter power amplifiers analysis and design, and smooth to the amplifier relevant indicators, and compared to similar foreign par. However, there is still insufficient and need to improve the stability and reliability areas. Moreover, the above R & D only remain in the experiment stage. Can large-scale production, quality assurance, and still meet the corresponding indicators, but also a process that needs to grope in the production and summary.
R & D for the TV transmitter, the development of digital television transmitters focus transmitter power amplifier. TV transmitter in the clear demands of the power amplifier based on the analysis of the power amplifier and the main component of technical indicators and further study of the television transmitter power amplifier design, for our television transmission system for the development of certain research value.
Keywords: TV transmitter; PA; composition and design
1 Introduction
China’s major television stations in the analog transmitter is still the basic stage, the digital transmitter to the development work has just started. In the digital transmitter in general, digital television signals modulated by COFDM mode output frequency analog signals, amplified by the conversion into the part. The modulation, including IF-FT (8K) and IFFT (2K) modes, respectively 6817 and 1705 carriers, each carrier frequency spacing between the very close, so very easy to fall-modulation signal band, causing intermodulation distortion, linear amplifier will seriously affect poor digital transmitter of the key indicators MER (Modulation Error Rate) and the Shoulder (with shoulder). Therefore, the requirement to enlarge part of the linear state should be working on, to affect the signal quality. This content is produced according to the problem, study the basic television transmitter power amplifier module and the design of the corresponding program.
2 TV transmitter power amplifier overview
2.1 The basic requirements for TV transmitter power amplifier
Television broadcasting on the most basic requirement is that the transmitter power amplifier power gain should be high enough. Power amplifier is a broadband amplifier, can cover the entire UHF band, the launch of the channel changes without the need for by-channel adjustment, power amplifier modules at all levels of the most advanced solid-state amplification of imports. Basic requirements are: amplifiers with high linearity, wide dynamic range that the digital modulation signal peak in the dynamic range of the transmitter when there is still a good linearity; even if the defect in a state of the module, the transmitter can still broadcast; transmitter frequency should be high enough precision and frequency stability, low phase noise, to ensure that signals are transmitted at the lowest possible bit error rate and SNR.
2.2 TV Transmitter Power Amplifier Specifications
DVB_T terrestrial digital television transmitters in the conventional power amplifier of the specific indicator is as follows:
Frequency range: 470MHz to 860MHz; amplification by: AB linear amplification; input impedance: 50Ω; Output impedance: 50Ω; return loss: ≥ 15dB; input power: 1W (nominal); output power: 1000W (maximum); power input : 120VDC ± 15%; Power consumption: 1800W (maximum); RF input connector: N Head (Yam head); RF output interface: DIN7/16 head; Protection: RF reflected power is too large (≥ 50W), over current, overheating ; Cooling: Forced air cooling; ambient temperature: -5 ℃ to 45 ℃; Altitude: Up to 3000m; Weight: about 15kg.
2.3 Composition of TV transmitter power amplifier
DVB_T terrestrial digital TV transmitter in the main 500 (W) and 1000 (W), of which 500 (W) with two power amplifier modules, 1000 (w) has four power amplifier modules. Power amplifier generally includes the following nine modules: RF control module; 100W power amplifier module (RF drive level); four-way power divider; 270W power amplifier module (radio frequency amplifier stage); four-way power combiner; power supply soft start; 3.5kw DC / DC switching power supply; control and display module; cooling system.
3, TV transmitter power amplifier design
3.1 RF Control Module
RF amplifier input signal is then sent to the module, after processing, then amplified, and then sent to the 100W power amplifier module (RF drive level). Specific process, according to chronological order as follows:
(1) gain control circuit. Adjust amplifier gain adjustment on the front panel potentiometer, the control module through the power amplifier to produce gain control command. The command sent to the module. Adjust the gain of the RF signal. Adjustment range: +1-3 dB.
(2) phase adjustment circuit. Conditioning amplifier phase adjustment on the front panel potentiometer, the control module through the power amplifier to generate phase adjustment command. The command sent to the module, through the MB coupled device to adjust the RF signal phase. Adjustment range: +35-35 degrees.
(3) amplifier. The 3dB coupler RF signal after divided into 2-way, from MRF6522-10 Zoom, and then by 3dB coupler synthesis. These include: input RF signal power measurement, radio frequency drive signal power measurement and pre-bias control.
3.2 270W (final stage) amplifier module
As the digital signal is a wideband signal transmitter, power amplifier module has therefore asked a very good linearity and gain evenly. Class AB power amplifier module consists of two BLF861 LDMOS, bias circuits and matching networks, etc., and use the 3dB directional coupler points as second-class power divider and power combiner. 270W function module circuit diagram shown in Figure 1:
3.3 Control Module
The role of control modules: acquisition sampling signal from the RF module, for processing, production control amplifier source of the command module; the amplifier’s status through the display board, showing the front panel of the power amplifier; to accept commands from the controller, while the work of the state to the control amplifier to go.
Four-way splitter and four 3.4 Synthesizer
Four-way splitter and four synthesizers are realized using microstrip power distribution or power combining. 100W Power Amplifier Module for Quad splitter to the RF amplified output signal, assigned to four 270W input power amplifier module, power amplifier module as the excitation signal. Taking into consideration that each side of the isolation between China and Japan as well as the absorption of reflected power, installation of the balance resistor. In order to reduce the reflected power, designed multi-stage 1 / 4 of the impedance converting circuit.
Four synthesizer to 4 270W Power Amplifier Module RF output signal after amplification, were synthesized. In order to reduce the reflected power, also designed the impedance converting circuit. Four synthesizer input connector through L29, sent to the power combiner.
3.5 Power Supply System
Switching power supply system, will come from the power supply +140 VDC, through the switch transformation method, into a power amplifier module required +32 VDC. Meanwhile, the current and voltage input and output sample signals to the control panel, under the control board’s order, to provide overcurrent protection and overvoltage protection. Switching power supply system block diagram shown in Figure 2:
4 Conclusion
With the further promotion of digital television and popular demand for terrestrial digital television transmitters will be increasing. The current most of the domestic digital TV transmitter also uses imported parts, the transmitter power amplifier is the core component, the localization of the power amplifier will greatly reduce costs. This is the main power in the terrestrial digital television transmitters R & D projects, the completion of the TV transmitter power amplifiers analysis and design, and smooth to the amplifier relevant indicators, and compared to similar foreign par. However, there is still insufficient and need to improve the stability and reliability areas. Moreover, the above R & D only remain in the experiment stage. Can large-scale production, quality assurance, and still meet the corresponding indicators, but also a process that needs to grope in the production and summary.
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