RF Encyclopedia
RF Encyclopedia, here you should find the basic terminology however there is always a need to spread this section so send your comments and let me know what I should add.
Band-stop filter
In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a range to very low levels. It is the opposite of a band-pass filter. A notch filter is a band-stop filter with a narrow stopband (high Q factor).
Other names include ‘band limit filter’, ‘T-notch filter’, ‘band-elimination filter’, and ‘band-rejection filter’.
Typically, the width of the stopband is less than 1 to 2 decades (that is, the highest frequency attenuated is less than 10 to 100 times the lowest frequency attenuated). In the audio band, a notch filter uses high and low frequencies that may be only semitones apart.
RF example 1: Non-linearities of power amplifiers For instance, when measuring non-linearities of power amplifiers a very narrow notch filter could be very useful to avoid the carrier so maximum input power will not be exceeded.
High-pass filter
A high-pass filter is a filter that passes high frequencies well, but attenuates (or reduces) frequencies lower than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a low-cut filter; the terms bass-cut filter or rumble filter are also used in audio applications. A high-pass filter is the opposite of a low-pass filter, and a bandpass filter is a combination of a high-pass and a low-pass.
It is useful as a filter to block any unwanted low frequency components of a complex signal while passing the higher frequencies. Of course, the meanings of ‘low’ and ‘high’ frequencies are relative to the cutoff frequency chosen by the filter designer.
Applications: Such a filter could be used to direct high frequencies to a tweeter speaker while blocking bass signals which could interfere with or damage the speaker. A low-pass filter, using a coil instead of a capacitor, could simultaneously be used to direct low frequencies to the woofer. See audio crossover.
High-pass and low-pass filters are also used in digital image processing to perform transformations in the frequency domain.
Most high-pass filters have zero gain (-inf dB) at DC. Such a high-pass filter with very low cutoff frequency can be used to block DC from a signal that is undesired in that signal (and pass nearly everything else). These are sometimes called DC blocking filters.
Low-pass filter
A low-pass filter is a filter that passes low frequencies well, but attenuates (or reduces) frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter when used in audio applications.
A high-pass filter is the opposite, and a bandpass filter is a combination of a high-pass and a low-pass.
The concept of a low-pass filter exists in many different forms, including electronic circuits (like a hiss filter used in audio), digital algorithms for smoothing sets of data, acoustic barriers, blurring of images, and so on. Low-pass filters play the same role in signal processing that moving averages do in some other fields, such as finance; both tools provide a smoother form of a signal which removes the short-term oscillations, leaving only the long-term trend.
Band-pass filter
A band-pass filter is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. An example of an analogue electronic band-pass filter is an RLC circuit (a resistor-inductor-capacitor circuit). These filters can also be created by combining a low-pass filter with a high-pass filter.
An ideal filter would have a completely flat passband (e.g. with no gain/attenuation throughout) and would completely attenuate all frequencies outside the passband. Additionally, the transition out of the passband would be instantaneous in frequency. In practice, no bandpass filter is ideal. The filter does not attenuate all frequencies outside the desired frequency range completely; in particular, there is a region just outside the intended passband where frequencies are attenuated, but not rejected. This is known as the filter roll-off, and it is usually expressed in dB of attenuation per octave of frequency. Generally, the design of a filter seeks to make the roll-off as narrow as possible, thus allowing the filter to perform as close as possible to its intended design. However, as the roll-off is made narrower, the passband is no longer flat and begins to “ripple.” This effect is particularly pronounced at the edge of the passband in an effect known as the Gibbs phenomenon.
LNB (Low-noise block) converter
A low-noise block converter (LNB, for low-noise block, or sometimes LNC, for low-noise converter) is used in communications satellite (usually broadcast satellite) reception. The LNB is usually fixed on or in the satellite dish, for the reasons outlined below.
Satellites use comparatively high radio frequencies to transmit their signals.
As microwave satellite signals do not easily pass through walls, roofs, or even glass windows, satellite antennas are required to be outdoors, and the signal needs to be passed indoors via cables. When radio signals are sent through coaxial cables, the higher the frequency, the more losses occur in the cable per unit of length. The signals used for satellite are of such high frequency (in the multiple gigahertz range) that special (costly) cable types or waveguide would be required and any significant length of cable leaves very little signal left on the receiving end.
The job of the LNB is to use the superheterodyne principle to take a wide block (or band) of relatively high frequencies, amplify and convert them to similar signals carried at a much lower frequency (called intermediate frequency or IF). These lower frequencies travel through cables with much less attenuation of the signal, so there is much more signal left on the satellite receiver end of the cable. It is also much easier and cheaper to design electronic circuits to operate at these lower frequencies, rather than the very high frequencies of satellite transmission.
The “low-noise” part means that special electronic engineering techniques are used, that the amplification and mixing takes place before cable attenuation and that the block is free of additional electronics like a power supply or a digital receiver. This all leads to a signal, which has less noise (unwanted signals) on the output than would be possible with less stringent engineering. Generally speaking, the higher the frequencies with which an electronic component has to operate, the more critical it is that noise be controlled. If low noise engineering techniques were not used, the sound and picture of satellite TV would be very low quality, if it could even be received at all without a much larger dish reflector. The low-noise quality of an LNB is expressed as the noise figure or noise temperature.
For the reception of wideband satellite television carriers, typically 27 MHz wide, the accuracy of the frequency of the LNB local oscillator need only be in the order of ±500kHz, so low cost dielectric oscillators (DRO) may be used. For the reception of narrow bandwidth carriers or ones using advanced modulation techniques, such as 16-QAM, highly stable and low phase noise LNB local oscillators are required. These use an internal crystal oscillator or an external 10 MHz reference from the indoor unit and a phase-locked loop (PLL) oscillator.
Satellite dish
A satellite dish is a type of parabolic antenna designed with the specific purpose of transmitting signals to and/or receiving from satellites. A satellite dish is a particular type of microwave antenna. Satellite dishes come in varying sizes and designs, and are most commonly used to receive satellite television.
The parabolic shape of a dish reflects the signal to the dish’s focal point. Mounted on brackets at the dish’s focal point is a device called a feedhorn. This feedhorn is essentially the front-end of a waveguide that gathers the signals at or near the focal point and ‘conducts’ them to a low-noise block downconverter or LNB. The LNB converts the signals from electromagnetic or radio waves to electrical signals and shifts the signals from the downlinked C-band and/or Ku-band to the L-band range. Direct broadcast satellite dishes use an LNBF, which integrates the feedhorn with the LNB. (A new form of omnidirectional satellite antenna, which does not use a directed parabolic dish and can be used on a mobile platform such as a vehicle, was recently announced by the University of Waterloo. [1])
Modern dishes intended for home television use are generally 43 cm (18″) to 80 cm (31″) in diameter, and are fixed in one position, for Ku-band reception from one orbital position. Prior to the existence of Direct broadcast satellite services, home users would generally have a motorised C-band satellite dish of up to 3 metres in diameter for reception of channels from different satellites. Overly small dishes can still cause problems, however, including rain fade and interference from adjacent satellites.
Motorised satellite dishes are still popular with enthusiasts, and three competing standards, which are often all supported by a set-top box, DiSEqC, USALS, and 36v Positioners.
Valve amplifier
A valve amplifier (UK and Aus.) or tube amplifier (U.S.), is a device for electrically amplifying the power of an electrical signal, typically (but not exclusively) sound or radio frequency signals.
Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and 1970s, and replacement valves are no longer produced in the same large quantities as they were in the past. Specially constructed valves are still in use at high power levels, especially at microwave frequencies; see the Microwave amplifiers section.
Valves are high voltage/low current devices in comparison with transistors (and especially MOSFETs) and their transfer characteristics show very flat anode current vs. anode voltage indicating high output impedances.
The high working voltage makes them well suited for radio transmitters, for example, and valves remain in use today for very high power radio transmitters, where there is still no other technology available. However, for most applications requiring an appreciable output current, a matching transformer is required. The transformer is a critical component and heavily influences the performance (and cost) of the amplifier.
Many power valves have good open-loop linearity, but only modest gain or transconductance. As a result, valve amplifiers usually need only modest levels of feedback. Signal amplifiers using tubes are capable of very high frequency response ranges - up to radio frequency. Indeed, many of the Directly Heated Single Ended Triode (DH-SET) audio amplifiers are in fact radio transmitting tubes designed to operate in the megahertz range. In practice, however, tube amplifier designs typically “couple” stages either capacitively, limiting bandwidth at the low end, or inductively with transformers, limiting the bandwidth at high end.
Circuit advantages of valves
* Good for high power systems
* Electrically very robust, they can tolerate overloads for minutes which would destroy bipolar transistor systems in milliseconds.
Disadvantages of valves
* Heater supplies are required for the cathodes
* Dangerously high voltages are required for the anodes
* Valved audio equipment is normally heavy because of the weight of transformers
* Valves often have a shorter working life than solid state parts because the heaters tend to fail
* Valves are fragile and break if hit, since they are usually made of glass. Solid state components don’t have this problem.
Modulation
Modulation is the process of varying a carrier signal in order to use that signal to convey information. The three key parameters of a sinusoid are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal. A device that performs modulation is known as a modulator and a device that performs the inverse operation of demodulation is known as a demodulator. A device that can do both operations is a modem (a contraction of the two terms).
In digital modulation, the changes in the signal are chosen from a fixed list (the modulation alphabet) each entry of which conveys a different possible piece of information (a symbol). The alphabet is often conveniently represented on a constellation diagram.
In analog modulation, the change is applied continuously in response to the data signal. The modulation may be applied to various aspects of the signal as the lists below indicate.
Amplitude modulation
Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting audio signals. It works by varying the strength of the transmitted signal in relation to the information being sent, for example, changes in the signal strength can be used to reflect sounds being reproduced. (Contrast this with frequency modulation, in which the transmitting frequency is varied; and phase modulation, in which the phase is varied.)
In the mid-1870s, a form of amplitude modulation—initially called “undulatory currents”—was the first method to successfully produce quality audio over telephone lines. Beginning in the early 1900s, it was also the original method used for audio radio transmissions, and remains in use by some forms of radio communication—”AM” is often used to refer to the mediumwave broadcast band (see AM radio).
As originally developed for the electric telephone, amplitude modulation was used to add audio information to the low-powered direct current flowing from a telephone transmitter to a receiver. As a simplified explanation, at the transmitting end, a telephone microphone was used to vary the strength of the transmitted current, according to the frequency and loudness of the sounds received. Then, at the receiving end of the telephone line, the transmitted electrical current affected an electromagnet, which strengthened and weakened in response to the strength of the current. In turn, the electromagnet produced vibrations in the receiver diaphragm, thus reproducing the frequency and loudness of the sounds originally heard at the transmitter.
In contrast to the telephone, in radio communication what is modulated is a continuous wave radio signal (carrier wave) produced by a radio transmitter. In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands. Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. Thus, most of the power output by an AM transmitter is effectively wasted: half the power is concentrated at the carrier frequency, which carries no useful information (beyond the fact that a signal is present); the remaining power is split between two identical sidebands, only one of which is needed.
To increase transmitter efficiency, the carrier can be removed (suppressed) from the AM signal. This produces a reduced-carrier transmission or double-sideband suppressed carrier (DSBSC) signal. If the carrier is only partially suppressed, a double-sideband reduced carrier (DSBRC) signal results. DSBSC and DSBRC signals need their carrier to be regenerated (by a beat frequency oscillator, for instance) to be demodulated using conventional techniques.
Even greater efficiency is achieved—at the expense of increased transmitter and receiver complexity—by completely suppressing both the carrier and one of the sidebands. This is single-sideband modulation, widely used in amateur radio due to its efficient use of both power and bandwidth.
A simple form of AM often used for digital communications is on-off keying, a type of amplitude-shift keying by which binary data is represented as the presence or absence of a carrier wave. This is commonly used at radio frequencies to transmit Morse code, referred to as continuous wave (CW) operation.
Frequency modulation
Frequency modulation (FM) is a form of modulation which represents information as variations in the instantaneous frequency of a carrier wave. (Contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant.) In analog applications, the carrier frequency is varied in direct proportion to changes in the amplitude of an input signal. Digital data can be represented by shifting the carrier frequency among a set of discrete values, a technique known as frequency-shift keying.
FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech (see FM broadcasting). Normal (analog) TV sound is also broadcast using FM. A narrowband form is used for voice communications in commercial and amateur radio settings. The type of FM used in broadcast is generally called wide-FM, or W-FM. In two-way radio, narrowband narrow-fm (N-FM) is used to conserve bandwidth. In addition, it is used to send signals into space.
FM is also used at intermediate frequencies by most analog VCR systems, including VHS, to record the luminance (black and white) portion of the video signal. FM is the only feasible method of recording video to and retrieving video from magnetic tape without extreme distortion, as video signals have a very large range of frequency components — from a few hertz to several megahertz, too wide for equalisers to work with due to electronic noise below -60 dB. FM also keeps the tape at saturation level, and therefore acts as a form of noise reduction, and a simple limiter can mask variations in the playback output, and the FM capture effect removes print-through and pre-echo. A continuous pilot-tone, if added to the signal — as was done on V2000 and many Hi-band formats — can keep mechanical jitter under control and assist timebase correction.
FM is also used at audio frequencies to synthesize sound. This technique, known as FM synthesis, was popularized by early digital synthesizers and became a standard feature for several generations of personal computer sound cards.
Phase modulation
Phase modulation (PM) is a form of modulation which represents information as variations in the instantaneous phase of a carrier wave.
Unlike its more popular counterpart, frequency modulation (FM), PM is not very widely used (except perhaps for in the inappropriately named FM-synthesis for musical instruments, introduced by Yamaha around 1982.) This is because it tends to require more complex receiving hardware and there can be ambiguity problems with determining whether, for example, the signal has 0° phase or 180° phase.
Phase-shift keying
Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave).
Any digital modulation scheme uses a finite number of distinct signals to represent digital data. In the case of PSK, a finite number of phases are used. Each of these phases is assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal — such a system is termed coherent.
Alternatively, instead of using the bit patterns to set the phase of the wave, it can instead be used to change it by a specified amount. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK since there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (it is a non-coherent scheme). In exchange, it produces more erroneous demodulations. The exact requirements of the particular scenario under consideration determine which scheme is used.
Frequency-shift keying
Frequency-shift keying (FSK) is a form of frequency modulation in which the modulating signal shifts the output frequency between predetermined values.
Usually, the instantaneous frequency is shifted between two discrete values termed the mark frequency and the space frequency.
Continuous phase forms of FSK exist in which there is no phase discontinuity in the modulated signal. The example shown at right is of such a form.
Other names for FSK are frequency-shift modulation and frequency-shift signaling.
Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly spectrally efficient form of coherent frequency-shift keying. In MSK the difference between the higher and lower frequency is identical to half the bit rate. As a result, the waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier period. This is the smallest FSK modulation index that can be chosen such that the waveforms for 0 and 1 are orthogonal. A variant of MSK called GMSK is used in the GSM mobile phone standard.
Minimum-shift keying
Minimum-shift keying (MSK) is a type of continuous phase frequency-shift keying.
Similarly to OQPSK, MSK is encoded with bits alternating between quarternary components, with the Q component delayed by half a bit period. However, instead of square pulses as OQPSK uses, MSK encodes each bit as a half sinusoid. This results in a constant-modulus signal, which reduces problems caused by non-linear distortion.
Gaussian minimum shift keying
Gaussian minimum shift keying or GMSK is a kind of continuous phase frequency-shift keying. The baseband modulation is generated by starting with a bitstream 0/1 and a bit-clock giving a timeslice for each bit. This is the type of modulation used in Global System for Mobile Communications (GSM).
The baseband signal is generated by first transforming the zero/one encoded bits into -1/+1 encoded bits. This -1/+1 signal is then filtered in such a way that the “boxcar” shaped +1/-1 pulses are transformed into Gaussian-shaped signals. The baseband signal is then modulated using frequency modulation, producing a complete GMSK signal. If the Gaussian shapes do not overlap, then the modulation form is called 1-GMSK. If the slots overlap 50% (½), the modulation is called 2-GMSK, and so on.
The more the bits overlap, the more significant intersymbol interference (ISI) from adjacent bits will be, and for 4-GMSK and up, the ISI seen at any particular point in time is stronger than the signal from the bit currently being decoded. By looking at greater parts of the signal using advanced decoder techniques (including Viterbi algorithm decoders), high density codings can be decoded efficiently. Currently the highest density coding being used is 5-GMSK.
Audio frequency-shift keying
Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented as changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. Normally, the transmitted audio alternates between two tones: one, the “mark”, represents a binary one; the other, the “space”, represents a binary zero.
AFSK differs from regular frequency-shift keying in that the modulation is performed at baseband frequencies. In radio applications, the AFSK-modulated signal is normally used to modulate an RF carrier (using a conventional technique, such as AM FM or ACSSB(R)(LM Mode(R)) for transmission.
AFSK is not generally used for high-speed data communications, as it is less efficient than other modulation modes. In addition to its simplicity, however, AFSK has the advantage that encoded signals will pass through AC-coupled links, including most equipment originally designed to carry music or speech.
Most early telephone-line modems used audio frequency-shift keying to send and receive data, up to rates of about 300 bits per second. The common Bell 103 modem used this technique, for example. Some early microcomputers used a specific form of AFSK modulation, the Kansas City standard, to store data on audio cassettes. AFSK is still widely used in amateur radio, as it allows data transmission through unmodified voiceband equipment.
AFSK is also used in the United States’ Emergency Alert System to transmit warning information. It is used at higher bitrates for Weathercopy used on Weatheradio by NOAA in the U.S., and more extensively by Environment Canada.
The CHU shortwave radio station in Ottawa, Canada broadcasts a Exclusive digital time signal encoded using AFSK modulation.
Amplitude-shift keying
Amplitude-shift keying (ASK) is a form of modulation which represents digital data as variations in the amplitude of a carrier wave.
The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal), keeping frequency and phase constant. The level of amplitude can be used to represent binary logic 0s and 1s. We can think of a carrier signal as an ON or OFF switch. In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given.
Like AM, ASK is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes in PSTN, etc. It requires excessive bandwidth and is therefore a waste of power. Both ASK modulation and demodulation processes are relatively inexpensive. This type of modulation can be used to transmit digital data over fiber.
Quadrature amplitude modulation
Quadrature amplitude modulation (QAM) is a modulation scheme which conveys data by changing (modulating) the amplitude of two carrier waves. These two waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers — hence the name of the scheme.
As with all modulation schemes, QAM conveys data by changing some aspect of a carrier signal, or the carrier wave, (usually a sinusoid) in response to a data signal. In the case of QAM, the amplitude of two waves, 90 degress out-of-phase with each other (in quadrature) are changed (modulated or keyed) to represent the data signal.
Phase modulation (analogue PM) and phase-shift keying (digital PSK) can be regarded as a special case of QAM, where the amplitude of the modulating signal is constant, with only the phase varying. This can also be extended to frequency modulation (FM) and frequency-shift keying (FSK), as these can be regarded as a special case of phase modulation.
Although analogue QAM is possible, this article focuses on digital QAM. Analogue QAM is used in NTSC, PAL and SECAM television systems, where the I- and Q-signals carry the components of chroma (colour) information. “Compatible QAM” or C-QUAM is used in AM stereo radio to carry the stereo difference information.
Continuous phase modulation
Continuous phase modulation (CPM) is a method for modulation of data commonly used in wireless modems. In contrast to other coherent digital phase-modulation techniques where the carrier phase abruptly resets to zero at the start of every symbol (e.g. M-PSK), with CPM the carrier phase is modulated in a continuous manner. For instance, with QPSK the carrier instantaneously jumps from a sine to a cosine (i.e. a 90 degree phase shift) whenever one of the two message bits of the current symbol differs from the two message bits of the previous symbol. This discontinuity requires a relatively large percentage of the power to occur outside of the intended band (e.g., high fractional out-of-band power), leading to poor spectral efficiency. Furthermore, CPM is typically implemented as a constant-envelope waveform, i.e. the transmitted carrier power is constant. Therefore, CPM is attractive because the phase continuity yields high spectral efficiency, and the constant-envelope yields excellent power efficiency. The primary drawback is the high implementation complexity required for an optimal receiver.
Demodulator
A demodulator is an electronic circuit used to recover the information content from the carrier wave of a signal. The term is usually used in connection with radio receivers, but there are many kinds of demodulators used in many other systems. Another common one is in a modem, which is a contraction of the terms modulator/demodulator.
Transmitter
A transmitter (sometimes abbreviated XMTR) is an electronic device which with the aid of an antenna propagates an electromagnetic signal such as radio, television, or other telecommunications.
A transmitter usually has a power supply, an oscillator, a modulator, and amplifiers for audio frequency (AF) and radio frequency (RF). The modulator is the device which piggybacks (or modulates) the signal information onto the carrier frequency, which is then broadcast. Sometimes a device (for example, a cell phone) contains both a transmitter and a radio receiver, with the combined unit referred to as a transceiver.
More generally and in communications and information processing, a “transmitter” is any object (source) which sends information to an observer (receiver). When used in this more general sense, vocal cords may also be considered an example of a “transmitter”.
In industrial process control a “transmitter” is any device which converts measurements from a sensor into a signal to be received, usually sent via wires, by some display or control device located a distance away. Typically in process control applications the “transmitter” will output a 4-20 mA current loop or digital protocol to represent a measured variable within a range. For example, a pressure transmitter might use 4 ma as a representation for 50 psig of pressure and 20 ma as 1000 psig of pressure and any value in between proportionately ranged between 50 and 1000 psig. Older technology transmitters used pneumatic pressure typically ranged between 3 to 15 psig (20 to 100 kPa) to represent a process variable.
Receiver (radio)
In radio terminology, a receiver is an electronic circuit that receives a radio signal from an antenna and decodes the signal for use as sound, pictures, navigational-position information, etc. Radio and radio receiver are often used specifically for receivers whose output consists only of sound, although other types of receivers,such as television receivers, are technically radio receivers as well.
A radio receiver is a real world example of a receiver in the information theoretic sense.
As an audio appliance, “receiver” refers to a tuner, a preamplifier, and a power amplifier all on the same chassis. Audiophiles will refer to such a device as an integrated receiver, while a single chassis that implements only one of the three component functions is called a discrete component. Some audio purists still prefer three discreet units - tuner, preamplifier and power amplifier - but the integrated receiver has, for some years, been the mainstream choice for music listening. The first integrated stereo receiver was made by the Harman Kardon company, and came onto the market in 1958. It had undistingushed performance, but it represented a breakthrough to the “all in one” concept of a receiver, and rapidly improving designs gradually made the receiver the mainstay of the marketplace.
Most older receivers also came with a loudspeaker (see photo). Today AV receivers are a common component in a high-fidelity or home-theatre system. The receiver is generally the nerve centre of a sophisticated home-theatre system providing selectable inputs for a number of different audio components like turntables, compact-disc players, and tape decks and video components like video-cassette recorders, DVD players, video-game systems, and televisions. With the decline of vinyl discs, modern receivers tend to omit inputs for turntables, which have separate requirements of their own. All other common audio/visual components can use any of the identical line-level inputs on the receiver for playback, regardless of how they are marked (the “name” on each input is mostly for the convienience of the user.) For instance, a second CD player can be plugged into an “Aux” input, and will work the same as it will in the “CD” input jacks. Some receivers can also provide signal processors to give a more realistic illusion of listening in a concert hall. Digital audio S/PDIF connections are also common today.
Some modern integrated receivers can send audio out to seven loudspeakers and an additional channel for a subwoofer and often include connections for headphones. Receivers vary greatly in price, and support stereophonic or surround sound. A high-quality receiver for dedicated audio-only listening (two channel stereo) can be relatively inexpensive; excellent ones can be purchased for $300 US or less. Because modern receivers are purely electronic devices with no moving parts unlike electromechanical devices like turntables and cassette decks, they tend to offer many years of trouble-free service. In recent years, the home theater in a box has become common, which often integrates a surround-capable receiver with a DVD player. The user simply connects it to a television, perhaps other components, and a set of loudspeakers.
Self-powered radios (clockwork radio) with a hand-cranked generator are used in developing nations or as part of an emergency/disaster preparedness kit.
Electromagnetism
Electromagnetism is the physics of the electromagnetic field; a field encompassing all of space which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles.
It is often convenient to understand the electromagnetic field in terms of two separate fields: the electric field and the magnetic field.
The electric field is produced by the presence of electrically charged particles, and causes the electric force. Electric force is the force observed as static electricity, and causes the flow of electric charge (electric current) in electrical conductors.
The magnetic field is produced by the motion of electric charges, i.e. electric current. The magnetic field causes the magnetic force associated with magnets.
The term “electromagnetism” comes from the fact that electrical and magnetic forces are involved simultaneously. A changing magnetic field produces an electric field (this is the phenomenon of electromagnetic induction, which provides for the operation of electrical generators, induction motors, and transformers). Similarly, a changing electric field generates a magnetic field. Because of this interdependence of the electric and magnetic fields, it makes sense to consider them as a single coherent entity — the electromagnetic field.
This unification, which was completed by James Clerk Maxwell, is one of the triumphs of 19th century physics. It had far-reaching consequences, one of which was the understanding of the nature of light. As it turns out, what is thought of as “light” is actually a propagating oscillatory disturbance in the electromagnetic field, i.e., an electromagnetic wave. Different frequencies of oscillation give rise to the different forms of electromagnetic radiation, from radio waves at the lowest frequencies, to visible light at intermediate frequencies, to gamma rays at the highest frequencies.
The theoretical implications of electromagnetism led to the development of special relativity by Albert Einstein in 1905.
Signal
In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying quantity. Signals are often scalar-valued functions of time (waveforms), but may be vector valued and may be functions of any other relevant independent variable.
The concept is broad, and hard to define precisely. Definitions specific to subfields are common. For example, in information theory, a signal is a codified message, ie, the sequence of states in a communications channel that encodes a message. In a communications system, a transmitter encodes a message into a signal, which is carried to a receiver by the communications channel. For example, the words “Mary had a little lamb” might be the message spoken into a telephone. The telephone transmitter converts the sounds into an electrical voltage signal. The signal is transmitted to the receiving telephone by wires; and at the receiver it is reconverted into sounds.
Signals can be categorized in various ways. The most common distinction is between discrete and continuous spaces that the functions are defined over, for example discrete and continuous time domains. Discrete-time signals are often referred to as time series in other fields. Continuous-time signals are often referred to as continuous signals even when the signal functions are not continuous; an example is a square-wave signal.
A second important distinction is between discrete-valued and continuous-valued. Digital signals are discrete-valued, but are often derived from an underlying continuous-valued physical process.
Noise
In common use the word noise means unwanted sound or noise pollution. In electronics noise can refer to the electronic signal corresponding to acoustic noise (in an audio system) or the electronic signal corresponding to the (visual) noise commonly seen as ’snow’ on a degraded television or video image. In signal processing or computing it can be considered data without meaning; that is, data that is not being used to transmit a signal, but is simply produced as an unwanted by-product of other activities. In Information Theory, however, noise is still considered to be information. In a broader sense the film grain or even advertisements in web pages can be considered noise.
Radio noise is interference with radio transmissions caused either by thermal noise from receiver input circuits or by radiated electromagnetic noise picked up by the receiver’s antenna. If no noise was picked up with radio signals, even weak transmissions could be received at virtually any distance by making a radio receiver that was sensitive enough. In practice this doesn’t work, and a point is reached where the only way to extend the range of a transmission is to increase the transmitter power.
Thermal noise can be made lower by cooling the circuits, but this is only usually worthwhile on radio telescopes. In other applications the limiting noise source depends on the frequency range in use. At low freqencies (longwave or mediumwave) and at high frequencies (shortwave), interference caused by lightning or by nearby electrical impulses in electrical switches, motors, vehicle ignition circuits, computers, and other man-made sources tends to swamp transmissions with thermal noise. These noises are often referred to as static. At very high frequency and ultra high frequency these sources can still be important, but at a much lower level, such that thermal noise is usually the limiting factor. In the microwave region, cosmic background noise may be relevant.
Electromagnetic noise can interfere with electronic equipment in general, causing malfunction, and in recent years standards have been laid down for the levels of electromagnetic radiation that electronic equipment is permitted to radiate. These standards are aimed at ensuring what is referred to as electromagnetic compatibility, or EMC.
Frequency mixer
In telecommunication, a mixer is a nonlinear circuit or device that accepts as its input two different frequencies and presents at its output (a) a signal equal in frequency to the sum of the frequencies of the input signals, (b) a signal equal in frequency to the difference between the frequencies of the input signals, and, if they are not filtered out, (c) the original input frequencies.
Electronic oscillator
An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave.
A low-frequency oscillator (or LFO) is an electronic oscillator that generates an AC waveform between 0.1 Hz and 10 Hz. This term is typically used in the field of audio synthesizers, to distinguish it from an audio frequency oscillator.
Shortwave
Shortwave radio operates between the frequencies of 2,310 kHz and 30 MHz (30,000 kHz) [1] and came to be referred to as such in the early days of radio because the wavelengths associated with this frequency range were shorter than those commonly in use at that time. An alternate name is HF or high frequency radio. Short wavelengths are associated with high frequencies because there is an inverse relationship between frequency and wavelength.
Longwave
The Longwave radio broadcasting band are those frequencies between 153 - 279 kHz, which correspond to wavelengths longer than 600 meters. This range is included within the low frequency band (but the low frequency band extends above and below longwave signals). Longwave signals have the property of following the curvature of the earth, making them ideal for continuous, continental communications. Unlike shortwave radio, longwave signals do not reflect or refract using the ionosphere, so there are fewer interference-caused fadeouts. Instead, the D-layer of the ionosphere and the surface of the earth serve as a waveguide directing the signal.
The earliest radio transmitters were all longwave transmitters, because propagation of radio waves of higher frequency was not yet understood. Radio alternator or spark-gap transmitters were commonly used to generate the radio frequency carrier wave.
Mediumwave
Mediumwave (MW) radio transmissions serves as the most common band for broadcasting. The standard AM broadcast band is 525 kHz to 1715 kHz in North America, but remains only up to 1615 kHz elsewhere.
In most of the Americas, mediumwave stations are separated by 10 kHz and have two sidebands of ±5 kHz. In the rest of the world, the separation is 9 kHz, with sidebands of ±4.5 kHz. Both provide adequate audio quality for voice, but are insufficient for high-fidelity broadcasting, which is common on the VHF FM bands. In the US the maximum transmitter power is restricted to 50 kilowatts, while in Europe there are medium wave stations with transmitter power up to 2.5 megawatts.
Mediumwave signals have the property of following the curvature of the earth (the groundwave) at all times, and also reflecting off the ionosphere at night (skywave). This makes this frequency band ideal for both local and continent-wide service, depending on the time of day. For example, during the day a radio receiver in the state of Maryland is able to receive reliable but weak signals from high-power stations WFAN, 660 kHz, and WOR, 710 kHz, 400 km away in New York City, due to groundwave propagation. The effectiveness of groundwave signals largely depends on ground conductivity—higher conductivity results in better propagation. At night, the same receiver picks up signals as far away as Mexico City and Chicago reliably.
Many North American stations are required to shut down or reduce power at night in order to make way for clear channel stations that can then be received over a wider range.
In Europe, each country is allocated a number of frequencies on which high power (up to 2.5 MW) can be used; the maximum power is also subject to international agreement. Other countries may only operate low-powered transmitters on the same frequency, again subject to agreement. For example, Russia operates a high-powered transmitter, located in its Kaliningrad exclave and used for external broadcasting, on 1323 kHz. The same frequency is also used by low-powered local radio stations in England; other parts of England can still receive the Russian broadcast. International mediumwave broadcasting in Europe has decreased markedly with the end of the Cold War and the increased availability of satellite and Internet TV and radio, although the cross-border reception of neighbouring countries’ broadcasts by expatriates and other interested listeners still takes place.
Due to the high demand for frequencies in Europe, many countries operate single frequency networks; in Britain, BBC Radio 5 broadcasts from various transmitters on either 693 or 909 kHz. These transmitters are carefully synchronised to minimise interference from more distant transmitters on the same frequency.
Decibel
The decibel (dB) is a measure of the ratio between two quantities, and is used in a wide variety of measurements in acoustics, physics and electronics. While originally only used for power and intensity ratios, it has come to be used more generally in engineering. The decibel is widely used in measurements of the loudness of sound. It is a “dimensionless unit” like percent. Decibels are useful because they allow even very large or small ratios to be represented with a conveniently small number (similar to scientific notation). This is achieved by using a logarithm.
Antenna
An antenna or aerial is an electrical device designed to transmit or receive radio waves or, more generally, any electromagnetic waves. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, radar, and space exploration. Antennas usually work in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies.
Physically, an antenna is an arrangement of conductors that generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. Some antenna devices (parabola, horn antenna) just adapt the free space to another type of antenna.
Antennas were used for the first time, in 1889, by Heinrich Hertz (1857-1894) to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. He even placed the emitter dipole in the focal point of a parabolic reflector. He published his work and installation drawings in Annalen der Physik und Chemie (vol. 36, 1889).
