0 Introduction The BGTV4152 all-solid-state TV transmitter is a new type of transmitter produced by Beiguang Technology Co., Ltd. In 2007, Shanxi Radio and Television Wireless Management Center put into use four of the transmitters in four directly affiliated alpine stations. After nearly two years of operation, the transmitter works stably and safely. Ensure safe, high quality broadcast and effective coverage of the launch pad.
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1 Working principle
1.1 Basic composition and working principle The BGTV4152 all-solid-state TV transmitter adopts modular design, advanced technology, complete interface, high quality index, stable performance and convenient maintenance. The transmitter is mainly composed of an exciter, a power splitter, a power amplifying module, a power synthesizer, a control system, a filter, a cooling system, and a power supply (as shown in FIG. 1).
The video and audio signals from the signal source are sent to the TV exciter to form a composite radio frequency signal. The transmitter uses a dual exciter configuration. The automatic switching of the double exciter is performed by the adder, and the exciter output signals are distributed into two paths, which are respectively sent to two 5 kW power amplifier cabinets. The power splitter divides the high frequency signal from the exciter into 8 channels and sends it to the power amplifier unit. After the high-frequency signal is amplified by the power amplifier unit, the power synthesis technology is used to output 5 kW of RF power. Then the high-frequency signals of the two 5 kW power amplifier cabinets are combined into one way, so that the final stage outputs 10 kW of synchronous top power. Finally, the filter and directional coupler output to the antenna system.
1.2 Working principle and technical characteristics of each functional module
1.2.1 Exciters Exciters are an important part of the transmitter. It is a functional part of the transmitter that performs video and audio signal processing and is modulated into a radio frequency signal. Its performance determines the performance and performance of the transmitter.
The video signal sent by the signal source is processed by amplification, delay, clamp, white level clipping, synchronous reproduction, etc., and then sent to the image intermediate frequency modulator for amplitude modulation. The amplitude modulated image IF signal is formed by the surface acoustic wave filter. After the sideband is output to the intermediate frequency corrector, the audio signal is sent to the audio circuit, and after being processed by pre-emphasis and filtering amplification, the audio signal is sent to the audio intermediate frequency modulator, and the modulated intermediate frequency signal is also output to the intermediate frequency corrector. The IF image has been modulated by the DG, the low-frequency nonlinear distortion correction and the frequency response adjustment circuit are combined with the audio IF signal in the IF corrector capsule to form a composite TV IF signal with an image sound power ratio of 10:1. The intermediate frequency signal of the TV is corrected by intermediate frequency, intermodulation correction, group delay correction, DP correction, ICPM correction, and then sent to the upper inverter to become the RF signal and filter out the out-of-band clutter, and the amplifier is amplified to 160 mW output. The ALC automatic level control circuit is provided in the exciter to make the exciter output power very stable. In addition to the stable power, there are two protection functions: First, once the video signal is interrupted, the image carrier is maintained at the black level amplitude, and is not pulled to the sync top level by the peak detection AGC in the corrector. The stage amplifier is overloaded. Second, there is a "soft start" function. When the power is turned on or the RF signal is turned on and off, the amplitude of the RF signal is always started slowly and then slowly, which avoids the impact on the power amplifier.
1.2.2 Power splitter The transmitter is powered by two 5 kW power amplifier cabinets with an output of 10 kW. Each 5 kW power amplifier cabinet consists of eight 750 W high power amplifier units. The power splitter distributes the signal from the exciter into eight outputs and sends them to the RF power amplifier for amplification. In addition to signal distribution, the power splitter also plays a role in matching the modules before and after.
1.2.3 Power amplifier module The power amplifier module uses LDMOS devices for power synthesis with an output power of 750 W. Its main features are:
(1) The imported MOSFET tube BLF368 is used for power amplification in the unit. The gain is high, the linearity is good, and it can operate under high reflection power. It has good temperature characteristics and the temperature coefficient is negative.
(2) The power amplifier tube is connected with a high-power circulator at the output end. When the power amplifier output is open, the power amplifier tube is not easy to be damaged; it is easy to maintain;
(3) Allow hot swapping;
(4) Perfect working status detection function. Not only provides the switching status and analog quantity of the working state and the fault signal to the transmitter microcomputer control system; meanwhile, the working state detecting interface is provided on the front panel;
(5) The phase trimming circuit ensures accurate power synthesis and reduces the synthesis loss. The block diagram of the working principle of the power amplifier module is shown in Figure 2.
The power amplifier consists of a three-stage power amplifier with power synthesis technology and a total gain of approximately 40 dB. The first stage pushes the power amplifier consisting of a wideband power amplifier module and two Class A power amplifier tubes. The distribution and synthesis use a 3 dB coupling line to achieve a good match between the input and output terminals. Has a very high linearity index. The second stage pushes the power amplifier to use two BLF368 power tubes, which work in Class A. The BLF368 power amplifier tube is a push-pull tube that requires balanced input and output. The input/output of the circuit is balanced/unbalanced by a semi-rigid coaxial cable, so that the input and output of the power amplifier tube are matched. Class A power amplifiers have good linearity indicators and large dynamic range, which lays a good foundation for digital TV. The final stage power amplifier uses the same type of LDMOS power tube as the second stage, sharing 4 combinations. Working in class AB, the combined output power is 750W. The first level two synthesis is synthesized by strip line orthogonal bridge, the second stage and the third stage are synthesized in the same direction; the corresponding first and second stages of the distribution network adopt the same phase distributor. Matching is also achieved with balanced/unbalanced conversion at the input and output of each tube.
Each power amplifier module is equipped with a separate switching power supply.
Each power amplifier module is equipped with protection devices such as over-excitation, over-heat, overload, over-voltage, and over-reflection protection, and the work is stable and reliable.
1.2.4 Power Synthesizer After the high-frequency signal is amplified by the power amplifier unit, the power synthesis technology is used to combine the high-frequency signals of several power amplifiers into one way, so that the final output meets the required synchronous top power. It consists of a passive network with absorbing resistors that balance them by absorption when the output power of each power amplifier is unbalanced.
1.2.5 Harmonic Filters Harmonic filters are used to suppress one or more harmonic components so that the out-of-band harmonic components meet the specifications.
1.2.6 Directional Coupler The output directional coupler detects the incident and reflected waves of the output signal and feeds it back to the control system as a power indication and fault detection signal.
1.2.7 Monitoring of Transmitter This transmitter has two sets of control, protection and detection systems. One set is a microcomputer monitoring system, which can realize manual/automatic opening and closing procedures and can monitor output power, reflected power and each power amplifier module. The current, temperature and other parameters of the inner tube. Implement fault detection and alarm, record, and print. It can also be equipped with a PC to remotely control and remotely operate the transmitter. The system also records the historical status information of the transmitter and stores it in the data table file for users to view when needed, which facilitates the maintenance of the equipment room and system, and also facilitates the maintenance of the transmitter; the other is the traditional switch control, which is a microcomputer. Controlled backup. Switching machine operation is also possible.
2 Transmitter use and maintenance
2.1 Transmitter normal operating procedures
2.1.1 Inspection before start-up After the first installation or repair, the machine should be inspected before starting up. First check whether the power supply and the safety ground wire are correct; check whether all the RF connection cables are connected and secure; check whether the ventilation system is normal; especially the image output load or antenna feeder. In short, you should check that the wiring of the system is correct before turning it on.
2.1.2 Power-on procedure First connect the three-phase 380 V power supply to the unit correctly. Before turning on the power, first place the air switch QT in the "off" position, and then determine if the 380 V voltage is normal. Place the QT in the "closed" position. At this time, the +24V/+12V regulated power supply works in the exciter, the monitoring unit sends power, and the whole machine is in the state to be turned on. The exciter works normally.
Under the condition that the video signal and the audio signal are not sent, the "video alarm" in the image modulator is red, and the "audio" red light of the sound modulator is on. The test meter installed on the front panel of the exciter has no indication value. The frequency offset test header on the front panel of the exciter is also displayed without frequency offset. If both the video signal and the audio signal are sent to the IF modulation box, the "video alarm" in the image modulator is off, the "output" green light is on, the audio modulator "audio" is off, and the exciter is mounted on the front panel. The meter has indication values ​​indicating the degree of modulation (around 87.5%) and frequency offset (about 50 kHz).
Press the power button to observe whether the rotation of each fan is normal and whether there is any abnormal sound. Whether the 32 V DC regulated power supply and the 750 W power amplifier module work normally, the power reaches the normal value after a few seconds.
After the above operation steps are completed, the machine enters the normal working state. Press the output power button. Look at the power meter above the LCD screen of the machine to indicate 10 000W synchronous top output power. Press the reflection output button, the table indication is small, indicating that the antenna feeder system is also normal. In the computer room, the TV receiver is used to view the effect. If the image and sound are good, the machine is working normally.
2.1.3 Transmitter Shutdown Procedure Press the shutdown button. At this time, the power meter has no indication value. The indicator light of the power amplifier module does not light, the fan stops blowing air cooling, and the 32 V DC regulated power supply does not work. Finally, cut off the 380 V power supply for this unit.
2.2 Use and maintenance In order to ensure the safe operation of the machine, to prevent machine failure and component damage, and to ensure the safety of the check-in staff, the following aspects should be noted.
(1) The ambient temperature and air volume of the transmitter should be guaranteed to be cleaned regularly and the main indicators should be checked.
(2) Due to vibration factors such as blowing, the fasteners can be loosened and should be regularly tested and reinforced.
(3) During the operation of the machine, it is not easy to pull out the power amplifier box or disconnect the high-frequency cable. Otherwise, it will cause abnormal work or even partial damage to the equipment.
(4) Careful maintenance records should be taken to help prevent potential hazards and analyze equipment failures.
(5) Be sure to connect the antenna feeder system to ensure a good match (the standing wave ratio is less than 1.2).
(6) When operating the FET, be sure to take anti-static measures.
3 Conclusion
After more than two years of operation, the transmitter performance is stable, and no major faults have occurred. The system indicators have also reached the ministerial level A standard, and the wireless coverage in each coverage area is good. All solid-state transmitters have obvious advantages such as cost saving, small maintenance, safety and reliability, but the technical requirements for technicians are greatly improved in use and maintenance, and more standardized operating procedures and maintenance management systems are required. We must strengthen business learning in peacetime. , timely review the maintenance experience to ensure proper maintenance and operation of the transmitter to ensure safe and high quality broadcast.
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