Radio transmitter design - AbsoluteAstronomy.com

Radio transmitter design is a complex topic which can be broken down into a series of smaller topics. A radio communication system

Radio communication system

A radio communication system sends signals by radio. Types of radio communication systems deployed depend on technology, standards, regulations, radio spectrum allocation, user requirements, service positioning, and investment....

requires two tuned circuits each at the transmitter

Transmitter

In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

and receiver

Receiver (radio)

A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...

, all four tuned to the same frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

. The transmitter is an electronic

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

device

Machine

A machine manages power to accomplish a task, examples include, a mechanical system, a computing system, an electronic system, and a molecular machine. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work...

which, usually with the aid of an antenna, propagates an electromagnetic

Electromagnetic radiation

Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

signal such as radio

Radio

Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

, television

Television

Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

, or other telecommunications.

Methods

At the beginning of the 20th century, there were four chief methods of arranging the transmitting circuits:

The transmitting system consists of two tuned circuits such that the one containing the spark-gap is a persistent oscillator; the other, containing the aerial structure, is a free radiator maintained in oscillation by being coupled to the first (Nikola Tesla
Nikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...

and Guglielmo Marconi
Guglielmo Marconi
Guglielmo Marconi was an Italian inventor, known as the father of long distance radio transmission and for his development of Marconi's law and a radio telegraph system. Marconi is often credited as the inventor of radio, and indeed he shared the 1909 Nobel Prize in Physics with Karl Ferdinand...

).
The oscillating system, including the aerial structure with its associated inductance-coils and condensers, is designed to be both a sufficiently persistent oscillator and a sufficiently active radiator (Oliver Lodge).
The transmitting system consists of two electrically coupled circuits, one of which, containing the air-gap, is a powerful but not persistent oscillator, being provided with a device for quenching the spark so soon as it has imparted sufficient energy to the other circuit containing the aerial structure, this second circuit then independently radiating the train of slightly damped waves at its own period (Oliver Joseph Lodge and Wilhelm Wien
Wilhelm Wien
Wilhelm Carl Werner Otto Fritz Franz Wien was a German physicist who, in 1893, used theories about heat and electromagnetism to deduce Wien's displacement law, which calculates the emission of a blackbody at any temperature from the emission at any one reference temperature.He also formulated an...

).
The transmitting system, by means either of an oscillating arc (Valdemar Poulsen
Valdemar Poulsen
Valdemar Poulsen was a Danish engineer who developed a magnetic wire recorder in 1899.-Biography:He was born on 23 November 1869 in Copenhagen...

) or a high-frequency alternator (Rudolf Goldschmidt
Rudolf Goldschmidt
Rudolf Goldschmidt was a German engineer and inventor.Goldschmidt was born in Neubukow and earned an engineering degree in 1878. He spent the following decade working in England with major firms such as Westinghouse. Subsequently he returned to Germany and eventually became a professor at...

), emits a persistent train of undamped waves interrupted only by being broken up into long and short groups by the operator's key.

Fixed frequency systems

For a fixed frequency

Frequency

transmitter one commonly used method is to use a resonant quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

in a Crystal oscillator

Crystal oscillator

A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...

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
Variable-frequency oscillator
A variable frequency oscillator in electronics is an oscillator whose frequency can be tuned over some range. It is a necessary component in any tunable radio receiver or transmitter that works by the superheterodyne principle, and controls the frequency to which the apparatus is...

(VFO)
Phase-locked loop
Phase-locked loop
A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...

frequency synthesiser
Direct digital synthesis
Direct digital synthesis
Direct Digital Synthesizer is a type of frequency synthesizer used for creating arbitrary waveforms from a single, fixed-frequency reference clock...

Frequency multiplication

For VHF transmitters, it is often not possible to operate the oscillator at the final output frequency. In such cases, for reasons including frequency stability, it is better to multiply the frequency of the free running oscillator up to the final, required frequency.

If the output of an amplifier stage is tuned to a multiple of the frequency with which the stage is driven, the stage will give a larger harmonic output than a linear amplifier

Linear amplifier

A linear amplifier is an electronic circuit whose output is proportional to its input, but capable of delivering more power into a load. The term usually refers to a type of radio-frequency power amplifier, some of which have output power measured in kilowatts, and are used in amateur radio...

. 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 (if one valve was removed) are canceled by the second valve. In the diagrams, bias supplies and neutralization measure have been omitted for clarity. In a real system, it is likely that tetrode

Tetrode

A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...

s would be used, as plate-to-grid capacitance in a tetrode is lower, thereby reducing stage instability.

In a push-pull stage, the output will contain only odd harmonics because of the canceling effect.

Frequency mixing and modulation

The task of many transmitters is to transmit some form of information using a radio signal (carrier wave) which has been modulated to carry the intelligence. A few rare types of transmitter do not carry information: the RF generator in a microwave oven

Microwave oven

A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...

, electrosurgery

Electrosurgery

Electrosurgery is the application of a high-frequency electric current to biological tissue as a means to cut, coagulate, desiccate, or fulgurate tissue. . Its benefits include the ability to make precise cuts with limited blood loss...

, and induction heating

Induction heating

Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...

.
RF transmitters that do not carry information are required by law to operate in an ISM band

ISM band

The industrial, scientific and medical radio bands are radio bands reserved internationally for the use of radio frequency energy for industrial, scientific and medical purposes other than communications....

AM modes

In many cases the carrier wave is mixed with another electrical signal to impose information upon it. This occurs in Amplitude modulation

Amplitude modulation

Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...

(AM).
Amplitude Modulation:
In Amplitude modulation the instantaneous change in the amplitude of the carrier Frequency with respect to the amplitude of the modulating or Base band signal.

Low level

Here a small audio

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

stage is used to modulate

Modulation

In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

a low power stage, the output of this stage is then amplified using a linear

Linear amplifier

RF amplifier.

Advantages

The advantage of using a linear RF amplifier is that the smaller early stages can be modulated, which only requires a small audio amplifier

Audio amplifier

An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...

to drive the modulator.

Disadvantages

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 class C amplifiers cannot be employed.

An approach which marries the advantages of low-level modulation with the efficiency of a Class C power amplifier chain is to arrange a feedback system to compensate for the substantial distortion of the AM envelope. A simple detector at the transmitter output (which can be little more than a loosely coupled diode

Diode

In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...

) recovers the audio signal, and this is used as negative feedback

Negative feedback

Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

to the audio modulator stage. The overall chain then acts as a linear amplifier as far as the actual modulation is concerned, though the RF amplifier itself still retains the Class C efficiency. This approach is widely used in practical medium power transmitters, such as AM radiotelephone

Radiotelephone

A radiotelephone is a communications system for transmission of speech over radio. Radiotelephone systems are not necessarily interconnected with the public "land line" telephone network. "Radiotelephone" is often used to describe the usage of radio spectrum where it is important to distinguish the...

High level

Advantages

One advantage of using class C amplifiers in a broadcast AM transmitter is that only the final stage needs to be modulated, and that all the earlier stages can be driven at a constant level. These class C stages will be able to generate the drive for the final stage for a smaller DC

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

power input. However, in many designs in order to obtain better quality AM the penultimate RF stages will need to be subject to modulation as well as the final stage.

Disadvantages

A large audio amplifier will be needed for the modulation stage, at least equal to the power of the transmitter output itself. Traditionally the modulation is applied using an audio transformer, and this can be bulky. Direct coupling from the audio amplifier is also possible (known as a cascode

Cascode

The cascode is a two-stage amplifier composed of a transconductance amplifier followed by a current buffer. Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input-output isolation, higher input impedance, high output impedance,...

arrangement), though this usually requires quite a high DC supply voltage (say 30 V or more), which is not suitable for mobile units.

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 still use valves.

Plate AM modulators

When the valve at the top conducts more than the potential difference between the anode and cathode of the lower valve (RF valve) will increase. The two valves can be thought of as two resistors in a potentiometer

Potentiometer

A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...

Screen AM modulators

Under steady state conditions (no audio driven) the stage will be a simple RF amplifier where the grid

Grid

Grid or The Grid may refer to:In entertainment and media:* The Grid, an electronic dance group* Kevorkian Death Cycle, a music group formerly called Grid* The Grid , a 2001 third person shooter...

bias is set by the cathode current. When the stage is modulated the screen potential changes and so alters the gain of the stage.

Single-sideband modulation

SSB, or SSB-AM single-sideband full carrier modulation, is very similar to single-sideband suppressed carrier modulation

Single-sideband modulation

Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....

(SSB-SC)

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. By convention it is normal to use the upper sideband (USB) in communication systems, except for HAM radio when the carrier frequency is below 10 MHz here the lower side band (LSB) is normally used.

Phasing method

This method is an alternative method for the generation of single sideband signals. One of the weaknesses of this method is the need for a network which imposes a constant 90^o phase shift on audio signals throughout the entire audio spectrum. By reducing the audio bandwidth the task of designing the phaseshift network can be made more easy.

Imagine that the audio is a single sine wave E = E° sine (ωt)

The audio signal is passed through the phase shift network to give two identical signals which differ by 90^o.

So as the audio input is a single sine wave the outputs will be

and

These audio outputs are mixed in non linear mixers with a carrier, the carrier drive for one of these mixers is shifted by 90°. The output of these mixers is combined in a linear circuit to give the SSB signal.

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

Strictly speaking the commonly used 'AM' is double-sideband full carrier. Morse

Morse

Morse can refer to:* Morse code, a method of coding messages into long and short beeps-Places:Canada* Morse , Saskatchewan* Morse, Saskatchewan, a hamlet* Morse No...

is often sent using on-off keying of an unmodulated carrier (Continuous wave

Continuous wave

A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...

), this can be thought of as an AM mode.

FM modes

Angle modulation

Angle modulation is a class of analog modulation. These techniques are based on altering the angle of a sinusoidal carrier wave to transmit data, as opposed to varying the amplitude, such as in AM transmission....

is the proper term for modulation by changing the instantaneous frequency or phase of the carrier signal. True FM and phase modulation

Phase modulation

Phase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Unlike its more popular counterpart, frequency modulation , PM is not very widely used for radio transmissions...

are the most commonly employed forms of analogue angle modulation.

Direct FM

Direct FM (true Frequency modulation

Frequency modulation

In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...

) is where the frequency of an oscillator is altered to impose the modulation upon the carrier wave. This can be done by using a voltage-controlled capacitor (Varicap

Varicap

In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode which has a variable capacitance that is a function of the voltage impressed on its terminals....

diode

Diode

) 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.

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

. The modulated signal from a phase-modulated stage can be understood with an FM receiver, but for good audio quality, the audio is applied to the phase modulation stage. The amount of modulation is referred to as the deviation

Frequency deviation

Frequency deviation is used in FM radio to describe the maximum instantaneous difference between an FM modulated frequency and the nominal carrier frequency...

, being the amount that the frequency of the carrier instantaneously deviates from the centre carrier frequency.

In some indirect FM solid state circuits, an RF drive is applied to the base of a transistor

Transistor

A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

. The tank circuit (LC), connected to the collector via a capacitor, contains a pair of varicap

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 6dB/octave high-pass filter

High-pass filter

A high-pass filter is a device that passes high frequencies and attenuates frequencies lower than its cutoff frequency. A high-pass filter is usually modeled as a linear time-invariant system...

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 and de-emphasis 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.

Sigma-delta modulation (∑Δ)

Valves

For high power systems it is normal to use valves, please see Valve RF amplifier

Valve RF amplifier

A valve RF amplifier or tube amplifier , is a device for electrically amplifying the power of an electrical signal, typically radio frequency signals....

for details of how valved RF power stages work.

Advantages of valves

Good for high power systems
Electrically very robust, they can tolerate overloads for minutes which would destroy bipolar
Bipolar
-Medicine:* Bipolar cell* Bipolar cell of the retina* Bipolar disorder** Bipolar I disorder** Bipolar II disorder** Bipolar NOS* Bipolar spectrum-Astronomy:* Bipolar nebula, a two-lobed, axially symmetric nebula...

transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

systems in milliseconds

Disadvantages of valves

Heater supplies are required for the cathodes
High voltages (risk of death) are required for the anodes
Valves often have a shorter working life than solid state parts because the heaters tend to fail

Solid state

For low and medium power it is often the case that solid state power stages are used. For higher power systems these cost more per watt

Watt

The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

of output power than a valved system.

Linking the transmitter to the aerial

The majority of modern transmitting equipment is designed to operate with a resistive

Electrical resistance

The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

load fed via coaxial cable

Coaxial cable

Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...

of a particular characteristic impedance

Characteristic impedance

The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...

, often 50 ohm

Ohm

The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...

s. To connect the aerial to this coaxial cable transmission line

Transmission line

In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...

a matching network and/or a balun

Balun

A balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...

may be required. Commonly an SWR meter

SWR meter

The SWR meter or VSWR meter measures the standing wave ratio in a transmission line. The meter can be used to indicate the degree of mismatch between a transmission line and its load , or evaluate the effectiveness of impedance matching efforts.-Directional SWR Meter:A directional SWR meter...

and/or an antenna analyzer

Antenna analyzer

An antenna analyzer is a device used for measuring the efficiency of antenna systems in radio electronics applications.-Theory of operation:...

are used to check the extent of the match between the aerial system and the transmitter via the transmission line (feeder). An SWR meter indicates forward power, reflected power, and the ratio between them.

See Antenna tuner

Antenna tuner

An antenna tuner, transmatch or antenna tuning unit is a device connected between a radio transmitter or receiver and its antenna to improve the efficiency of the power transfer between them by matching the impedance of the equipment to the antenna...

and balun

Balun

for details of matching networks and baluns respectively.

EMC matters

While this section was written from the point of view of an amateur radio operator

Amateur radio operator

An amateur radio operator is an individual who typically uses equipment at an amateur radio station to engage in two-way personal communications with other similar individuals on radio frequencies assigned to the amateur radio service. Amateur radio operators have been granted an amateur radio...

with relation to television interference it applies to the construction and use of all radio transmitters, and other electronic devices which generate high RF powers with no intention of radiating these. For instance a dielectric heater might contain a 2000 watt

Watt

27 MHz source within it, if the machine operates as intended then none of this RF power will leak out. However, if the device is subject to a fault then when it operates RF will leak out and it will be now a transmitter. Also computers are RF devices, if the case is poorly made then the computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

will radiate at VHF. For example if you attempt to tune into a weak FM radio station (88 to 108 MHz, band II) at your desk you may lose reception when you switch on your PC. Equipment which is not intended to generate RF, but does so through for example sparking

Electrostatic discharge

Electrostatic discharge is a serious issue in solid state electronics, such as integrated circuits. Integrated circuits are made from semiconductor materials such as silicon and insulating materials such as silicon dioxide...

at switch

Switch

In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

contacts is not considered here.

RF leakage (defective RF shielding)

All equipment using RF electronics

Electronics

should be inside a screened metal box, all connections in or out of the metal box should be filtered to avoid the ingress or egress 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

Capacitor

A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

. This is a capacitor which is mounted in a hole in the shield, one terminal of the capacitor is its metal body which touches the shielding of the box while the other two terminal of the capacitor are the on either side of the shield. The feed through capacitor can be thought of as a metal rod which has a dielectric sheath which in turn has a metal coating.

In addition to the feed through capacitor, either a resistor

Resistor

A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...

or RF choke

Choke (electronics)

A choke is a coil of insulated wire, often wound on a magnetic core, used as a passive inductor which blocks higher-frequency alternating current in an electrical circuit while passing signals of much lower frequency and direct current by having an impedance largely determined by reactance, which...

can be used to increase the filtering on the lead. In transmitters it is vital to prevent RF from entering the transmitter through any lead such as an electric power

Electric power

Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

, microphone

Microphone

A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...

or control

Control theory

Control theory is an interdisciplinary branch of engineering and mathematics that deals with the behavior of dynamical systems. The desired output of a system is called the reference...

connection. If RF does enter a transmitter in this way then an instability known as Motorboating (electronics) can occur. Motorboating is an example of a self inflicted EMC problem.

If a transmitter is suspected of being responsible for a television interference problem, then it should be run into a dummy load

Dummy load

A dummy load is a device used to simulate an electrical load, usually for testing purposes.-Radio:In radio this device is also known as a dummy antenna or a radio frequency termination. It is a device used in place of an antenna to aid in testing a radio transmitter...

; 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 does not cause interference during this test, then it is safe to assume that a signal has to be radiated from the antenna

Antenna (radio)

An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

to cause a problem. If the transmitter

Transmitter

does cause interference during this test then a path exists by which RF power is leaking out of the equipment, this can be due to bad shielding. This is a rare but insidious problem and it is vital that it be tested for. Such leakage is most likely to occur on homemade equipment or equipment that has been modified. RF leakage from microwave oven

Microwave oven

A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...

s may also sometimes be observed, especially when the oven's RF seal has been compromised.

Spurious emissions

Early in the development of radio technology it was recognised that the signals emitted by transmitters had to be 'pure'. For instance Spark-gap transmitter
Spark-gap transmitter
A spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...

s were quickly outlawed as they give an output which is so wide in terms of frequency. In modern equipment there are three main types of spurious emissions.

The term spurious emission
Spurious emission
A spurious emission is any radio frequency not deliberately created or transmitted, especially in a device which normally does create other frequencies...

s refers to any signal which comes out of a transmitter other than the wanted signal. The spurious emissions include harmonics, out of band mixer products which are not fully suppressed and leakage from the local oscillator
Local oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...

and other systems within the transmitter.

Harmonics

These are multiples of the operation frequency of the transmitter, they can be generated in a stage of the transmitter even if it is driven with a perfect sine wave because no real life amplifier is perfectly linear.

Avoiding harmonic generation

It is best if these harmonics are designed out at an early stage. For instance a push-pull amplifier consisting of two tetrode

Tetrode

valves attached to an anode

Anode

An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

tank resonant LC circuit which has a coil which is connected to the high voltage DC supply at the centre (Which is also RF ground) will only give a signal for the fundamental and the odd harmonics.

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

Low pass

Low Pass may refer to*Low Pass, Oregon*Low-pass filter* Low Pass in Mountain passes in Montana...

filter to reduce the level of the harmonics.

Detection

The harmonics can be tested for using an RF spectrum analyser (expensive) or with an absorption wavemeter

Absorption wavemeter

The Absorption wavemeter is a simple device for the measurement of radio frequency energy at different frequencies.The device can be used by the users of radio equipment to check the approximate frequency of a strong signal source, and also to check the output of a transmitter for harmonics. Many...

(cheap). If a harmonic is found which is at the same frequency as the frequency of the signal wanted at the receiver then this spurious emission can prevent the wanted signal from being received.

Local oscillators and unwanted mixing products

Imagine a transmitter, which has an intermediate frequency

Intermediate frequency

In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...

(IF) of 144 MHz, which is mixed with 94 MHz to create a signal at 50 MHz, which is then amplified and transmitted. If the local oscillator signal was to enter the power amplifier and not be adequately suppressed then it could be radiated. It would then have the potential to interfere with radio signals at 94 MHz in the FM audio (band II) broadcast band. Also the unwanted mixing product at 238 MHz could in a poorly designed system be radiated. Normally with good choice of the intermediate and local oscillator frequencies this type of trouble can be avoided, but one potentially bad situation is in the construction of a 144 to 70 MHz converter, here the local oscillator

Local oscillator

A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...

is at 74 MHz which is very close to the wanted output. Good well made units have been made which use this conversion but their design and construction has been challenging, for instance in the late 1980s Practical Wireless

Practical Wireless

Practical Wireless is a British radio and electronics magazine, now published monthly by PW Publishing of Broadstone, Dorset. The current editor is Rob Mannion....

published a design (Meon-4) for such a transverter

Transverter

A transverter is a radio frequency device that consists of an upconverter and a downconverter in one unit. Transverters are used in conjunction with transceivers to change the range of frequencies over which the transceiver can communicate....

http://pjm.dyndns.org/meon/meon4-1%20001.jpg http://pjm.dyndns.org/meon/. This problem can be thought of as being related to the Image response

Image response

Image response is a measure of performance of a radio receiver, particularly one that operates on the super-heterodyne principle....

problem which exists in receivers.

One method of reducing the potential for this transmitter defect is the use of balance and double balanced mixers. If the equation is assumed to be

E = E₁ E₂

and is driven by two simple sine waves, f₁ and f₂ then the output will be a mixture of four frequencies

f₁

f₁+f₂

f₁-f₂

f₂

If the simple mixer is replaced with a balanced mixer then the number of possible products is reduced. Imagine that two mixers which have the equation {I = E₁ E₂} are wired up so that the current outputs are wired to the two ends of a coil (the centre of this coil is wired to ground) then the total current flowing through the coil is the difference between the output of the two mixer stages. If the f₁ drive for one of the mixers is phase shifted by 180° then the overall system will be a balanced mixer.
E = K . E_f2 . ΔE_f1

So the output will now have only three frequencies

f₁+f₂

f₁-f₂

f₂

Now as the frequency mixer has fewer outputs the task of making sure that the final output is clean will be simpler.

Instability and parasitic oscillations

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. Another sign is if the output power suddenly increases wildly when the input power is increased slightly, it is noteworthy that in a class C stage that this behaviour can be seen under normal conditions. The best defence against this transmitter defect is a good design, also it is important to pay good attention to the neutralization of the valves or transistors.

Methods

Fixed frequency systems

Variable frequency systems

Frequency multiplication

Frequency mixing and modulation

AM modes

Low level

High level

Types of AM modulators

Plate AM modulators

Screen AM modulators

Single-sideband modulation

Filter method

Phasing method

Vestigial-sideband modulation

Morse

FM modes

Direct FM

Indirect FM

Valves

Advantages of valves

Disadvantages of valves

Solid state

Linking the transmitter to the aerial

EMC matters

RF leakage (defective RF shielding)

Spurious emissions

Harmonics

Avoiding harmonic generation

Removal of harmonics with filters

Detection

Local oscillators and unwanted mixing products

Instability and parasitic oscillations

See also