Tuesday, May 25, 2010
Tuesday, May 18, 2010
Triangular Wave Generator Circuit
This is a circuit for a constant amplitude triangular-wave generator. A variable frequency triangular wave whose amplitude is independent of frequency is provided by this circuit. This circuit is based on LM101.
Triangular Wave Generator Circuit
As a reset circuit, the generator embodies an integrator as a ramp generator and a threshold detector with hysteresis. There is no further explanation is needed because the integrator has been described in a previous section. It’s a latch circuit with a large dead zone, on that case, the threshold amplifier detector is similar to a Schmidt Trigger. By using positive feedback around an operational amplifier, we can implement this function. The positive feedback network provides a voltage at the non-inverting input which is determined by the attenuation of the feedback loop and the saturation voltage amplifier when the amplifier output is in either the positive or negative saturated state. The voltage at the input of the amplifier must be caused to change polarity by an amount in excess of the amplifier input offset voltage to cause the amplifier to change states. When this is done, until the voltage at its input again reverse, the amplifier saturates in the opposite direction and remains in that state. By examining the operation with the output of the threshold detector in the positive state, the complete circuit operation may be understood. To cause a current I+ to flow, we have to apply the detector positive saturation voltage to the integrator summing junction through the combination R3 and R4.Then, a negative-going ramp with a rate of I+/C1 volts per second until its output equals the negative trip point of the threshold detector is generated by the integrator. After that, at the integrator summing point , the threshold detector changes to the negative output state and supplies a negative current, I-. A positive-going ramp with a rate of I-/C1 volts per second is generated by the integrator until its output equals the positive trip point of the threshold detector where the detector again changes output state and the cycle repeats. R3, R4, C1, and the positive and negative saturation voltages of the amplifier A1 determine the triangular-wave frequency. Amplitude is determined by the threshold detector saturation voltages and the ratio of R5 to the combination of R1 and R2. If the detector has equal positive and negative saturation voltages, positive and negative ramp rates are equal and positive and negative peaks are equal. If the inverting input of the threshold detector, A1, is offset with respect to ground, the output waveform may be offset with respect to ground. If the detector is clamped with matched Zener diodes as shown in following figure, the generator may be made independent of temperature and supply voltage. If power supply impedance causes oscillation during its transition time, the detector may be compensated and the integrator should be compensated for unity-gain. For maximum symmetry, the current into the integrator should be large with respect to Ibias and offset voltage should be small with respect to Voutpeak.
[Schematic circuit source: National Semiconductor Application Note]
Linearized Platinum RTD Signal Conditioner
The offsetting and scaled gain are provided by A2. Therefore, the A2’s output will swing 0.00V to 10.000V for 0.00 C to 100.00 C temperature swing at the RTD. The noise pick-up is limited by the 1uF capacitor. Returning a small portion of the circuit’s output to A1 will correct the 0.4 C non linearity error due to the RTD’s imperfect response. This circuit is calibrated by substituting the sensor with a precision decade box. The deviation of the each sensor is different depends on the manufacture that produced it. The purity of the material fix the gain slope of the platinum. [Source: Linear Technology Application Note]
TV transmitter power amplifier design of the composition
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.
Monday, May 17, 2010
History Of Cable Television
Plasma Television
The modern days can be truly called �digital age� as most of the everyday gadgets like TVs, cameras, microwaves, washing machines etc. are all microprocessor based � the tiny devices that store a host of instructions and control the operation of the appliance. The capture of images through camera, transmission and processing of images are all digitalized now. Consumers have widely adapted to miniaturization of the gadgets that digitalization has brought about whether it is digital camera, laptop, cell phone etc. They have come to expect better quality, ease of use, portability and enhanced features. Their older counterparts like the manual camera or the huge computer system or the bulky radio walkie-talkies have all been dumped into history. Everyone prefers to use the technologically advanced high tech models that are more convenient to use. Even children demand buying latest technology gadgets and are at ease using them.
Televisions are no exceptions to the digitalization wave that is sweeping across various industry and plasma televisions have come to replace the old bulky cathode ray tube based TV sets. Better quality TVs are now available everywhere that are increasing the market for TVs with more buyers. The plasma televisions have been the result of years of development of the plasma technology now integrated with digital processing of signals inside the TV. Before buying a plasma television set, the user has to be aware of the pros and cons of the new technology as well as the features that come with the plasma TV. One has to be an informed buyer � knowing the advantages and disadvantages, if any of the new technology. Some of them are discussed in the paragraph below.
The plasma televisions have higher resolution than conventional cathode ray tube TVs, more true to life colors due to the advanced plasma panels and digital signal processing inside the TV. They also have flat screens that eliminate distortions in the image, less strain of viewing on the eyes and thin design saving storage space. The plasma television screen has higher aspect ratio and brightness that is uniform throughout. The plasma panel has tiny picture elements � pixels for short that are equally energized giving the screen uniform brightness. Wider viewing angles, capability to accept wide variety of video inputs, immunity to influence of nearby magnetic fields like loud speakers on picture quality, light in weight etc. are some of the other advantages of plasma televisions.
The plasma TVs are truly a consumer�s delight with superior picture quality, ease of use and handling and enhanced features. Engineers are continuing to improve plasma television technology through research to meet high customer expectations and making it more user friendly with increasing number of features.
First 3D television sets go on sale in Britain
The 3D TV revolution hit Britain yesterday as the first sets go on sale, in one of the biggest changes to home viewing since the introduction of colour television.
Samsung won the race to sell the first 3D hardware and it's C7000 47-inch television will hit the shelves today for £1,800.
The television will be available from flagship John Lewis stores, but in order to experience the technology consumers will also need to shell out £200 for special 3D glasses and a cable.
A woman tests out the new Samsung 40-inch 3D television in John Lewis, the first store to offer the new technology
Viewers will also need a Blu Ray player in order to have anything to watch, as the visual effect won't be available on TV until June, when Sky launches its 3D channel.
Manufacturers hope blockbuster 3D movies such as Avatar and sports events such as the 2010 World Cup in South Africa will help drive demand.So far, 3D screenings of Premier League matches in pubs have been successful - although fans have been known to use half-time to 'get their eyes back'.
However, consumers may be deterred by a lack of programmes and the need to use special eyewear, a reason that thwarted previous attempts to push 3D.
Samsung's version will soon be followed by rival versions. Sony is already taking pre-orders in Harrods for its sets, due to launch in June, while LG and Panasonic sets will arrive next month.
Curry's and Comet will stock Korean giant UE40's C700 across Britain from next week.
John Kempner, buyer for vision at John Lewis said, '3D TV is an incredibly exciting technology, and once again John Lewis is first to market. Manufacturers like to use us as their launch partner because our customers are always early adopters of new technology.'
Customers who want to get their hands on the Samsung 3D TV will also need to buy £200 special glasses
However, the store admitted that at present there is very little to actually watch - although purchasers will also get a copy of animated Dreamworks film Monsters vs Aliens on DVD.
'It is worth remembering that even though 3DTV content is limited at the moment, this new breed of TV provides fantastic picture quality for regular 2D viewing, so a 3DTV is a great investment purchase,' said Mr Kempner.
Three-dimensional sets sold out in their first week on sale in the U.S..
Panasonic became the first major TV maker to sell 3D sets in America on March 10, selling a 50-inch HD TV with a pair of glasses and a 3D Blu-ray player for just under $3,000.
Samsung, the world’s largest TV maker, quickly followed on March 14 with a 55-inch 3D model for $3,300. Sony plans to start selling 3D Bravia TVs from June.
First Television Set Article
LCD televisions use LCD panels for displaying the images � thin and flat devices consisting of a no. of pixels or picture elements that illuminate depending on the signal passed through them. These pixels are made up of thin film transistors (TFT). The TFT is a kind of field effect transistor containing layers of metallic, dielectric and semiconductor material. The LCD technology functions differently from the LED (Light emitting Diodes), plasma or cathode ray tube technology for displaying images.
Early LCD technology had drawbacks like ghost images when images are moving fast, narrow viewing angles that made it hard to view even when positioned slightly away from the monitor, etc. but they have been overcome now. They compare favorably with plasma panel technology and have become very popular. LCD technology is also more energy efficient and less bulky than cathode ray tube televisions. The images are more pleasing and less strenuous to the eyes than CRT based displays. The images are distortion free and are of higher quality than the latter.
The panels can be conveniently mounted on walls thus saving space. The display sizes were small in the early years of development of LCD technology but large size LCD television screens have been recently manufactured by companies like Sony, Sharp, Samsung, LG-Philips, Westinghouse Digital etc. Seventh generation LCD panels as large as 45 inches have been available since October 2004, while even larger sizes like 65 inches have been introduced by Sharp Corporation, 82 inches by Samsung in March 2005 and LG-Philips introduced 100 inches panel in August 2006.
Recent LCD television is adaptable to the variety of display standards that are in vogue around the world today like NTSC, PAL and SECAM. They also have built in electronic transformers to handle various voltages like 110 and 230V, universal adapter plugs etc. The LCD panels apart from TV can be used as computer monitors that work with VGA and DVI signals and have wide variety of resolutions. The market for LCD television is expected to grow considerably in the next few years and the cost is expected to come down with various manufacturers investing heavily for producing these panels.
Subscribe to:
Posts (Atom)
