Automatic gain control
Automatic gain control called automatic volume control, is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of, to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the input. The average or peak output signal level is used to dynamically adjust the gain of the amplifiers, enabling the circuit to work satisfactorily with a greater range of input signal levels, it is used in most radio receivers to equalize the average volume of different radio stations due to differences in received signal strength, as well as variations in a single station's radio signal due to fading. Without AGC the sound emitted from an AM radio receiver would vary to an extreme extent from a weak to a strong signal. In a typical receiver the AGC feedback control signal is taken from the detector stage and applied to control the gain of the IF or RF amplifier stages; the signal to be gain controlled goes to a diode & capacitor, which produce a peak-following DC voltage.
This is fed to the RF gain blocks thus altering their gain. Traditionally all the gain-controlled stages came before the signal detection, but it is possible to improve gain control by adding a gain-controlled stage after signal detection. In 1925, Harold Alden Wheeler obtained a patent. Karl Küpfmüller published an analysis of AGC systems in 1928. By the early 1930s most new commercial broadcast receivers included automatic volume control. AGC is a departure from linearity in AM radio receivers. Without AGC, an AM radio would have a linear relationship between the signal amplitude and the sound waveform – the sound amplitude, which correlates with loudness, is proportional to the radio signal amplitude, because the information content of the signal is carried by the changes of amplitude of the carrier wave. If the circuit were not linear, the modulated signal could not be recovered with reasonable fidelity. However, the strength of the signal received will vary depending on the power and distance of the transmitter, signal path attenuation.
The AGC circuit keeps the receiver's output level from fluctuating too much by detecting the overall strength of the signal and automatically adjusting the gain of the receiver to maintain the output level within an acceptable range. For a weak signal, the AGC operates the receiver at maximum gain, it is disadvantageous to reduce the gain of the RF front end of the receiver on weaker signals as low gain can worsen signal-to-noise ratio and blocking. Since the AM detector diode produces a DC voltage proportional to signal strength, this voltage can be fed back to earlier stages of the receiver to reduce gain. A filter network is required so that the audio components of the signal don't appreciably influence gain. Communications receivers may have more complex AVC systems, including extra amplification stages, separate AGC detector diodes, different time constants for broadcast and shortwave bands, application of different levels of AGC voltage to different stages of the receiver to prevent distortion and cross-modulation.
Design of the AVC system has a great effect on the usability of the receiver, tuning characteristics, audio fidelity, behavior on overload and strong signals. FM receivers though they incorporate limiter stages and detectors that are insensitive to amplitude variations, still benefit from AGC to prevent overload on strong signals. A related application of AGC is as a method of overcoming unwanted clutter echoes; this method relies on the fact. The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter. While this does not help detect targets masked by stronger surrounding clutter, it does help to distinguish strong target sources. In the past, radar AGC was electronically controlled and affected the gain of the entire radar receiver; as radars evolved, AGC became computer-software controlled, affected the gain with greater granularity, in specific detection cells. Many radar countermeasures use a radar's AGC to fool it, by "drowning out" the real signal with the spoof, as the AGC will regard the weaker, true signal as clutter relative to the strong spoof.
An audio tape generates a certain amount of noise. If the level of the signal on the tape is low, the noise is more prominent, i.e. the signal-to-noise ratio is lower than it could be. To produce the least noisy recording, the recording level should be set as high as possible without being so high as to clip or distort the signal. In professional high-fidelity recording the level is set manually using a peak-reading meter; when high fidelity is not a requirement, a suitable recording level can be set by an AGC circuit which reduces the gain as the average signal level increases. This allows a usable recording to be made for speech some distance from the microphone of an audio recorder. Similar considerations apply with VCRs. A potential disadvantage of AGC is that when recording something like music with quiet and loud passages such as classical music, the AGC will tend to make the quiet passages louder and the loud passages quieter, compressing the dynamic range. So
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing power as a result of the modulation process. The sidebands carry the information transmitted by the signal; the sidebands consist of all the Fourier components of the modulated signal except the carrier. All forms of modulation produce sidebands. Amplitude modulation of a carrier signal results in two mirror-image sidebands; the signal components above the carrier frequency constitute the upper sideband, those below the carrier frequency constitute the lower sideband. For example, if a 900 kHz carrier is amplitude modulated by a 1 kHz audio signal, there will be components at 899 kHz and 901 kHz as well as 900 kHz in the generated radio frequency spectrum. In conventional AM transmission, as used by broadcast band AM stations, the original audio signal can be recovered by either synchronous detector circuits or by simple envelope detectors because the carrier and both sidebands are present.
This is sometimes called double sideband amplitude modulation, but not all variants of DSB are compatible with envelope detectors. In some forms of AM, the carrier may be reduced; the term DSB reduced-carrier implies enough carrier remains in the transmission to enable a receiver circuit to regenerate a strong carrier or at least synchronise a phase-locked loop but there are forms where the carrier is removed producing double sideband with suppressed carrier. Suppressed carrier systems require more sophisticated circuits in the receiver and some other method of deducing the original carrier frequency. An example is the stereophonic difference information transmitted in stereo FM broadcasting on a 38 kHz subcarrier where a low-power signal at half the 38-kHz carrier frequency is inserted between the monaural signal frequencies and the bottom of the stereo information sub-carrier; the receiver locally regenerates the subcarrier by doubling a special 19 kHz pilot tone. In another example, the quadrature modulation used for chroma information in PAL television broadcasts, the synchronising signal is a short burst of a few cycles of carrier during the "back porch" part of each scan line when no image is transmitted.
But in other DSB-SC systems, the carrier may be regenerated directly from the sidebands by a Costas loop or squaring loop. This is common in digital transmission systems such as BPSK. If part of one sideband and all of the other remain, it is called vestigial sideband, used with television broadcasting, which would otherwise take up an unacceptable amount of bandwidth. Transmission in which only one sideband is transmitted is called single-sideband modulation or SSB. SSB is the predominant voice mode on shortwave radio other than shortwave broadcasting. Since the sidebands are mirror images, which sideband is used is a matter of convention. In SSB, the carrier is suppressed reducing the electrical power without affecting the information in the sideband; this makes for more efficient use of transmitter power and RF bandwidth, but a beat frequency oscillator must be used at the receiver to reconstitute the carrier. If the reconstituted carrier frequency is wrong the output of the receiver will have the wrong frequencies, but for speech small frequency errors are no problem for intelligibility.
Another way to look at an SSB receiver is as an RF-to-audio frequency transposer: in USB mode, the dial frequency is subtracted from each radio frequency component to produce a corresponding audio component, while in LSB mode each incoming radio frequency component is subtracted from the dial frequency. Sidebands can interfere with adjacent channels; the part of the sideband that would overlap the neighboring channel must be suppressed by filters, before or after modulation. In broadcast band frequency modulation, subcarriers above 75 kHz are limited to a small percentage of modulation and are prohibited above 99 kHz altogether to protect the ±75 kHz normal deviation and ±100 kHz channel boundaries. Amateur radio and public service FM transmitters utilize ±5 kHz deviation. Frequency modulation generates sidebands, the bandwidth consumed depending on the modulation index - requiring more bandwidth than DSB. Bessel functions can be used to calculate the bandwidth requirements of FM transmissions.
Independent sideband Out-of-band communications involve a channel other than the main communication channel. Side lobe Sideband computing is a distributed computing method using a channel separate from the main communication channel. TV transmitter
Homodyne detection is a method of extracting information encoded as modulation of the phase and/or frequency of an oscillating signal, by comparing that signal with a standard oscillation that would be identical to the signal if it carried null information. "Homodyne" signifies a single frequency, in contrast to the dual frequencies employed in heterodyne detection. When applied to processing of the reflected signal in remote sensing for topography, homodyne detection lacks the ability of heterodyne detection to determine the size of a static discontinuity in elevation between two locations. Homodyne detection is more applicable to velocity sensing. In optical interferometry, homodyne signifies that the reference radiation is derived from the same source as the signal before the modulating process. For example, in a laser scattering measurement, the laser beam is split into two parts. One is the local oscillator and the other is sent to the system to be probed; the scattered light is mixed with the local oscillator on the detector.
This arrangement has the advantage of being insensitive to fluctuations in the frequency of the laser. The scattered beam will be weak, in which case the steady component of the detector output is a good measure of the instantaneous local oscillator intensity and therefore can be used to compensate for any fluctuations in the intensity of the laser. In radio technology, the distinction is not the source of the local oscillator, but the frequency used. In heterodyne detection, the local oscillator is frequency-shifted, while in homodyne detection it has the same frequency as the radiation to be detected. See direct conversion receiver. Lock-in amplifiers are homodyne detectors integrated into measurement equipment or packaged as stand-alone laboratory equipment for sensitive detection and selective filtering of weak or noisy signals. Homodyne/lock-in detection has been one of the most used signal processing methods across a wide range of experimental disciplines for decades. Homodyne and heterodyne techniques are used in thermoreflectance techniques.
In the processing of signals in some applications of magnetic resonance imaging, homodyne detection can offer advantages over magnitude detection. The homodyne technique can suppress excessive noise and undesired quadrature components, provide stable access to information that may be encoded into the phase or polarity of images. An encrypted secure communication system can be based on quantum key distribution. An efficient receiver scheme for implementing QKD is balanced homodyne detection using a positive-intrinsic-negative diode. Homodyne detection was one of the key techniques in demonstrating quantum entanglement. Optical heterodyne detection Heterodyne detection Heterodyne Su, Shi-Lei. "Generating a four-photon polarization-entangled cluster state with homodyne measurement via cross-Kerr nonlinearity". Chinese Physics B. 21: 044205. Bibcode:2012ChPhB..21d4205S. Doi:10.1088/1674-1056/21/4/044205. ISSN 1674-1056
Radio receiver design
Radio receiver design includes the electronic design of different components of a radio receiver which processes the radio frequency signal from an antenna in order to produce usable information such as audio. The complexity of a modern receiver and the possible range of circuitry and methods employed are more covered in electronics and communications engineering; the term radio receiver is understood in this article to mean any device, intended to receive a radio signal in order to generate useful information from the signal, most notably a recreation of the so-called baseband signal which modulated the radio signal at the time of transmission in a communications or broadcast system. Design of a radio receiver must consider several fundamental criteria to produce a practical result; the main criteria are gain, selectivity and stability. The receiver must contain a detector to recover the information impressed on the radio carrier signal, a process called modulation. Gain is required because the signal intercepted by an antenna will have a low power level, on the order of femtowatts.
To produce an audible signal in a pair of headphones requires this signal to be amplified a trillion-fold or more. The magnitudes of the required gain are so great that the logarithmic unit decibel is preferred - a gain of 1 trillion times the power is 120 decibels, a value achieved by many common receivers. Gain is provided by one or more amplifier stages in a receiver design. Selectivity is the ability to "tune in" to just one station of the many that may be transmitting at any given time. An adjustable bandpass filter is a typical stage of a receiver. A receiver may include several stages of bandpass filters to provide sufficient selectivity. Additionally, the receiver design must provide immunity from spurious signals that may be generated within the receiver that would interfere with the desired signal. Broadcasting transmitters in any given area are assigned frequencies so that receivers can properly select the desired transmission. Sensitivity is the ability to recover the signal from the background noise.
Noise is generated in the path between transmitter and receiver, but is significantly generated in the receiver's own circuits. Inherently, any circuit above absolute zero generates some random noise that adds to the desired signals. In some cases, atmospheric noise is far greater than that produced in the receiver's own circuits, but in some designs, measures such as cryogenic cooling are applied to some stages of the receiver, to prevent signals from being obscured by thermal noise. A good receiver design may have a noise figure of only a few times the theoretical minimum for the operating temperature and desired signal bandwidth; the objective is to produce a signal-to-noise ratio of the recovered signal sufficient for the intended purpose. This ratio is often expressed in decibels. A signal-to-noise ratio of 10 dB might be usable for voice communications by experienced operators, but a receiver intended for high-fidelity music reproduction might require 50 dB or higher signal-to-noise ratio.
Stability is required in at least two senses. Frequency stability. Additionally, the great magnitude of gain generated must be controlled so that spurious emissions are not produced within the receiver; these would lead to distortion of the recovered information, or, at worst, may radiate signals that interfere with other receivers. The detector stage recovers the information from the radio-frequency signal, produces the sound, video, or data, impressed on the carrier wave initially. Detectors may be as simple as an "envelope" detector for amplitude modulation, or may be more complex circuits for more developed techniques such as frequency-hopping spread spectrum. While not fundamental to a receiver, automatic gain control is a great convenience to the user, since it automatically compensates for changing received signal levels or different levels produced by different transmitters. Many different approaches and fundamental receiver "block diagrams" have developed to address these several, sometimes contradictory, factors.
Once these technical objectives have been achieved, the remaining design process is still complicated by considerations of economics, patent rights, fashion. A crystal radio uses no active parts: it is powered only by the radio signal itself, whose detected power feeds headphones in order to be audible at all. In order to achieve a minimal sensitivity, a crystal radio is limited to low frequencies using a large antenna, it relies on detection using some sort of semiconductor diode such as the original cat's-whisker diode discovered long before the development of modern semiconductors. A crystal receiver is simple and can be easy to make or improvise, for example, the foxhole radio. However, the crystal radio needs a long antenna to operate, it displays poor selectivity. The tuned radio frequency receiver consists of a radio frequency amplifier having one or more stages all tuned to the desired reception frequency; this is followed by a detector an envelope detector using a diode, followed by audio amplification.
This was developed after the invention of the triode vacuum tube improving the reception of radio signals using
QST is a magazine for amateur radio enthusiasts, published by the American Radio Relay League. It is a membership journal, included with membership in the ARRL; the publisher claims that circulation of QST in the United States is higher than all other amateur radio-related publications in the United States combined. Although an exact number for circulation is not published by the American Radio Relay League, the organization claimed 154,627 members at the end of 2008 all of whom receive the magazine monthly, in addition to issues delivered to libraries and newsstands; the magazine name is derived from the radio Q signal meaning "calling all stations", its first issue was dated December 1915. QST suspended publication after September 1917 due to World War One, but has been continuously published since its resumption in May 1919. Supplemental content to the magazine is available on the ARRL web site, including a complete archive in PDF format, available to ARRL members starting in 2008. QST includes projects for the amateur radio enthusiast, articles and reports on ARRL affairs.
Particular interest is given to amateur radio's role in emergency communications such as in the hours after the September 11 attacks and in Hurricane Katrina. The magazine was first published in December 1915, with its first three issues financed by American Radio Relay League founder Hiram Percy Maxim and secretary Clarence D. Tuska, with an expectation that increased membership would finance its continued existence. In October 1916, the editors announced the formation of The QST Publishing Company to insulate Maxim and Tuska from possible litigation risks. Publication of QST was temporarily suspended after the September 1917 issue. In April 1917, the United States government, following its entrance into World War I, banned all amateur radio activities, a large percentage of the magazine's subscribers had entered military service; the ban on amateur radio was lifted after the conclusion of the war. QST returned in May 1919 with only 8 pages long. At a meeting in New York on March 29, a group that included Maxim and nine others decided to finance its return in this form and make a plea for membership and subscription renewals.
The June 1919 issue, still without a cover, announced that the war time ban on receiving had been lifted. In July 1919, QST resumed its previous format, although amateurs would not be permitted back on the air until that fall, when a supplement to the October issue proclaimed “BAN OFF”. By September 1920, QST was back up to 100 pages, a size not seen since April 1917. Publication continued throughout World War II, despite amateur radio's hiatus by order of the U. S. government. During both wars, amateurs were in high demand as military radio operators, QST's staff pitched in for the war effort; as part of its centennial celebration in 2014, ARRL published two volumes of QST reprints from 1915-2013: one on Amateur Radio technology and the other on advertising. Official website
An envelope detector is an electronic circuit that takes a high-frequency amplitude modulated signal as input and provides an output, the envelope of the original signal. The capacitor in the circuit above stores charge on the rising edge and releases it through the resistor when the input signal amplitude falls; the diode in series rectifies the incoming signal, allowing current flow only when the positive input terminal is at a higher potential than the negative input terminal. Most practical envelope detectors use either half-wave or full-wave rectification of the signal to convert the AC audio input into a pulsed DC signal. Filtering is used to smooth the final result; this filtering is perfect and some "ripple" is to remain on the envelope follower output for low frequency inputs such as notes from a bass instrument. Reducing the filter cutoff frequency gives a smoother output, but decreases the high frequency response. Therefore, practical designs must reach a compromise. Any AM FM signal x can be written in the following form x = R cos In the case of AM, φ is constant and can be ignored.
In AM, the carrier frequency ω is constant. Thus, all the information in the AM signal is. Hence an AM signal is given by the function x = cos with m representing the original audio frequency message, C the carrier amplitude and R equal to C + m. So, if the envelope of the AM signal can be extracted, the original message can be recovered. In the case of FM, the transmitted x can be ignored. However, many FM receivers measure the envelope anyway for received signal strength indication; the simplest form of envelope detector is the diode detector, shown above. A diode detector is a diode between the input and output of a circuit, connected to a resistor and capacitor in parallel from the output of the circuit to the ground. If the resistor and capacitor are chosen, the output of this circuit should approximate a voltage-shifted version of the original signal. A simple filter can be applied to filter out the DC component. An envelope detector can be constructed using a precision rectifier feeding into a low-pass filter.
The envelope detector has several drawbacks: The input to the detector must be band-pass filtered around the desired signal, or else the detector will demodulate several signals. The filtering can be done with a tunable filter or, more a superheterodyne receiver It is more susceptible to noise than a product detector If the signal is overmodulated, distortion will occurMost of these drawbacks are minor and are acceptable tradeoffs for the simplicity and low cost of using an envelope detector. An envelope detector can be used to demodulate a modulated signal by removing all high frequency components of the signal; the capacitor and resistor form a low-pass filter to filter out the carrier frequency. Such a device is used to demodulate AM radio signals because the envelope of the modulated signal is equivalent to the baseband signal. An envelope detector is sometimes referred to as an envelope follower in musical environments, it is still used to detect the amplitude variations of an incoming signal to produce a control signal that resembles those variations.
However, in this case the input signal is made up of audible frequencies. Envelope detectors are a component of other circuits, such as a compressor or an auto-wah or envelope-followed filter. In these circuits, the envelope follower is part of what is known as the "side chain", a circuit which describes some characteristic of the input, in this case its volume. Both expanders and compressors use the envelope's output voltage to control the gain of an amplifier. Auto-wah uses the voltage to control the cutoff frequency of a filter; the voltage-controlled filter of an analog synthesizer is a similar circuit. Modern envelope followers can be implemented: directly as electronic hardware, or as software using either a digital signal processor or on a general purpose CPU. Analytic signal Attack-decay-sustain-release envelope Envelope detector Envelope and envelope recovery
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier wave 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 heterodyning, resulting in a signal at the difference or beat frequency. Intermediate frequencies are used in superheterodyne radio receivers, in which an incoming signal is shifted to an IF for amplification before final detection is done. Conversion to an intermediate frequency is useful for several reasons; when several stages of filters are used, they can all be set to a fixed frequency, which makes them easier to build and to tune. Lower frequency transistors have higher gains so fewer stages are required. It's easier to make selective filters at lower fixed frequencies. There may be several such stages of intermediate frequency in a superheterodyne receiver. Intermediate frequencies are used for three general reasons.
At high frequencies, signal processing circuitry performs poorly. Active devices such as transistors cannot deliver much amplification. Ordinary circuits using capacitors and inductors must be replaced with cumbersome high frequency techniques such as striplines and waveguides. So a high frequency signal is converted to a lower IF for more convenient processing. For example, in satellite dishes, the microwave downlink signal received by the dish is converted to a much lower IF at the dish, to allow a inexpensive coaxial cable to carry the signal to the receiver inside the building. Bringing the signal in at the original microwave frequency would require an expensive waveguide. A second reason, in receivers that can be tuned to different frequencies, is to convert the various different frequencies of the stations to a common frequency for processing, it is difficult to build multistage amplifiers and detectors that can have all stages track in tuning different frequencies, but it is comparatively easy to build tunable oscillators.
Superheterodyne receivers tune in different frequencies by adjusting the frequency of the local oscillator on the input stage, all processing after, done at the same fixed frequency, the IF. Without using an IF, all the complicated filters and detectors in a radio or television would have to be tuned in unison each time the frequency was changed, as was necessary in the early tuned radio frequency receivers. A more important advantage is; the bandwidth of a filter is proportional to its center frequency. In receivers like the TRF in which the filtering is done at the incoming RF frequency, as the receiver is tuned to higher frequencies its bandwidth increases; the main reason for using an intermediate frequency is to improve frequency selectivity. In communication circuits, a common task is to separate out or extract signals or components of a signal that are close together in frequency; this is called filtering. Some examples are, picking up a radio station among several that are close in frequency, or extracting the chrominance subcarrier from a TV signal.
With all known filtering techniques the filter's bandwidth increases proportionately with the frequency. So a narrower bandwidth and more selectivity can be achieved by converting the signal to a lower IF and performing the filtering at that frequency. FM and television broadcasting with their narrow channel widths, as well as more modern telecommunications services such as cell phones and cable television, would be impossible without using frequency conversion; the most used intermediate frequencies for broadcast receivers are around 455 kHz for AM receivers and 10.7 MHz for FM receivers. In special purpose receivers other frequencies can be used. A dual-conversion receiver may have two intermediate frequencies, a higher one to improve image rejection and a second, lower one, for desired selectivity. A first intermediate frequency may be higher than the input signal, so that all undesired responses can be filtered out by a fixed-tuned RF stage. In a digital receiver, the analog to digital converter operates at low sampling rates, so input RF must be mixed down to IF to be processed.
Intermediate frequency tends to be lower frequency range compared to the transmitted RF frequency. However, the choices for the IF are most dependent on the available components such as mixer, filters and others that can operate at lower frequency. There are other factors involved in deciding the IF frequency, because lower IF is susceptible to noise and higher IF can cause clock jitters. Modern satellite television receivers use several intermediate frequencies; the 500 television channels of a typical system are transmitted from the satellite to subscribers in the Ku microwave band, in two subbands of 10.7 - 11.7 and 11.7 - 12.75 GHz. The downlink signal is received by a satellite dish. In the box at the focus of the dish, called a low-noise block downconverter, each block of frequencies is converted to the IF range of 950 - 2150 MHz by two fixed frequency local oscillators at 9.75 and 10.6 GHz. One of the two blocks is selected by a control signal from the set top box inside, which switches on one of the local oscillators.
This IF is carried into the building to the television receiver on a coaxial cable. At the cable company's set top box, the signal is converted to a lower IF of 480 MHz for filtering, by a variable frequency oscillator; this is sent through a 30 MHz bandpass filter, which selects the signal from one of the transponders on the satellite, which carries several channels. Further