radio transmitter
In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna with the purpose of signal transmissio ...
or just ''transmitter'' is an
electronic device
Electronics is a scientific and engineering discipline that studies and applies the principles of physics to design, create, and operate devices that manipulate electrons and other electrically charged particles. It is a subfield of physics and ...
which produces
radio wave
Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...
electromagnetic wave
In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength, ...
GHz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...
. The transmitter itself generates a
radio frequency
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
alternating current
Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in w ...
, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves. Transmitters are necessary parts of all systems that use
radio
Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...
:
radio
Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...
and
television broadcasting
A television broadcaster or television network is a telecommunications network for the distribution of television content, where a central operation provides programming to many television stations, pay television providers or, in the United ...
,
cell phone
A mobile phone or cell phone is a portable telephone that allows users to make and receive calls over a radio frequency link while moving within a designated telephone service area, unlike fixed-location phones ( landline phones). This radio ...
s,
wireless network
A wireless network is a computer network that uses wireless data connections between network nodes. Wireless networking allows homes, telecommunications networks, and business installations to avoid the costly process of introducing cables int ...
s,
radar
Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
,
two way radio
A two-way radio is a radio transceiver (a radio that can both transmit and receive radio waves), which is used for bidirectional person-to-person voice communication with other users with similar radios, in contrast to a broadcast receiver, whi ...
s like
walkie talkie
A walkie-talkie, more formally known as a handheld transceiver, HT, or handheld radio, is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald Hings, radio engineer ...
s,
radio navigation
Radio navigation or radionavigation is the application of radio waves to geolocalization, determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of Radiodetermination-satellite servi ...
systems like GPS, remote entry systems, among numerous other uses.
A transmitter can be a separate piece of equipment, or an
electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or Conductive trace, traces through which electric current can flow. It is a t ...
within another device. Most transmitters consist of an
electronic oscillator
An electronic oscillator is an electronic circuit that produces a periodic, oscillating or alternating current (AC) signal, usually a sine wave, square wave or a triangle wave, powered by a direct current (DC) source. Oscillators are found ...
which generates an oscillating
carrier wave
In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or freq ...
, a
modulator
Signal modulation is the process of varying one or more properties of a periodic waveform in electronics and telecommunication for the purpose of transmitting information.
The process encodes information in form of the modulation or message ...
which impresses an information bearing modulation signal on the carrier, and an
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power su ...
which increases the power of the signal. To prevent
interference
Interference is the act of interfering, invading, or poaching. Interference may also refer to:
Communications
* Interference (communication), anything which alters, modifies, or disrupts a message
* Adjacent-channel interference, caused by extra ...
between different users of the
radio spectrum
The radio spectrum is the part of the electromagnetic spectrum with frequencies from 3 Hz to 3,000 GHz (3 THz). Electromagnetic waves in this frequency range, called radio waves, are widely used in modern technology, particula ...
, transmitters are strictly regulated by national radio laws, and are restricted to certain
frequencies
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
and power levels, depending on use. The design must typically be certificated (formerly type approved) before sale. An important legal requirement is that the circuit does not radiate significant radio wave power outside its assigned frequency band, called ''
spurious emission
In radio communication, a spurious emission is any component of a radiated radio frequency signal, the complete suppression of which, would not impair the integrity of the modulation type, or the information being transmitted. A radiated signal ...
''.
Design issues
A radio transmitter design has to meet certain requirements. These include the frequency of operation, the type of
modulation
Signal modulation is the process of varying one or more properties of a periodic waveform in electronics and telecommunication for the purpose of transmitting information.
The process encodes information in form of the modulation or message ...
, the stability and purity of the resulting signal, the efficiency of power use, and the power level required to meet the system design objectives. High-power transmitters may have additional constraints with respect to radiation safety, generation of X-rays, and protection from high voltages.
Typically a transmitter design includes generation of a
carrier signal
In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or frequ ...
, which is normally
sinusoidal
A sine wave, sinusoidal wave, or sinusoid (symbol: ∿) is a periodic wave whose waveform (shape) is the trigonometric sine function. In mechanics, as a linear motion over time, this is '' simple harmonic motion''; as rotation, it correspond ...
, optionally one or more frequency multiplication stages, a modulator, a power amplifier, and a filter and matching network to connect to an antenna. A very simple transmitter might contain only a continuously running oscillator coupled to some antenna system. More elaborate transmitters allow better control over the modulation of the emitted 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 loading presented by the antenna from altering the frequency of the oscillator.
Determining the frequency
Fixed frequency systems
For a fixed
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
transmitter one commonly used method is to use a
resonant
Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
in a
crystal oscillator
A crystal oscillator is an electronic oscillator Electrical circuit, circuit that uses a piezoelectricity, piezoelectric crystal as a frequency selective surface, frequency-selective element. The oscillator frequency is often used to keep trac ...
to fix the frequency. Where the frequency has to be variable, several options can be used.
Variable frequency systems
* An array of crystals – used to enable a transmitter to be used on several different frequencies; rather than being a truly variable frequency system, it is a system which is fixed to several different frequencies (a subset of the above).
*
Variable-frequency oscillator
A variable frequency oscillator (VFO) in electronics is an oscillator whose frequency can be tuned (i.e., varied) over some range. It is a necessary component in any tunable radio transmitter and in receivers that work by the superheterodyne pr ...
(VFO)
*
Phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and ou ...
While modern frequency synthesizers can output a clean stable signal up through UHF, for many years, especially at higher frequencies, it was not practical to operate the oscillator at the final output frequency. For better frequency stability, it was common to multiply the frequency of the oscillator up to the final, required frequency. This was accommodated by allocating the short wave amateur and marine bands in harmonically related frequencies such as 3.5, 7, 14 and 28 MHz. Thus one 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 tuned to a multiple of the frequency with which the stage is driven, the stage will give a large harmonic output. Many transmitters have used this simple approach successfully. However these more complex circuits will do a better job. In a push-push stage, the output will only contain ''even'' harmonics. This is because the currents which would generate the fundamental and the odd harmonics in this circuit are canceled by the second device. In a push-pull stage, the output will contain only ''odd'' harmonics because of the canceling effect.
Adding modulation to the signal
The task of a transmitter is to convey some form of information using a radio signal (carrier wave) which has been modulated to carry the information. The RF generator in a
microwave oven
A microwave oven, or simply microwave, is an electric oven that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces Dipole#Molecular dipoles, polar molecules in the food to rotate and ...
,
electrosurgery
Electrosurgery is the application of a high-frequency (radio frequency) alternating polarity, electrical current to biological tissue as a means to cut, coagulate, desiccate, or fulgurate tissue.Hainer BL, "Fundamentals of electrosurgery", '' ...
, and
induction heating
Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an inductor that creates an electromagnetic field within the coi ...
are similar in design to transmitters, but usually not considered as such in that they do not intentionally produce a signal that will travel to a distant point. Such RF devices are required by law to operate in an
ISM band
The ISM radio bands are portions of the radio spectrum reserved internationally for ''industrial, scientific, and medical'' (ISM) purposes, excluding applications in telecommunications.
Examples of applications for the use of radio frequency (RF ...
where interference to radio communications will not occur. Where communications is the object, one or more of the following methods of incorporating the desired signal into the radio wave is used.
AM modes
When a radio frequency wave is varied in amplitude in a manner which follows the modulating signal, usually voice, video or data, we have
amplitude modulation
Amplitude modulation (AM) is a signal modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the instantaneous amplitude of the wave is varied in proportion t ...
(AM).
Low level and high level
In low level modulation a small
audio
Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to:
Sound
*Audio signal, an electrical representation of sound
*Audio frequency, a frequency in the audio spectrum
*Digital audio, representation of sound ...
stage is used to modulate a low power stage. The output of this stage is then amplified using a
linear
In mathematics, the term ''linear'' is used in two distinct senses for two different properties:
* linearity of a '' function'' (or '' mapping'');
* linearity of a '' polynomial''.
An example of a linear function is the function defined by f(x) ...
RF amplifier. The great disadvantage of this system is that the amplifier chain is less efficient, because it has to be linear to preserve the modulation. Hence high efficiency class C amplifiers cannot be employed, unless a
Doherty amplifier
The Doherty amplifier is a modified class B radio frequency amplifier invented by William H. Doherty of Bell Telephone Laboratories Inc in 1936. Whereas conventional class B amplifiers can clip on high input-signal levels, the Doherty power ampli ...
, EER (Envelope Elimination and Restoration) or other methods of predistortion or
negative feedback
Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused ...
are used. High level modulation uses class C amplifiers in a broadcast AM transmitter and only the final stage or final two stages are modulated, and all the earlier stages can be driven at a constant level. When modulation is applied to the plate of the final tube, a large audio amplifier is needed for the modulation stage, equal to 1/2 of the DC input power of the modulated stage. Traditionally the modulation is applied using a large audio transformer. However many different circuits have been used for high-level AM modulation.
Types of AM modulators
A wide range of different circuits have been used for AM. While it is perfectly possible to create good designs using solid-state electronics, valved (tube) circuits are shown here. In general, valves are able to easily yield RF powers far in excess of what can be achieved using solid state. Most high-power broadcast stations below 3 MHz use solid state circuits, but higher power stations above 3 MHz still use valves.
=Plate AM modulators
=
High level plate modulation consists of varying the voltage on the plate (anode) of the valve so that it swings from nearly zero to double the resting value. This will produce 100% modulation and can be done by inserting a transformer in series with the high voltage supply to the anode so that the vector sum of the two sources, (DC and audio) will be applied. A disadvantage is the size, weight and cost of the transformer as well as its limited audio frequency response, especially for very powerful transmitters.
Alternatively a series regulator can be inserted between the DC supply and the anode. The DC supply provides twice the average voltage the anode sees. The regulator can allow none or all of the voltage to pass, or any intermediate value. The audio input operates the regulator in such a way as to produce the instantaneous anode voltage needed to reproduce the modulation envelope. An advantage of the series regulator is that it can set the anode voltage to any desired value. Thus the power output of the transmitter can be easily adjusted, allowing the use of dynamic carrier control. The use of PDM switching regulators makes this system very efficient, whereas the original analog regulators were very inefficient and also non linear. Series PDM modulators are used in solid state transmitters also, but the circuits are somewhat more complex, using push pull or bridge circuits for the RF section.
These simplified diagrams omit such details as filament, screen and grid bias supplies, and the screen and cathode connections to RF ground.
=Screen AM modulators
=
Under carrier conditions (no audio) the stage will be a simple RF amplifier where the screen voltage is set lower than normal to limit the RF output to about 25% of full power. When the stage is modulated the screen potential changes and so alters the gain of the stage. It takes much less audio power to modulate the screen, but final stage efficiency is only about 40%, compared to 80% with plate modulation. For this reason screen modulation was used only in low power transmitters and is now effectively obsolete.
AM related modes
Several derivatives of AM are in common use. These are
Single-sideband modulation
SSB, or SSB-AM single-sideband full carrier modulation, is very similar to single-sideband suppressed carrier modulation (SSB-SC). It is used where it is necessary to receive the audio on an AM receiver, while using less bandwidth than with double sideband AM. Due to high distortion, it is seldom used.
Either SSB-AM or SSB-SC are produced by the following methods.
=Filter method
=
Using a balanced mixer a double side band signal is generated, this is then passed through a very narrow bandpass filter to leave only one side-band.Pappenfus, Bruene and Schoenike ''Single sideband principles and circuits'' McGraw-Hill, 1964, chapter 6 By convention it is normal to use the upper sideband (USB) in communication systems, except for amateur radio when the carrier frequency is below 10 MHz. There the lower side band (LSB) is normally used.
=Phasing method
=
The phasing method for the generation of single sideband signals uses a network which imposes a constant 90° phase shift on audio signals over the audio range of interest. This was difficult with analog methods but with DSP is very simple.
These audio outputs are each mixed in a linear balanced mixer with a carrier. The carrier drive for one of these mixers is also shifted by 90°. The outputs of these mixers are added in a linear circuit to give the SSB signal by phase cancellation of one of the sidebands. Connecting the 90° delayed signal from either the audio or the carrier (but not both) to the other mixer will reverse the sideband, so either USB or LSB is available with a simple DPDT switch.
Vestigial-sideband modulation
Vestigial-sideband modulation (VSB, or VSB-AM) is a type of modulation system commonly used in analogue TV systems. It is normal AM which has been passed through a filter which reduces one of the sidebands. Typically, components of the lower sideband more than 0.75 MHz or 1.25 MHz below the carrier will be heavily attenuated.
Morse
Morse code
Morse code is a telecommunications method which Character encoding, encodes Written language, text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code i ...
is usually sent using on-off keying of an unmodulated carrier (
Continuous wave
A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration. It may refer to e.g. a laser or particl ...
). No special modulator is required.
This interrupted carrier may be analyzed as an AM-modulated carrier. On-off keying produces sidebands, as expected, but they are referred to as "key-clicks". Shaping circuits are used to turn the transmitter on and off smoothly instead of instantly in order to limit the bandwidth of these sidebands and reduce interference to adjacent channels.
FM modes
Angle modulation
Angle modulation is a class of signal modulation that is used in telecommunication transmission systems using carrier waves. The class comprises frequency modulation (FM) and phase modulation (PM), and is based on altering the frequency or the ph ...
is the proper term for modulation by changing the instantaneous frequency or phase of the carrier signal. True FM and
phase modulation
Phase modulation (PM) is a signal modulation method for conditioning communication signals for transmission. It encodes a message signal as variations in the instantaneous phase of a carrier wave. Phase modulation is one of the two principal f ...
are the most commonly employed forms of analogue angle modulation.
Direct FM
Direct FM (true
Frequency modulation
Frequency modulation (FM) is a signal modulation technique used in electronic communication, originally for transmitting messages with a radio wave. In frequency modulation a carrier wave is varied in its instantaneous frequency in proporti ...
) is where the frequency of an
oscillator
Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
is altered to impose the modulation upon the carrier wave. This can be done by using a voltage-controlled capacitor ( varicap
diode
A diode is a two-Terminal (electronics), terminal electronic component that conducts electric current primarily in One-way traffic, one direction (asymmetric electrical conductance, conductance). It has low (ideally zero) Electrical resistance ...
) in a crystal-controlled oscillator or frequency synthesiser. The frequency of the oscillator is then multiplied up using a frequency multiplier stage, or is translated upwards using a mixing stage, to the output frequency of the transmitter. The amount of modulation is referred to as the deviation, being the amount that the frequency of the carrier instantaneously deviates from the centre carrier frequency.
Indirect FM
Indirect FM employs a varicap diode to impose a phase shift (which is voltage-controlled) in a tuned circuit that is fed with a plain carrier. This is termed
phase modulation
Phase modulation (PM) is a signal modulation method for conditioning communication signals for transmission. It encodes a message signal as variations in the instantaneous phase of a carrier wave. Phase modulation is one of the two principal f ...
.
In some indirect FM solid state circuits, an RF drive is applied to the base of a
transistor
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...
. The tank circuit (LC), connected to the collector via a capacitor, contains a pair of varicap diodes. As the voltage applied to the varicaps is changed, the phase shift of the output will change.
Phase modulation is mathematically equivalent to direct Frequency modulation with a 6 dB/octave
high-pass filter
A high-pass filter (HPF) is an electronic filter that passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency ...
applied to the modulating signal. This high-pass effect can be exploited or compensated for using suitable frequency-shaping circuitry in the audio stages ahead of the modulator. For example, many FM systems will employ
pre-emphasis
In signal processing, pre-emphasis is a technique to protect against anticipated noise and loss. The idea is to boost (and hence distort) the frequency range that is most susceptible to noise and loss beforehand, so that after a noisy and lossy ...
and
de-emphasis
In signal processing, pre-emphasis is a technique to protect against anticipated noise and loss. The idea is to boost (and hence distort) the frequency range that is most susceptible to noise and loss beforehand, so that after a noisy and lossy ...
for noise reduction, in which case the high-pass equivalency of phase modulation automatically provides for the pre-emphasis. Phase modulators are typically only capable of relatively small amounts of deviation while remaining linear, but any frequency multiplier stages also multiply the deviation in proportion.
Digital modes
Transmission of digital data is becoming more and more important. Digital information can be transmitted by AM and FM modulation, but often digital modulation consists of complex forms of modulation using aspects of both AM and FM.
COFDM
In telecommunications, orthogonal frequency-division multiplexing (OFDM) is a type of digital transmission used in digital modulation for encoding digital (binary) data on multiple Carrier wave, carrier frequencies. OFDM has developed into a popul ...
is used for
DRM
DRM may refer to:
Government, military and politics
* Defense reform movement, U.S. campaign inspired by Col. John Boyd
* Democratic Republic of Madagascar, a former socialist state (1975–1992) on Madagascar
* Direction du renseignement militair ...
broadcasts. The transmitted signal consists of multiple carriers each modulated in both amplitude and phase. This allows very high bit rates and makes very efficient use of bandwidth. Digital or pulse methods also are used to transmit voice as in cell phones, or video as in terrestrial TV broadcasting. Early text messaging such as RTTY allowed the use of class C amplifiers, but modern digital modes require linear amplification.
See also Sigma-delta modulation (ΣΔ)
Amplifying the signal
Valves
For high power, high frequency systems it is normal to use valves, see Valve RF amplifier for details of how valved RF power stages work. Valves are electrically very robust, they can tolerate overloads which would destroy bipolar
transistor
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...
systems in milliseconds. As a result, valved amplifiers may resist mistuning, lightning and power surges better. However, they require a heated cathode which consumes power and will fail in time due to loss of emission or heater burn out. The high voltages associated with valve circuits are dangerous to persons. For economic reasons, valves continue to be used for the final power amplifier for transmitters operating above 1.8 MHz and with powers above about 500 watts for amateur use and above about 10 kW for broadcast use.
Solid state
Solid state devices, either discrete transistors or integrated circuits, are universally used for new transmitter designs up to a few hundred watts. The lower level stages of more powerful transmitters are also all solid state. Transistors can be used at all frequencies and power levels, but since the output of individual devices is limited, higher power transmitters must use many transistors in parallel, and the cost of the devices and the necessary combining networks can be excessive. As new transistor types become available and the price drops, solid state may eventually replace all valve amplifiers.
Linking the transmitter to the aerial
The majority of modern transmitting equipment is designed to operate with a
resistive
The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paral ...
load fed via
coaxial cable
Coaxial cable, or coax (pronounced ), is a type of electrical cable consisting of an inner Electrical conductor, conductor surrounded by a concentric conducting Electromagnetic shielding, shield, with the two separated by a dielectric (Insulat ...
of a particular
characteristic impedance
The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a wave travelling in one direction along the line in the absence of reflections in th ...
, often 50
ohm
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (1 ...
s. To connect the power stage of the transmitter to this coaxial cable
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
a matching network is required. For solid state transmitters this is typically a broadband transformer which steps up the low impedance of the output devices to 50 ohms. A tube transmitter will contain a tuned output network, most commonly a PI network, that steps the load impedance which the tube requires down to 50 ohms. In each case the power producing devices will not transfer power efficiently if the network is detuned or badly designed or if the antenna presents other than 50 ohms at the transmitter output. Commonly an SWR meter and/or directional wattmeter are used to check the extent of the match between the aerial system and the transmitter via the transmission line (feeder). A directional wattmeter indicates forward power, reflected power, and often SWR as well. Each transmitter will specify a maximum allowable mismatch based on efficiency, distortion, and possible damage to the transmitter. Many transmitters have automatic circuits to reduce power or shut down if this value is exceeded.
Transmitters feeding a balanced transmission line will need a
balun
A balun (from "balanced to unbalanced", originally, but now derived from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A ba ...
. This transforms the single ended output of the transmitter to a higher impedance balanced output. High power short wave transmission systems typically use 300 ohm balanced lines between the transmitter and antenna. Amateurs often use 300–450 ohm balanced antenna feeders.
See
Antenna tuner
An antenna tuner, a matchbox, transmatch, antenna tuning unit (ATU), antenna coupler, or feedline coupler is a device connected between a radio transmitter or receiver and its antenna to improve power transfer between them by matching the imped ...
and
balun
A balun (from "balanced to unbalanced", originally, but now derived from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A ba ...
for details of matching networks and baluns respectively.
EMC matters
Many devices depend on the transmission and reception of radio waves for their operation. The possibility for mutual interference is great. Many devices not intended to transmit signals may do so. For instance a dielectric heater might contain a 2000
watt
The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
27 MHz source within it. If the machine operates as intended then none of this RF power will leak out. However, if due to poor design or maintenance it allows RF to leak out, it will become a transmitter or unintentional radiator.
RF leakage & shielding
All equipment using RF
electronics
Electronics is a scientific and engineering discipline that studies and applies the principles of physics to design, create, and operate devices that manipulate electrons and other Electric charge, electrically charged particles. It is a subfield ...
should be inside a screened conductive box and all connections in or out of the box should be filtered to avoid the passage of radio signals. A common and effective method of doing so for wires carrying DC supplies, 50/60 Hz AC connections, audio and control signals is to use a feedthrough
capacitor
In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...
, whose job is to short circuit any RF on the wire to ground. The use of ferrite beads is also common.
If an intentional transmitter produces interference, then it should be run into a dummy load; this is a resistor in a screened box or can which will allow the transmitter to generate radio signals without sending them to the antenna. If the
transmitter
In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna (radio), antenna with the purpose of sig ...
continues to cause interference during this test then a path exists by which RF power is leaking out of the equipment and this can be due to bad shielding. Such leakage is most likely to occur on homemade equipment or equipment that has been modified or had covers removed. RF leakage from
microwave oven
A microwave oven, or simply microwave, is an electric oven that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces Dipole#Molecular dipoles, polar molecules in the food to rotate and ...
s, while rare, may occur due to defective door seals, and may be a health hazard.
Spurious emissions
Early in the development of radio technology it was recognized that the signals emitted by transmitters had to be 'pure'.
Spark-gap transmitter
A spark-gap transmitter is an obsolete type of transmitter, radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the m ...
s were outlawed once better technology was available as they give an output which is very wide in terms of frequency. The term
spurious emission
In radio communication, a spurious emission is any component of a radiated radio frequency signal, the complete suppression of which, would not impair the integrity of the modulation type, or the information being transmitted. A radiated signal ...
s refers to any signal which comes out of a transmitter other than the wanted signal. In modern equipment there are three main types of spurious emissions:
harmonic
In physics, acoustics, and telecommunications, a harmonic is a sinusoidal wave with a frequency that is a positive integer multiple of the ''fundamental frequency'' of a periodic signal. The fundamental frequency is also called the ''1st har ...
s, out of band mixer products which are not fully suppressed and leakage from the
local oscillator
In electronics, the term local oscillator (LO) refers to an electronic oscillator when used in conjunction with a Frequency mixer, mixer to change the frequency of a signal. This frequency conversion process, also called Heterodyne, heterodyning ...
and other systems within the transmitter.
Harmonics
These are multiples of the operation frequency of the transmitter, they can be generated in any stage of the transmitter which is not perfectly linear and must be removed by filtering.
=Avoiding harmonic generation
=
The difficulty of removing harmonics from an amplifier will depend on the design. A push-pull amplifier will have fewer harmonics than a single ended circuit. A class A amplifier will have very few harmonics, class AB or B more, and class C the most. In the typical class C amplifier, the resonant tank circuit will remove most of the harmonics, but in either of these examples, a low pass filter will likely be needed following the amplifier.
=Removal of harmonics with filters
=
In addition to the good design of the amplifier stages, the transmitter's output should be filtered with a
low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
to reduce the level of the harmonics. Typically the input and output are interchangeable and match to 50 ohms. Inductance and capacity values will vary with frequency. Many transmitters switch in a suitable filter for the frequency band being used. The filter will pass the desired frequency and reduce all harmonics to acceptable levels.
The harmonic output of a transmitter is best checked using an RF
spectrum analyzer
A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most co ...
or by tuning a receiver to the various harmonics. If a harmonic falls on a frequency being used by another communications service then this spurious emission can prevent an important signal from being received. Sometimes additional filtering is used to protect a sensitive range of frequencies, for example, frequencies used by aircraft or services involved with protection of life and property. Even if a harmonic is within the legally allowed limits, the harmonic should be further reduced.
Oscillators and mix products
When mixing signals to produce a desired output frequency, the choice of
Intermediate frequency
In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in Transmission (telecommunications), transmission or reception. The intermediate frequency is ...
and
local oscillator
In electronics, the term local oscillator (LO) refers to an electronic oscillator when used in conjunction with a Frequency mixer, mixer to change the frequency of a signal. This frequency conversion process, also called Heterodyne, heterodyning ...
is important. If poorly chosen, a spurious output can be generated. For example, if 50 MHz is mixed with 94 MHz to produce an output on 144 MHz, the third harmonic of the 50 MHz may appear in the output. This problem is similar to the Image response problem which exists in receivers.
One method of reducing the potential for this transmitter defect is the use of balanced and double balanced mixers. A simple mixer will pass both of the input frequencies and all of their harmonics along with the sum and difference frequencies. If the simple mixer is replaced with a balanced mixer then the number of possible products is reduced. If the frequency mixer has fewer outputs the task of making sure that the final output is ''clean'' will be simpler.
Instability and parasitics
If a stage in a transmitter is unstable and is able to oscillate then it can start to generate RF at either a frequency close to the operating frequency or at a very different frequency. One good sign that it is occurring is if an RF stage has a power output even without being driven by an exciting stage. Output power should increase smoothly as input power is increased, although with Class C, there will be a noticeable threshold effect. Various circuits are used for parasitic suppression in a good design. Proper neutralization is also important.
Control and protection
The simplest transmitters such as
RFID
Radio-frequency identification (RFID) uses electromagnetic fields to automatically identify and track tags attached to objects. An RFID system consists of a tiny radio transponder called a tag, a radio receiver, and a transmitter. When tri ...
devices require no external controls. Simple tracking transmitters may have only an on-off switch. Many transmitters must have circuits that allow them to be turned on and off and the power output and frequency adjusted or modulation levels adjusted. Many modern multi-featured transmitters allow the adjustment of many different parameters. Usually these are under microprocessor control via multilevel menus, thus reducing the required number of physical knobs. Often a display screen provides feedback to the operator to assist in adjustments. The user friendliness of this interface will often be one of the main factors in a successful design.
Microprocessor controlled transmitters also may include software to prevent off frequency or other illegal operation. Transmitters using significant power or expensive components must also have protection circuits which prevent such things as overload, overheating or other abuse of the circuits. Overload circuits may include mechanical relays, or electronic circuits. Simple fuses may be included to protect expensive components. Arc detectors may shut off the transmitter when sparks or fires occur.
Protection features must also prevent the human operator and the public from encountering the high voltages and power which exist inside the transmitter. Tube transmitters typically use DC voltages between 600 and 30,000 volts, which are deadly if contacted. Radio frequency power above about 10 watts can cause burning of human tissue through contact and higher power can actually cook human flesh without contact. Metal shielding is required to isolate these dangers. Properly designed transmitters have doors or panels which are interlocked, so that open doors activate switches which do not allow the transmitter to be turned on when the dangerous areas are exposed. In addition, either resistors which bleed off the high voltages or shorting relays are employed to insure that capacitors do not retain a dangerous charge after turn off.
With large high power transmitters, the protective circuits can comprise a significant fraction of the total design complexity and cost.
Power supplies
Some RFID devices take power from an external source when it interrogates the device, but most transmitters either have self-contained batteries, or are mobile systems which typically operate directly from the 12 volt vehicle battery. Larger fixed transmitters will require power from the mains. The voltages used by a transmitter will be AC and DC of many different values. Either AC transformers or DC power supplies are required to provide the values of voltage and current needed to operate the various circuits. Some of these voltages will need to be regulated. Thus a significant part of the total design will consist of power supplies. Power supplies will be integrated into the control and protection systems of the transmitter, which will turn them on in the proper sequence and protect them from overloads. Often rather complicated logic systems will be required for these functions.
American Radio Relay League
The American Radio Relay League (ARRL) is the largest membership association of amateur radio enthusiasts in the United States. ARRL is a non-profit organization and was co-founded on April 6, 1914, by Hiram Percy Maxim and Clarence D. Tuska of ...
. (2012).The ARRL Handbook for Radio Communications., , url=http://www.arrl.org/arrl-handbook-2013
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Radio Society of Great Britain
The Radio Society of Great Britain (RSGB) is the United Kingdom's recognised national society for amateur radio operators. The society was founded in 1913 as the London Wireless Club, making it one of the oldest organisations of its kind in the ...