the design of the radio transmitter From Wikipedia , the free encyclopedia Contents [ hide ] 1 Design issues 2 Determination of the frequency Fixed frequency systems 2.1 2.2 systems with variable frequency Frequency multiplying 2.3 3 Adding modulation signal 3.1 modes AM 3.1.1 Low level and high level 3.1.2 Types of AM modulators 3.2 Related AM modes 3.2.1 Single-sideband modulation 3.2.2 vestigial sideband modulation 3.3 Morse 3.4 FM modes 3.4.1 Direct FM 3.4.2 FM indirect 3.5 Numerical methods 4 amplify the signal 4.1 Valves 4.2 Solid state 5 connecting the transmitter to the antenna 6 EMC 6.1 Leakage and RF shielding 6.2 Spurious 6.2.1 Harmonics 6.2.2 oscillators and mixing products 6.2.3 Instability and parasitic 7 Control and Protection 8 Power 9 See also 10 References A radio transmitter is an electronic device which, when connected to an antenna, an electromagnetic signals produced as in radio and television broadcasting, two-way communications or radar. Heating appliances , like a microwave oven, while similar in design, are not usually called transmitters, in that they use electromagnetic locally rather than passing it on to another place energy. Design problems [edit] A design of the radio transmitter must meet certain requirements . These include the operating frequency , modulation type , stability and purity of the resulting signal , the efficiency of energy consumption and the power level required to achieve the design goals of the system. [ 1 ] High-power transmitter can have additional constraints as regards the safety of radiation , the X-ray generation , and a protection against high voltages. [ 2 ] Typical design of the transmitter includes generating a carrier signal , possibly one or more frequency multiplying stages , a modulator , a power amplifier , and a filter network and adaptation to connect to an antenna . A simple transmitter may contain only an oscillator operating continuously coupled to an antenna system . More sophisticated transmitters allow better control of the modulation signal and improve the stability of the transmitted frequency. For example , the master oscillator power amplifier ( MOPA ) configuration inserts an amplifier stage between the oscillator and the antenna. This prevents changes in the load presented by the antenna from the change in the frequency of the oscillator. [3 ] Determining the frequency [edit] Systems fixed frequency [edit] For a fixed frequency transmitter one commonly used method is to use a resonant quartz crystal in a crystal oscillator to set the frequency. When the frequency must be variable , several options can be used . Systems with variable frequency [edit] An array of crystals - used to activate a transmitter to be used on several different frequencies, instead of a variable frequency system actually , it is a system that is attached to several different frequencies ( a subset of the foregoing ) . Variable frequency oscillator ( VFO ) PLL frequency synthesizer Direct Digital Synthesis frequency multiplication [edit] Frequency doubler a frequency doubler pushbutton . The output is set at twice the input frequency . frequency tripling At a triple frequency push-pull output is given three times the input frequency . While modern frequency synthesizers can make a stable clean signal through UHF , for many years , especially at higher frequencies, it was not practical to use the oscillator frequency of final output. For better frequency stability , it was common to multiply the frequency of the oscillator to the final , the required frequency. This was welcomed by assigning short wave amateur and marine bands in the harmonically related frequencies as 3.5 , 7, 14 and 28 MHz. Thus, a crystal or VFO could cover several bands. In simple equipment this approach is still used occasionally . If the output of an amplifier stage is simply adjusted to a multiple of the frequency at which the phase is driven, in step give a significant harmonic output . Many issuers have successfully used this simple approach. However, these more complex circuits do a better job . In a push-push stage, the output will contain only even harmonics. This is because the currents that could generate the fundamental and odd harmonics in this circuit are canceled by the second device. In a push-pull stage, the output will contain only odd harmonics due to the cancellation effect. Adding signal modulation [edit] The task of a transmitter is to transmit some form of information using a radio signal (carrier) that has been modulated to carry the intelligence. The RF generator in a microwave oven , electrosurgery , and induction heating are similar in design to the issuers , but generally not regarded as such by the fact that they do not intentionally produce a signal that will make a distant point . These RF devices are required by law to operate in an ISM band where interference to radio communications will not occur . Where the communication is subject one or more of the methods of incorporation of the desired signal in the following radio wave is used . AM modes [edit] When the amplitude of a radio frequency wave is changed in amplitude in a way that follows the modulation signal , usually voice , video or data , we amplitude modulation (AM ) . Low level and high level [edit] Modulation of a small low level audio stage is used to modulate a phase of low power. The output of this step is then amplified by a linear RF amplifier. The biggest drawback of this system is that the amplifier chain is less efficient , because it must be linear to preserve the modulation. C amplifiers therefore high efficiency class can not be used unless a Doherty amplifier , EER ( Envelope Elimination and Restoration ) or other methods of pre-distortion or negative feedback are used. High-level modulation using class C amplifiers , in an AM broadcast transmitter and the last step or final two steps are modulated , and all the previous steps can be driven at a constant level . When the modulation is applied to the tube end plate , a large audio amplifier is required for the modulation step , equal to 1/ 2 of the DC input power of the phase-modulated . Traditionally, the modulation is applied with a large audio transformer. However , many different circuits have been used for high- level modulation AM . See Amplitude Modulation . Types of modulators AM [edit] A wide range of different circuits have been used for the AM band. Although it is perfectly possible to create good models using electronic semiconductor valves circuits ( tube) are presented here. In general, the valves are able to easily produce RF beyond what can be achieved using solid state power. Most radio stations high power circuits under 3 MHz using the solid state, but the biggest stations than 3 MHz power still use valves. Plate AM modulators [ edit] Anode modulation using a transformer. The anode of the valve sees the vector sum of volts and audio anode voltage . A phase modulated in series. In modern transmitters , the series regulator will use PWM switching high efficiency. Historically , the regulator would have been a series of tube analog. Modulating the level plate is to vary the voltage on the plate ( anode ) of the valve so that it takes practically zero to twice the quiescent value . This will produce a modulation of 100 % and can be done by inserting a transformer in series with the high voltage to the anode so that the vector sum of these two sources , DC (and audio ) is applied power . One drawback is the size, weight and cost of the transformer and its limited audio frequency response , especially for very powerful transmitters . Alternatively a series regulator may be inserted between the DC power supply and the anode. The DC power supply provides twice the normal voltage anode sees. The controller can allow no or all of the voltage to be transmitted, or any intermediate value . The audio input to the controller works to produce the instantaneous anode voltage necessary to reproduce the modulation envelope . One advantage of the series regulator is that it can adjust the anode voltage to any desired value . Thus, the transmitter power can be easily adjusted , allowing the use of control of dynamic companies. The use of switching regulators PDM makes it very efficient, while the original analog regulators were also very inefficient and non-linear. PDM series modulators are used in semiconductor emitters as well, but the circuits are a little more complex , using push-pull or bridge circuits for the RF part . These simplified diagrams omit details such as filament, screen and supplies gate bias , and the screen and cathode connections to the RF ground. Screen AM modulators [ edit] Screen AM modulator . gate bias does not appear Payment in carrier (no audio ) will be a single stage RF amplifier when the voltage of the display is lower than normal for limiting the RF output to about 25 % of full power . When the phase is modulated screen potential changes and thus changes the gain of the stage . It takes much less power to modulate the audio screen, but the efficiency of the output stage is only about 40 % compared to 80% with a modulation of the plate. For this modulation screen reason has been used in low-power transmitters and is now effectively obsolete. AM Related modes [edit] Several derivatives are commonly used AM . These are For single sideband modulation [edit] Article: single sideband modulation SSB, or SSB -AM modulation single sideband full carrier , is very similar to the suppressed carrier modulation single sideband (SSB -SC) is used where it is necessary to receive audio on an AM receiver while using less bandwidth to double sideband AM . Due to the high distortion , it is rarely used . Or SSB -AM or SSB- SC are produced by the following methods. Filtering method [edit] The use of a balanced signal to a double sideband mixer is generated, it is then passed through a filter to leave only a very narrow bandwidth side of the strip . [4] By convention, it is normal to use the upper sideband (USB) in communication systems , with the exception of amateur radio when the carrier frequency below 10 MHz. It the lower sideband (LSB ) is normally used . Phasing method [edit] Progressive method of SSB generation A method for suppressing the generation of single-sideband signals , which uses a network requires a constant 90 ° phase shift on the audio signals in the audio range of interest . It was difficult with analog methods , but DSP is very simple. These audio outputs are each mixed in a linear medium with a balanced mixer . The supporting disc of one of the mixers is shifted by 90 ° . The outputs of the mixers are added to a linear circuit to obtain the SSB signal by phase cancellation of one of the sidebands . Connect the audio or carrier (but not both) the delayed signal from 90 ° to the other mixer reverse sideband , so either USB or LSB is available with a single DPDT switch. In vestigial sideband modulation [edit] Article: vestigial sideband modulation Vestigial sideband modulation (VSB , or VSB- AM) is a type of modulation system commonly used in analogue television systems . It is normal AM which was passed through a filter that reduces one of the sidebands . Typically , the components of the lower beyond 0.75 MHz or 1.25 MHz lower sideband , the carrier will be highly attenuated . Morse [edit] Morse code is usually sent using the on-off input of an unmodulated ( CW ) . No special modulator is required. This door interrupted can be analyzed as an AM-modulated carrier. On-off keying sidebands produced , as expected, but they are considered key clicks . Circuits formatting are used to switch the transmitter on and off gently instead of the moment, to limit the bandwidth of these sidebands and reduce adjacent channel interference . FM modes [edit] Angle modulation is the term corresponding to the modulation by changing the instantaneous frequency or phase of the carrier signal. True FM and phase modulation are the most commonly used forms of analog angle modulation. FM Direct [edit] Direct FM ( frequency modulation real ) is where the frequency of an oscillator is changed so as to impose on the modulation of the carrier wave . This can be done by using a voltage-controlled capacitor ( Varicap diode) in a quartz oscillator or frequency synthesizer. The frequency of the oscillator is multiplied with a frequency multiplier stage , or is translated upward using a mixing step, the output frequency of the transmitter. The amount of modulation is designated as the type , the quantity being the frequency of the carrier instantly moves away from the central carrier frequency. Indirect FM [edit] Solid state circuit indirect FM . FM indirect uses varactor to impose a phase shift (which is controlled by the voltage ) in a tuned circuit which is fed with an ordinary carrier. This is called phase modulation. In some solid-state circuits indirect FM, a high frequency drive is applied to the base of a transistor. The resonant circuit ( LC ) connected to the collector via a capacitor , includes a pair of varactor diodes. As the voltage applied to the varicap is changed, the phase shift of the output signal changes . Phase modulation is mathematically equivalent to the direct frequency modulation with a high-pass filter 6dB/octave applied to the modulation signal . This high-pass effect can be exploited or compensated using frequency appropriate shaping circuits in the audio stages before the modulator. For example , many FM systems employ pre-emphasis and de-emphasis for noise reduction , in this case , the equivalent high-pass phase modulation automatically provides pre-emphasis . phase modulators are generally capable of relatively small amounts of deflection but still linear , but all frequency multiplier stages are also increasing in proportion to the difference . Digital modes [edit] Transmission of digital data becomes increasingly important. Digital information can be transmitted by AM and FM modulation , digital modulation but often involves complex forms of modulation using aspects of both AM and FM . COFDM is used for DRM broadcasts. The transmitted signal is comprised of multiple carriers each modulated in both amplitude and phase. This allows very high data rates and allows a very efficient use of bandwidth . Digital or pulse methods are also used to transmit voice as in cell phones , or video as in terrestrial TV broadcasting . Text messaging early as RTTY allowed the use of Class C amplifiers , but modern digital modes require linear amplification . See also modulation sigma- delta ( ΣΔ ) Amplify the signal [edit] Valves [edit] For high power , high frequency systems , it is normal to use valves , refer to Valve RF amplifier how valve RF power stages work . The valves are electrically very strong , they can withstand loads that would destroy bipolar transistor systems in milliseconds. As a result, the amplifier valves can withstand detuning , lightning and surge better. However , they require a heated cathode which consumes energy and fail over time due to the loss of transmission or heating burnout . High voltages associated with valve circuits are dangerous to people . For economic reasons , the valves continue to be used for the final power amplifier for transmitters operating above 1.8 MHz with powers above 500 watts for amateur use and above about 10 kW for the use of broadcasting. The solid state [ edit] Solid state devices , either discrete transistors or integrated circuits are universally used for new designs of the transmitter to a few hundred watts. The more powerful transmitters lower level steps are all solid state. Transistors can be used at all frequencies and power levels, but since the release of individual devices are limited, high power transmitters must use multiple transistors in parallel, and the cost of devices and networks by combining required can be excessive. As new types of transistors become available and the price drops , solid state may eventually replace all tube amplifiers . Connecting the transmitter to the antenna [edit] The majority of modern transmission equipment is designed to operate with a load resistor powered via a coaxial cable of a particular characteristic impedance 50 ohms often . To connect the transmitter to the coaxial cable transmission line matching network may be necessary. Commonly an SWR meter and / or directional meter is used to verify the extent of correspondence between the antenna system and transmitter via the transmission line ( charger). A directional power meter shows the incident power , reflected power and SWR and often . Transmitters feeding a balanced transmission line will need a balun. This converts the single output of the transmitter to a higher output impedance balanced . Transmission systems shortwave generally use high power 300 ohm balanced line between the transmitter and the antenna. Amateurs often use 300-450 ohm balanced antenna feeders . See antenna tuner and balun for details of matching networks and baluns respectively . EMC [edit] Main article : Electromagnetic compatibility Many devices depend on the transmission and reception of radio waves for their operation. The possibility of mutual interference is great . Many devices are not designed to transmit signals can. For example, dielectric heating may contain 2000 Watt source of 27 MHz within it. If the machine works as expected then none of this RF power leak. However, if due to poor design or maintenance , it allows RF to escape, it becomes a transmitter or unintentional radiator. RF leakage and shielding [edit] Article: RF shielding All equipment using RF -mail should be inside a box shielded wire and all connections in or out of the box should be filtered to prevent the passage of radio signals. A common and effective method to do so for the son carrying supplies DC , 50/60 Hz , AC audio and control signals connections is to use a capacitor crossing, whose job it is to bypass any RF on the wire ground. The use of ferrite beads is also common. If a transmitter produces intentional interference, then it must be executed in a dummy load is a resistor in a shielded enclosure or may allow the issuer to generate radio signals without sending them to the antenna. If the transmitter continues to interfere in this test then there is a way by which the RF power is leaking out of the equipment and this may be due to poor protection. Such leakage is more likely to occur on the hardware or home that has been modified or had the covers equipment. RF leakage from microwave ovens , although rare, may occur due to faulty door seals , and can be a health hazard . Spurious radiation [edit] Early in the development of radio technology , it was recognized that the signals transmitted by the transmitters should be pure. spark gap transmitters were banned times better technology was available , because they give an output that is very broad in terms of frequency . The term spurious emissions refers to any signal that comes out of an issuer other than the useful signal . In modern equipment, there are three main types of spurious : harmonic -band mixer products that are not fully suppressed and the leakage of the local oscillator and other systems within the transmitter. Harmonics [edit] Are multiples of the operating frequency of the transmitter , which may be generated in any stage of the transmitter , which is not perfectly linear and must be removed by filtration. Avoid generation of harmonics [edit] This push-pull amplifier broadband uses a ferrite core transformer for coupling and matching . Both NPN transistors can be biased to class A , AB or C, and will always have very low to even multiples of the design frequency harmonics. Odd harmonics will be stronger , but still manageable . Class C will have the most harmonics. This amplifier uses a single ended circuitry anode closely listening to reduce harmonics when operating in Class AB or C. The difficulty in suppressing harmonics of an amplifier depends on the design. A push-pull amplifier has less harmonics than one circuit closed. A Class A amplifier has very little harmonic or class AB and class B plus C at most. In the typical class C amplifier , the resonant circuit will remove most of the harmonics, but low pass in these two examples , a filter will probably be necessary after the amplifier. Suppression of harmonics with filters [ edit] A simple low-pass filter suitable for harmonic reduction . In addition to the good design of the amplifier stages , the output of the transmitter must be filtered by a low pass filter to reduce the level of harmonics. Typically , the input and output are interchangeable and correspond to 50 ohms. values of inductance and capacitance varies with frequency . Many issuers pass through a filter matched to the frequency band used . The filter will pass the desired frequency and reduce the harmonics to acceptable levels . The harmonic output of a transmitter is better controlled using an RF spectrum analyzer or by adjusting a receiver for the different harmonics. If a harmonic falls on a frequency used by another communication service then this parasite transmission can prevent an important signal to be received . Sometimes additional filtering is used to protect a sensitive frequency range , for example, the frequencies used by aircraft or services involved in the protection of lives and property. Even if a harmonic is within the legally authorized harmonic should be further reduced. Oscillators and mixing products [edit] Simple but poor mixer. A diode is shown but any non-linear device may be used. A double balanced mixer using diodes matched . It is also possible to use active devices such as transistors or valves . When the mixing signals to produce a desired output frequency , the choice of the intermediate frequency and the local oscillator is important. If poorly chosen , a parasitic output can be generated. For example, if 50 MHz is mixed with 94 MHz to produce an output of 144 MHz, the third harmonic of the 50 MHz may appear in the output. This problem is similar to the visual response problem that exists in the receivers. A method of reducing the potential of this issuer default is to use mixers Balanced and double . A simple mixer will pass both input frequencies and all their harmonics with the sum and difference frequencies . If the single mixer is replaced by a balanced mixer , then the number of possible products is reduced. If the frequency mixer is less outputs of the task to ensure that the end result is clean will be easier . Instability and pests [edit] If a stage of a transmitter is unstable and is capable of swinging so it can start to generate RF is at a frequency close to the operating frequency or very different frequency . A good sign that this happens is that if a floor has an RF output power, even without being driven by an exciting stage. Output should increase smoothly as the power input is increased, but with class C , there will be a noticeable effect threshold. Different circuits are used for pest suppression in a good design. Appropriate neutralization is also important. Control and protection [edit] FT-817 transceiver control a button and two buttons control 52 different parameters. Single emitters such as RFID devices require no external controls. Simple monitoring transmitter may have only one power switch . Many issuers must have circuits that allow them to be switched off and the output power and frequency modulation levels adjusted or adjusted. Many modern multi -function transmitters can adjust many different parameters . Usually, these are under the control of the microprocessor via hierarchical menus , reducing the number of physical buttons. Often , a display screen provides information to the operator to help with adjustments. The usability of this interface is often a key factor in a successful design. Microprocessor issuers may also include software to prevent the cutoff frequency or other illegal operation. Transmitters using significant power or expensive components must also have protection circuits that prevent things such as overloading, overheating abuse or other circuits . circuits overload relay may include mechanical or electronic circuits. Simple fuse may be included to protect expensive components . arc sensors can turn off the transmitter when sparks or fire occur . With large high-power transmitters , protection circuits may include a significant portion of the design complexity and total cost . Power [edit] Some RFID take power from an external source devices when it queries the device , but most issuers or autonomous cells , or are mobile systems that generally operate directly from the 12 volt vehicle battery . Largest fixed transmitters require power network. The voltages used by an issuer will be AC and DC number of different values . Transformers or AC or DC power supplies are required to provide the voltage and current necessary for the operation of the various circuits . Some of these tensions must be regulated.
Posted on: Sat, 02 Nov 2013 13:19:36 +0000