Intermediate frequency
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In communications and
electronic engineering Electronics engineering is a sub-discipline of electrical engineering which emerged in the early 20th century and is distinguished by the additional use of active components such as semiconductor devices to amplify and control electric current ...
, an intermediate frequency (IF) is a
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
to which a
carrier wave In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...
is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a
local oscillator In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the frequenc ...
signal in a process called
heterodyning A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is u ...
, resulting in a signal at the difference or
beat frequency In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, ''perceived'' as a periodic variation in volume whose rate is the difference of the two frequencies. With tuning instruments that can produce ...
. Intermediate frequencies are used in superheterodyne radio receivers, in which an incoming signal is shifted to an IF for amplification before final
detection {{Unreferenced, date=March 2018 In general, detection is the action of accessing information without specific cooperation from with the sender. In the history of radio communications, the term " detector" was first used for a device that detected ...
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 generally have higher gains so fewer stages are required. It's easier to make sharply selective filters at lower fixed frequencies. There may be several such stages of intermediate frequency in a superheterodyne receiver; two or three stages are called ''
double A double is a look-alike or doppelgänger; one person or being that resembles another. Double, The Double or Dubble may also refer to: Film and television * Double (filmmaking), someone who substitutes for the credited actor of a character * ...
'' (alternatively, ''dual'') or '' triple conversion'', respectively.


Justification

Intermediate frequencies are used for three general reasons. At very high (
gigahertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
) frequencies, signal processing circuitry performs poorly. Active devices such as
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
s cannot deliver much amplification (
gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...
). Ordinary circuits using
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
s and
inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
s must be replaced with cumbersome high frequency techniques such as striplines and
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
s. 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 so that a relatively inexpensive coaxial cable can carry the signal to the receiver inside the building. Bringing the signal in at the original microwave frequency would require an expensive
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
. In receivers that can be tuned to different frequencies, a second reason is to convert the various different frequencies of the stations to a common frequency for processing. It is difficult to build multistage
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...
s,
filters Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component tha ...
, and
detector A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s that can have all stages track the tuning of 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, and all processing after that is 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 (TRF). A more important advantage is that it gives the receiver a constant bandwidth over its tuning range. 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 Selectivity may refer to: Psychology and behaviour * Choice, making a selection among options * Discrimination, the ability to recognize differences * Socioemotional selectivity theory, in social psychology Engineering * Selectivity (radio), a ...
. In communication circuits, a very 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 Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture (see YUV color model), separately from the accompanying luma signal (or Y' for short). Chrominance is usually represente ...
subcarrier from a TV signal. With all known filtering techniques the filter's
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
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 A television network or television broadcaster is a telecommunications network for distribution of television program content, where a central operation provides programming to many television stations or pay television providers. Until the mid- ...
with their narrow channel widths, as well as more modern telecommunications services such as cell phones and
cable television Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fibre-optic cables. This contrasts with bro ...
, would be impossible without using frequency conversion.


Uses

Perhaps the most commonly 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 even be higher than the input signal, so that all undesired responses can be easily filtered out by a fixed-tuned RF stage. In a digital receiver, the
analog-to-digital converter In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide ...
(ADC) 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, amplifiers and others that can operate at lower frequency. There are other factors involved in deciding the IF, because lower IF is susceptible to noise and higher IF can cause clock jitters. Modern
satellite television Satellite television is a service that delivers television programming to viewers by relaying it from a communications satellite orbiting the Earth directly to the viewer's location. The signals are received via an outdoor parabolic antenna comm ...
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 A low-noise block downconverter (LNB) is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish and converts them to a signal which is sent through a cable to the receiver i ...
(LNB), 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 A set-top box (STB), also colloquially known as a cable box and historically television decoder, is an information appliance device that generally contains a TV-tuner input and displays output to a television set and an external source of si ...
, 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
transponder In telecommunications, a transponder is a device that, upon receiving a signal, emits a different signal in response. The term is a blend of ''transmitter'' and ''responder''. In air navigation or radio frequency identification, a flight trans ...
s on the satellite, which carries several channels. Further processing selects the channel desired, demodulates it and sends the signal to the television.


History

An intermediate frequency was first used in the superheterodyne radio receiver, invented by American scientist Major Edwin Armstrong in 1918, during
World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
. A member of the Signal Corps, Armstrong was building radio
direction finding Direction finding (DF), or radio direction finding (RDF), isin accordance with International Telecommunication Union (ITU)defined as radio location that uses the reception of radio waves to determine the direction in which a radio stati ...
equipment to track German military signals at the then-very high frequencies of 500 to 3500 kHz. The triode vacuum tube amplifiers of the day would not amplify stably above 500 kHz, however, it was easy to get them to
oscillate Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
above that frequency. Armstrong's solution was to set up an oscillator tube that would create a frequency near the incoming signal and mix it with the incoming signal in a mixer tube, creating a
heterodyne A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is u ...
or signal at the lower difference frequency where it could be amplified easily. For example, to pick up a signal at 1500 kHz the local oscillator would be tuned to 1450 kHz. Mixing the two created an intermediate frequency of 50 kHz, which was well within the capability of the tubes. The name ''superheterodyne'' was a contraction of ''supersonic heterodyne'', to distinguish it from receivers in which the heterodyne frequency was low enough to be directly audible, and which were used for receiving continuous wave (CW) Morse code transmissions (not speech or music). After the war, in 1920, Armstrong sold the patent for the superheterodyne to Westinghouse, who subsequently sold it to
RCA The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Comp ...
. The increased complexity of the superheterodyne circuit compared to earlier regenerative or tuned radio frequency receiver designs slowed its use, but the advantages of the intermediate frequency for selectivity and static rejection eventually won out; by 1930, most radios sold were 'superhets'. During the development of
radar Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, we ...
in
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
, the superheterodyne principle was essential for downconversion of the very high radar frequencies to intermediate frequencies. Since then, the superheterodyne circuit, with its intermediate frequency, has been used in virtually all radio receivers.


Examples

* down to c. 20 kHz, 30 kHz (A. L. M. Sowerby and H. B. Dent), 45 kHz (first commercial superheterodyne receiver: RCA Radiola AR-812 of 1923/1924), c. 50 kHz, c. 100 kHz, c. 120 kHz * 110 kHz was used in European AM
longwave In radio, longwave, long wave or long-wave, and commonly abbreviated LW, refers to parts of the radio spectrum with wavelengths longer than what was originally called the medium-wave broadcasting band. The term is historic, dating from the e ...
broadcast receivers. * 175 kHz (early wide band and communications receivers before introduction of powdered iron cores) * 260 kHz (early standard broadcast receivers), 250–270 kHz * Copenhagen Frequency Allocations: 415–490 kHz, 510–525 kHz *
AM radio AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transmis ...
receivers: 450 kHz, 455 kHz (most common), 460 kHz, 465 kHz, 467 kHz, 470 kHz, 475 kHz, and 480 kHz. * FM radio receivers: 262 kHz (old car radios), 455 kHz, 1.6 MHz, 5.5 MHz, 10.7 MHz (most common), 10.8 MHz, 11.2 MHz, 11.7 MHz, 11.8 MHz, 13.45 MHz, 21.4 MHz, 75 MHz and 98 MHz. In double-conversion superheterodyne receivers, a first intermediate frequency of 10.7 MHz is often used, followed by a second intermediate frequency of 470 kHz (or 700 kHz with DYNAS). There are triple conversion designs used in police scanner receivers, high-end communications receivers, and many point-to-point microwave systems. Modern DSP chip consumer radios often use a ' low-IF' of 128 kHz for FM. * Narrowband FM receivers: 455 kHz (most common), 470 kHz * Shortwave receivers: 1.6 MHz, 1.6–3.0 MHz, 4.3 MHz (for 40–50 MHz-only receivers). In double-conversion superheterodyne receivers, a first intermediate frequency of 3.0 MHz is sometimes combined with a second IF of 465 kHz. * Analogue television receivers using system M: 41.25 MHz (audio) and 45.75 MHz (video). Note, the channel is flipped over in the conversion process in an intercarrier system, so the audio IF is lower than the video IF. Also, there is no audio local oscillator; the injected video carrier serves that purpose. * Analogue television receivers using system B and similar systems: 33.4 MHz for the aural and 38.9 MHz for the visual signal. (The discussion about the frequency conversion is the same as in system M.) * Satellite
uplink In a telecommunications network, a link is a communication channel that connects two or more devices for the purpose of data transmission. The link may be a dedicated physical link or a virtual circuit that uses one or more physical links or sha ...
-
downlink In a telecommunications network, a link is a communication channel that connects two or more devices for the purpose of data transmission. The link may be a dedicated physical link or a virtual circuit that uses one or more physical links or shar ...
equipment: 70 MHz, 950–1450 MHz (L-band) downlink first IF. * Terrestrial
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
equipment: 250 MHz, 70 MHz or 75 MHz. *
Radar Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, we ...
: 30 MHz. * RF test equipment: 310.7 MHz, 160 MHz, and 21.4 MHz.


See also

*
Mechanical filter A mechanical filter is a signal processing filter usually used in place of an electronic filter at radio frequencies. Its purpose is the same as that of a normal electronic filter: to pass a range of signal frequencies, but to block others. T ...
*
Low-IF receiver In a low-IF receiver, the RF signal is mixed down to a non-zero low or moderate intermediate frequency, typically a few megahertz (instead of 33–40 MHz) for TV, and even lower frequencies (typically 120–130 kHz instead of 10.7–10.8 ...
*
Zero-IF receiver A direct-conversion receiver (DCR), also known as homodyne, synchrodyne, or zero-IF receiver, is a radio receiver design that demodulates the incoming radio signal using synchronous detection driven by a local oscillator whose frequency is ide ...


References

{{Authority control Radio electronics Broadcast engineering