Intermediate frequency
Encyclopedia
In communications and electronic engineering
, an intermediate frequency (IF) is a frequency
to which a carrier frequency
is shifted as an intermediate step in transmission
or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator
signal in a process called 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. There may be several such stages of intermediate frequency in a superheterodyne, which is called double (or triple) conversion.
s cannot deliver much amplification (gain
) without becoming unstable. Ordinary circuits using capacitor
s and inductor
s must be replaced with cumbersome high frequency techniques such as stripline
s and waveguide
s. So a high frequency signal is converted to a lower IF for processing.
A second reason to use an IF, in receivers that can be tuned to different stations, is to convert the various different frequencies of the stations to a common frequency for processing. It is difficult to build amplifier
s, filters
, and detector
s that can be tuned to different frequencies, but easy to build tunable oscillators
. Superheterodyne receivers tune in different stations simply by adjusting the frequency of the local oscillator on the input stage, and all processing after that is done at the same 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 station was changed, as was necessary in the early tuned radio frequency receiver
s.
But the main reason for using an intermediate frequency is to improve frequency selectivity. 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
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.
Perhaps the most commonly used intermediate frequencies are around 455 kHz for AM receivers and 10.7 MHz for FM receivers. However, the intermediate frequency can range from 10–100 MHz. Intermediate frequency (IF) are generated by mixing the RF and LO frequency together to create a lower frequency called IF. Most of the ADC/DAC operates in 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 frequency, because lower IF is susceptible to noise and higher IF can cause clock jitters.
in 1918, during World War I
. A member of the Signal Corps, Armstrong was building radio direction finding
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
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' 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.
After the war, in 1920, Armstrong sold the patent for the superheterodyne to Westinghouse
, who subsequently sold it to RCA
. 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
in World War II
, 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.
Electronic engineering
Electronics engineering, also referred to as electronic engineering, is an engineering discipline where non-linear and active electrical components such as electron tubes, and semiconductor devices, especially transistors, diodes and integrated circuits, are utilized to design electronic...
, an intermediate frequency (IF) is a frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
to which a carrier frequency
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
is shifted as an intermediate step in transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator
Local oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...
signal in a process called heterodyning, resulting in a signal at the difference or beat frequency. Intermediate frequencies are used in superheterodyne
Superheterodyne receiver
In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency...
radio receivers, in which an incoming signal is shifted to an IF for amplification
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
before final detection
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
is done. There may be several such stages of intermediate frequency in a superheterodyne, which is called double (or triple) conversion.
Overview
Intermediate frequencies are used for three general reasons. At very high (gigahertz) frequencies, signal processing circuitry performs poorly. Active devices such as transistorTransistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s cannot deliver much amplification (gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
) without becoming unstable. Ordinary circuits using capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s and inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s must be replaced with cumbersome high frequency techniques such as stripline
Stripline
Stripline is a transverse electromagnetic transmission line medium, that was invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s.- Description :...
s and waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
s. So a high frequency signal is converted to a lower IF for processing.
A second reason to use an IF, in receivers that can be tuned to different stations, is to convert the various different frequencies of the stations to a common frequency for processing. It is difficult to build amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s, filters
Electronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
, and detector
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
s that can be tuned to different frequencies, but easy to build tunable oscillators
Electronic oscillator
An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...
. Superheterodyne receivers tune in different stations simply by adjusting the frequency of the local oscillator on the input stage, and all processing after that is done at the same 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 station was changed, as was necessary in the early tuned radio frequency receiver
Tuned radio frequency receiver
A tuned radio frequency receiver is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. Prevalent in the early 20th century, it can be difficult to operate because each stage must be individually tuned...
s.
But the main reason for using an intermediate frequency is to improve frequency selectivity. 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
Filter (signal processing)
In signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal...
. Some examples are, picking up a radio station among several that are close in frequency, or extracting the chrominance
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
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.
Perhaps the most commonly used intermediate frequencies are around 455 kHz for AM receivers and 10.7 MHz for FM receivers. However, the intermediate frequency can range from 10–100 MHz. Intermediate frequency (IF) are generated by mixing the RF and LO frequency together to create a lower frequency called IF. Most of the ADC/DAC operates in 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 frequency, because lower IF is susceptible to noise and higher IF can cause clock jitters.
History
An intermediate frequency was first used in the superheterodyne radio receiver, invented by American scientist Major Edwin ArmstrongEdwin Armstrong
Edwin Howard Armstrong was an American electrical engineer and inventor. Armstrong was the inventor of modern frequency modulation radio....
in 1918, during World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
. A member of the Signal Corps, Armstrong was building radio direction finding
Direction finding
Direction finding refers to the establishment of the direction from which a received signal was transmitted. This can refer to radio or other forms of wireless communication...
equipment to track German military signals at the then-very high frequencies of 500 to 3500 kHz. The triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
amplifiers of the day would not amplify stably above 500 kHz, however, it was easy to get them to oscillate
Electronic oscillator
An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...
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' 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.
After the war, in 1920, Armstrong sold the patent for the superheterodyne to Westinghouse
Westinghouse Electric (1886)
Westinghouse Electric was an American manufacturing company. It was founded in 1886 as Westinghouse Electric Company and later renamed Westinghouse Electric Corporation by George Westinghouse. The company purchased CBS in 1995 and became CBS Corporation in 1997...
, who subsequently sold it to RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
. The increased complexity of the superheterodyne circuit compared to earlier regenerative or tuned radio frequency receiver
Tuned radio frequency receiver
A tuned radio frequency receiver is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. Prevalent in the early 20th century, it can be difficult to operate because each stage must be individually tuned...
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
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
in World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, 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.
Commonly used intermediate frequencies
- TelevisionTelevisionTelevision is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
receivers: 30 MHz to 900 MHz - AnalogueAnalog transmissionAnalog transmission is a transmission method of conveying voice, data, image, signal or video information using a continuous signal which varies in amplitude, phase, or some other property in proportion to that of a variable...
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 intercarrierIntercarrier methodThe intercarrier method is a system in television that reduces the cost of transmitters and receiver sets by processing audio and video signals together and minimizing the number of separate stages for audio and video signals....
system, so the audio IF frequency is lower than the video IF frequency. Also, there is no audio local oscillator, the injected video carrier serves that purpose. - AnalogueAnalog transmissionAnalog transmission is a transmission method of conveying voice, data, image, signal or video information using a continuous signal which varies in amplitude, phase, or some other property in proportion to that of a variable...
television receivers using system B and similar systems: 33.4 MHz. for aural and 38.9 MHz. for visual signal. (The discussion about the frequency conversion is the same as in system M) - FM radio receivers: 262 kHz, 455 kHz, 1.6 MHz, 5.5 MHz, 10.7 MHz, 10.8 MHz, 11.2 MHz, 11.7 MHz, 11.8 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. There are triple conversion designs used in police scanner receivers, high-end communications receivers, and many point-to-point microwave systems.
- AM radio receivers: 450 kHz, 455 kHz, 460 kHz, 465 kHz, 470 kHz, 475 kHz, 480 kHz
- Satellite uplinkUplinkA telecommunications link is generally one of several types of information transmission paths such as those provided by communication satellites to connect two points on earth.-Uplink:...
-downlink equipment: 70 MHz, 950-1450 Downlink first IF - Terrestrial microwaveMicrowaveMicrowaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
equipment: 250 MHz, 70 MHz or 75 MHz - RadarRadarRadar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
: 30 MHz - RFRadio frequencyRadio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
Test Equipment: 310.7 MHz, 160 MHz, 21.4 MHz