![Superheterod radio](https://upload.wikimedia.org/wikipedia/commons/9/9d/Superheterod_radio.JPG)
A superheterodyne receiver, often shortened to superhet, is a type of
radio receiver
In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...
that uses
frequency mixing to convert a received signal to a fixed
intermediate frequency
In communications and electronic engineering, an intermediate frequency (IF) 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 sign ...
(IF) which can be more conveniently processed than the original
carrier frequency
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 a m ...
. It was long believed to have been invented by US engineer
Edwin Armstrong
Edwin Howard Armstrong (December 18, 1890 – February 1, 1954) was an American electrical engineer and inventor, who developed FM (frequency modulation) radio and the superheterodyne receiver system. He held 42 patents and received numerous awa ...
, but after some controversy the earliest patent for the invention is now credited to French radio engineer and radio manufacturer
Lucien Lévy
Lucien Lévy (11 March 1892 – 24 May 1965) was a French radio engineer and radio receiver manufacturer.
He invented the superheterodyne method of amplifying radio signals, used in almost all AM radio receivers.
His patent claim was at first disa ...
.
Virtually all modern radio receivers use the superheterodyne principle; except those
software-defined radios using ''direct sampling''.
History
Heterodyne
Early
Morse code
Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...
radio broadcasts were produced using an
alternator
An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature.Go ...
connected to a
spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conductor ...
. The output signal was at a
carrier frequency
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 a m ...
defined by the physical construction of the gap, modulated by the
alternating current
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
signal from the alternator. Since the output frequency of the alternator was generally in the audible range, this produces an audible
amplitude modulated
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to t ...
(AM) signal. Simple
radio detectors filtered out the high-frequency carrier, leaving the modulation, which was passed on to the user's
headphone
Headphones are a pair of small loudspeaker drivers worn on or around the head over a user's ears. They are electroacoustic transducers, which convert an electrical signal to a corresponding sound. Headphones let a single user listen to an au ...
s as an audible signal of dots and dashes.
In 1904,
Ernst Alexanderson
Ernst Frederick Werner Alexanderson (January 25, 1878 – May 14, 1975) was a Swedish-American electrical engineer, who was a pioneer in radio and television development. He invented the Alexanderson alternator, an early radio transmitter used b ...
introduced the
Alexanderson alternator
An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high-frequency alternating current for use as a radio transmitter. It was one of the first devices capable of generating the continuou ...
, a device that directly produced radio frequency output with higher power and much higher efficiency than the older spark gap systems. In contrast to the spark gap, however, the output from the alternator was a pure carrier wave at a selected frequency. When detected on existing receivers, the dots and dashes would normally be inaudible, or "supersonic". Due to the filtering effects of the receiver, these signals generally produced a click or thump, which were audible but made determining dot or dash difficult.
In 1905, Canadian inventor
Reginald Fessenden
Reginald Aubrey Fessenden (October 6, 1866 – July 22, 1932) was a Canadian-born inventor, who did a majority of his work in the United States and also claimed U.S. citizenship through his American-born father. During his life he received hundre ...
came up with the idea of using two Alexanderson alternators operating at closely spaced frequencies to broadcast two signals, instead of one. The receiver would then receive both signals, and as part of the detection process, only the
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 su ...
would exit the receiver. By selecting two carriers close enough that the beat frequency was audible, the resulting Morse code could once again be easily heard even in simple receivers. For instance, if the two alternators operated at frequencies 3 kHz apart, the output in the headphones would be dots or dashes of 3 kHz tone, making them easily audible.
Fessenden coined the term "
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 us ...
", meaning "generated by a difference" (in frequency), to describe this system. The word is derived from the Greek roots ''hetero-'' "different", and ''-dyne'' "power".
Regeneration
Morse code was widely used in the early days of radio because it was both easy to produce and easy to receive. In contrast to voice broadcasts, the output of the amplifier didn't have to closely match the modulation of the original signal. As a result, any number of simple amplification systems could be used. One method used an interesting side-effect of early
triode
A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 19 ...
amplifier tubes. If both the plate (anode) and grid were connected to resonant circuits tuned to the same frequency and the stage gain was much higher than
unity
Unity may refer to:
Buildings
* Unity Building, Oregon, Illinois, US; a historic building
* Unity Building (Chicago), Illinois, US; a skyscraper
* Unity Buildings, Liverpool, UK; two buildings in England
* Unity Chapel, Wyoming, Wisconsin, US; a h ...
, stray
capacitive coupling
Capacitive coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) nodes, induced by the electric field. This coupling can have an intentional or accidental ...
between the grid and the plate would cause the amplifier to go into oscillation.
In 1913,
Edwin Howard Armstrong
Edwin Howard Armstrong (December 18, 1890 – February 1, 1954) was an American electrical engineer and inventor, who developed FM (frequency modulation) radio and the superheterodyne receiver system. He held 42 patents and received numerous aw ...
described a receiver system that used this effect to produce audible Morse code output using a single triode. The output of the amplifier taken at the anode was connected back to the input through a "tickler", causing
feedback
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
that drove input signals well beyond unity. This caused the output to oscillate at a chosen frequency with great amplification. When the original signal cut off at the end of the dot or dash, the oscillation decayed and the sound disappeared after a short delay.
Armstrong referred to this concept as a
regenerative receiver
A regenerative circuit is an amplifier circuit that employs positive feedback (also known as regeneration or reaction). Some of the output of the amplifying device is applied back to its input so as to add to the input signal, increasing the am ...
, and it immediately became one of the most widely used systems of its era. Many radio systems of the 1920s were based on the regenerative principle, and it continued to be used in specialized roles into the 1940s, for instance in the
IFF Mark II
IFF Mark II was the first operational identification friend or foe system. It was developed by the Royal Air Force just before the start of World War II. After a short run of prototype Mark Is, used experimentally in 1939, the Mark II began wi ...
.
RDF
There was one role where the regenerative system was not suitable, even for Morse code sources, and that was the task of
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 station ...
, or RDF.
The regenerative system was highly non-linear, amplifying any signal above a certain threshold by a huge amount, sometimes so large it caused it to turn into a transmitter (which was the entire concept behind IFF). In RDF, the strength of the signal is used to determine the location of the transmitter, so one requires
linear amplification
A linear amplifier is an electronic circuit whose output is proportional to its input, but capable of delivering more power into a load. The term usually refers to a type of radio-frequency (RF) power amplifier, some of which have output power mea ...
to allow the strength of the original signal, often very weak, to be accurately measured.
To address this need, RDF systems of the era used triodes operating below unity. To get a usable signal from such a system, tens or even hundreds of triodes had to be used, connected together anode-to-grid. These amplifiers drew enormous amounts of power and required a team of maintenance engineers to keep them running. Nevertheless, the strategic value of direction finding on weak signals was so high that the
British Admiralty
The Admiralty was a department of the Government of the United Kingdom responsible for the command of the Royal Navy until 1964, historically under its titular head, the Lord High Admiral – one of the Great Officers of State. For much of it ...
felt the high cost was justified.
Superheterodyne
![Prototype Armstrong superheterodyne receiver 1920](https://upload.wikimedia.org/wikipedia/commons/c/ce/Prototype_Armstrong_superheterodyne_receiver_1920.jpg)
Although a number of researchers discovered the superheterodyne concept, filing patents only months apart (see below), American engineer
Edwin Armstrong
Edwin Howard Armstrong (December 18, 1890 – February 1, 1954) was an American electrical engineer and inventor, who developed FM (frequency modulation) radio and the superheterodyne receiver system. He held 42 patents and received numerous awa ...
is often credited with the concept. He came across it while considering better ways to produce RDF receivers. He had concluded that moving to higher "short wave" frequencies would make RDF more useful and was looking for practical means to build a linear amplifier for these signals. At the time, short wave was anything above about 500 kHz, beyond any existing amplifier's capabilities.
It had been noticed that when a regenerative receiver went into oscillation, other nearby receivers would start picking up other stations as well. Armstrong (and others) eventually deduced that this was caused by a "supersonic heterodyne" between the station's carrier frequency and the regenerative receiver's oscillation frequency. When the first receiver began to oscillate at high outputs, its signal would flow back out through the antenna to be received on any nearby receiver. On that receiver, the two signals mixed just as they did in the original heterodyne concept, producing an output that is the difference in frequency between the two signals.
For instance, consider a lone receiver that was tuned to a station at 300 kHz. If a second receiver is set up nearby and set to 400 kHz with high gain, it will begin to give off a 400 kHz signal that will be received in the first receiver. In that receiver, the two signals will mix to produce four outputs, one at the original 300 kHz, another at the received 400 kHz, and two more, the difference at 100 kHz and the sum at 700 kHz. This is the same effect that Fessenden had proposed, but in his system the two frequencies were deliberately chosen so the beat frequency was audible. In this case, all of the frequencies are well beyond the audible range, and thus "supersonic", giving rise to the name superheterodyne.
Armstrong realized that this effect was a potential solution to the "short wave" amplification problem, as the "difference" output still retained its original modulation, but on a lower carrier frequency. In the example above, one can amplify the 100 kHz beat signal and retrieve the original information from that, the receiver does not have to tune in the higher 300 kHz original carrier. By selecting an appropriate set of frequencies, even very high-frequency signals could be "reduced" to a frequency that could be amplified by existing systems.
For instance, to receive a signal at 1500 kHz, far beyond the range of efficient amplification at the time, one could set up an oscillator at, for example, 1560 kHz. Armstrong referred to this as the "
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 ...
" or LO. As its signal was being fed into a second receiver in the same device, it did not have to be powerful, generating only enough signal to be roughly similar in strength to that of the received station. When the signal from the LO mixes with the station's, one of the outputs will be the heterodyne difference frequency, in this case, 60 kHz. He termed this resulting difference the "
intermediate frequency
In communications and electronic engineering, an intermediate frequency (IF) 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 sign ...
" often abbreviated to "IF".
In December 1919, Major E. H. Armstrong gave publicity to an indirect method of obtaining short-wave amplification, called the super-heterodyne. The idea is to reduce the incoming frequency, which may be, for example 1,500,000 cycles (200 meters), to some suitable super-audible frequency that can be amplified efficiently, then passing this current through an intermediate frequency amplifier, and finally rectifying and carrying on to one or two stages of audio frequency amplification.
The "trick" to the superheterodyne is that by changing the LO frequency you can tune in different stations. For instance, to receive a signal at 1300 kHz, one could tune the LO to 1360 kHz, resulting in the same 60 kHz IF. This means the amplifier section can be tuned to operate at a single frequency, the design IF, which is much easier to do efficiently.
Development
![Radiola AR-812 superheterodyne ad](https://upload.wikimedia.org/wikipedia/commons/d/df/Radiola_AR-812_superheterodyne_ad.jpg)
Armstrong put his ideas into practice, and the technique was soon adopted by the military. It was less popular when commercial
radio broadcasting
Radio broadcasting is transmission of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based radio ...
began in the 1920s, mostly due to the need for an extra tube (for the oscillator), the generally higher cost of the receiver, and the level of skill required to operate it. For early domestic radios,
tuned radio frequency receiver
A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and usually an ...
s (TRF) were more popular because they were cheaper, easier for a non-technical owner to use, and less costly to operate. Armstrong eventually sold his superheterodyne patent to
Westinghouse, which then sold it to
Radio Corporation of America (RCA), the latter monopolizing the market for superheterodyne receivers until 1930.
Because the original motivation for the superhet was the difficulty of using the triode amplifier at high frequencies, there was an advantage in using a lower intermediate frequency. During this era, many receivers used an IF frequency of only 30 kHz.
These low IF frequencies, often using IF transformers based on the self-resonance of iron-core
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s, had poor
image frequency
A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carri ...
rejection, but overcame the difficulty in using triodes at radio frequencies in a manner that competed favorably with the less robust
neutrodyne
The Neutrodyne radio receiver, invented in 1922 by Louis Hazeltine, was a particular type of tuned radio frequency (TRF) receiver, in which the instability-causing inter-electrode capacitance of the triode RF tubes is cancelled out or "neutrali ...
TRF receiver. Higher IF frequencies (455 kHz was a common standard) came into use in later years, after the invention of the
tetrode
A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). Th ...
and
pentode
A pentode is an electronic device having five electrodes. The term most commonly applies to a three-grid amplifying vacuum tube or thermionic valve that was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode (called a ''tripl ...
as amplifying tubes, largely solving the problem of image rejection. Even later, however, low IF frequencies (typically 60 kHz) were again used in the ''second'' (or third) IF stage of
double or triple-conversion communications receivers to take advantage of the
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 ...
more easily achieved at lower IF frequencies, with image-rejection accomplished in the earlier IF stage(s) which were at a higher IF frequency.
In the 1920s, at these low frequencies, commercial IF filters looked very similar to 1920s audio interstage coupling transformers, had similar construction, and were wired up in an almost identical manner, so they were referred to as "IF transformers". By the mid-1930s, superheterodynes using much higher intermediate frequencies (typically around 440–470 kHz) used tuned transformers more similar to other RF applications. The name "IF transformer" was retained, however, now meaning "intermediate frequency". Modern receivers typically use a mixture of
ceramic resonator
A Ceramic Resonator is an electronic component consisting of a piece of a piezoelectric ceramic material with two or more metal electrodes attached. When connected in an electronic oscillator circuit, resonant mechanical vibrations in the device ge ...
s or
surface acoustic wave
A surface acoustic wave (SAW) is an acoustic wave traveling along the surface of a material exhibiting elasticity, with an amplitude that typically decays exponentially with depth into the material, such that they are confined to a depth of abou ...
resonators and traditional tuned-inductor IF transformers.
By the 1930s, improvements in vacuum tube technology rapidly eroded the TRF receiver's cost advantages, and the explosion in the number of broadcasting stations created a demand for cheaper, higher-performance receivers.
The introduction of an additional grid in a vacuum tube, but before the more modern screen-grid tetrode, included the
tetrode with two control grids; this tube combined the mixer and oscillator functions, first used in the so-called
autodyne
The autodyne circuit was an improvement to radio signal amplification using the Lee De Forest, De Forest Audion vacuum tube amplifier. By allowing the tube to oscillate at a frequency slightly different from the desired signal, the sensitivity ov ...
mixer. This was rapidly followed by the introduction of tubes specifically designed for superheterodyne operation, most notably the
pentagrid converter
The pentagrid converter is a type of radio receiving valve (vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver.
The pentagrid was part of a line of development of valves that were able to take an ...
. By reducing the tube count (with each tube stage being the main factor affecting cost in this era), this further reduced the advantage of TRF and regenerative receiver designs.
By the mid-1930s, commercial production of TRF receivers was largely replaced by superheterodyne receivers. By the 1940s, the vacuum-tube superheterodyne AM broadcast receiver was refined into a cheap-to-manufacture design called the "
All American Five
The term All American Five (abbreviated AA5) is a colloquial name for mass-produced, superheterodyne radio receivers that used five vacuum tubes in their design. These radio sets were designed to receive amplitude modulation (AM) broadcasts in the ...
" because it used five vacuum tubes: usually a converter (mixer/local oscillator), an IF amplifier, a detector/audio amplifier, audio power amplifier, and a rectifier. Since this time, the superheterodyne design was used for almost all commercial radio and TV receivers.
Patent battles
French engineer
Lucien Lévy
Lucien Lévy (11 March 1892 – 24 May 1965) was a French radio engineer and radio receiver manufacturer.
He invented the superheterodyne method of amplifying radio signals, used in almost all AM radio receivers.
His patent claim was at first disa ...
filed a patent application for the superheterodyne principle in August 1917 with brevet n° 493660.
Armstrong also filed his patent in 1917.
Levy filed his original disclosure about seven months before Armstrong's.
German inventor
Walter H. Schottky
Walter Hans Schottky (23 July 1886 – 4 March 1976) was a German physicist who played a major early role in developing the theory of electron and ion emission phenomena, invented the screen-grid vacuum tube in 1915 while working at Siemens ...
also filed a patent in 1918.
At first the US recognised Armstrong as the inventor, and his US Patent 1,342,885 was issued on 8 June 1920.
After various changes and court hearings Lévy was awarded US patent No 1,734,938 that included seven of the nine claims in Armstrong's application, while the two remaining claims were granted to Alexanderson of GE and Kendall of AT&T.
Principle of operation
![Superheterodyne receiver block diagram 2](https://upload.wikimedia.org/wikipedia/commons/3/3f/Superheterodyne_receiver_block_diagram_2.svg)
The diagram at right shows the block diagram of a typical single-conversion superheterodyne receiver. The diagram has blocks that are common to superheterodyne receivers,
with only the RF amplifier being optional.
The
antenna
Antenna ( antennas or antennae) may refer to:
Science and engineering
* Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves
* Antennae Galaxies, the name of two collid ...
collects the radio signal. The tuned RF stage with optional RF amplifier provides some initial selectivity; it is necessary to suppress the ''
image frequency
A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carri ...
'' (see below), and may also serve to prevent strong out-of-passband signals from saturating the initial amplifier. 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 ...
provides the mixing frequency; it is usually a variable frequency oscillator which is used to tune the receiver to different stations. The
frequency mixer
In electronics, a mixer, or frequency mixer, is an electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals are applied to a mixer, and it produces new signals at the sum and di ...
does the actual
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 us ...
that gives the superheterodyne its name; it changes the incoming radio frequency signal to a higher or lower, fixed,
intermediate frequency
In communications and electronic engineering, an intermediate frequency (IF) 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 sign ...
(IF). The IF
band-pass filter
A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.
Description
In electronics and signal processing, a filter is usually a two-por ...
and amplifier supply most of the gain and the narrowband filtering for the radio. The
demodulator
Demodulation is extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated ...
extracts the audio or other
modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
from the IF radio frequency. The extracted signal is then amplified by the audio amplifier.
Circuit description
To receive a radio signal, a suitable
antenna
Antenna ( antennas or antennae) may refer to:
Science and engineering
* Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves
* Antennae Galaxies, the name of two collid ...
is required. The output of the antenna may be very small, often only a few
microvolt
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
Defi ...
s. The signal from the antenna is tuned and may be amplified in a so-called radio frequency (RF) amplifier, although this stage is often omitted. One or more
tuned circuit
An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...
s at this stage block frequencies that are far removed from the intended reception frequency. To tune the receiver to a particular station, the frequency of the local oscillator is controlled by the tuning knob (for instance). Tuning of the local oscillator and the RF stage may use a
variable capacitor
A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio (therefo ...
, or
varicap diode
In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode designed to exploit the voltage-dependent capacitance of a reverse-biased p–n junction.
Applications
Va ...
.
The tuning of one (or more) tuned circuits in the RF stage must track the tuning of the local oscillator.
Local oscillator and mixer
The signal is then fed into a circuit where it is mixed with a sine wave from a variable frequency oscillator known as the
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 ...
(LO). The mixer uses a non-linear component to produce both sum and difference
beat frequencies signals,
each one containing the
modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
contained in the desired signal. The output of the mixer may include the original RF signal at ''f''
RF, the local oscillator signal at ''f''
LO, and the two new heterodyne frequencies ''f''
RF + ''f''
LO and ''f''
RF − ''f''
LO. The mixer may inadvertently produce additional frequencies such as third- and higher-order intermodulation products. Ideally, the IF
bandpass filter
A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.
Description
In electronics and signal processing, a filter is usually a two-port ...
removes all but the desired IF signal at ''f''
IF. The IF signal contains the original modulation (transmitted information) that the received radio signal had at ''f''
RF.
The frequency of the local oscillator ''f''
LO is set so the desired reception radio frequency ''f''
RF mixes to ''f''
IF. There are two choices for the local oscillator frequency because the dominant mixer products are at ''f''
RF ± ''f''
LO. If the local oscillator frequency is less than the desired reception frequency, it is called low-side injection (''f''
IF = ''f''
RF − ''f''
LO); if the local oscillator is higher, then it is called high-side injection (''f''
IF = ''f''
LO − ''f''
RF).
The mixer will process not only the desired input signal at f
RF, but also all signals present at its inputs. There will be many mixer products (heterodynes). Most other signals produced by the mixer (such as due to stations at nearby frequencies) can be
filtered
Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter m ...
out in the IF
tuned amplifier
A tuned amplifier is an electronic amplifier which includes bandpass filtering components within the amplifier circuitry. They are widely used in a variety of wireless applications.
Schemes
There are several tuning schemes in use,
* Staggere ...
; that gives the superheterodyne receiver its superior performance. However, if ''f''
LO is set to ''f''
RF + ''f''
IF, then an incoming radio signal at ''f''
LO + ''f''
IF will ''also'' produce a heterodyne at ''f''
IF; the frequency ''f''
LO + ''f''
IF is called the ''image frequency'' and must be rejected by the tuned circuits in the RF stage. The image frequency is 2 ''f''
IF higher (or lower) than the desired frequency ''f''
RF, so employing a higher IF frequency ''f''
IF increases the receiver's ''image rejection'' without requiring additional selectivity in the RF stage.
To suppress the unwanted image, the tuning of the RF stage and the LO may need to "track" each other. In some cases, a narrow-band receiver can have a fixed tuned RF amplifier. In that case, only the local oscillator frequency is changed. In most cases, a receiver's input band is wider than its IF center frequency. For example, a typical AM broadcast band receiver covers 510 kHz to 1655 kHz (a roughly 1160 kHz input band) with a 455 kHz IF frequency; an FM broadcast band receiver covers 88 MHz to 108 MHz band with a 10.7 MHz IF frequency. In that situation, the RF amplifier must be tuned so the IF amplifier does not see two stations at the same time. If the AM broadcast band receiver LO were set at 1200 kHz, it would see stations at both 745 kHz (1200−455 kHz) and 1655 kHz. Consequently, the RF stage must be designed so that any stations that are twice the IF frequency away are significantly attenuated. The tracking can be done with a multi-section variable capacitor or some
varactor
In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode designed to exploit the voltage-dependent capacitance of a reverse-biased p–n junction.
Applications
Vara ...
s driven by a common control voltage. An RF amplifier may have tuned circuits at both its input and its output, so three or more tuned circuits may be tracked. In practice, the RF and LO frequencies need to track closely but not perfectly.
In the days of
tube (valve) electronics, it was common for superheterodyne receivers to combine the functions of the local oscillator and the mixer in a single tube, leading to a savings in power, size, and especially cost. A single
pentagrid converter
The pentagrid converter is a type of radio receiving valve (vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver.
The pentagrid was part of a line of development of valves that were able to take an ...
tube would oscillate and also provide signal amplification as well as frequency mixing.
IF amplifier
The stages of an intermediate frequency amplifier ("IF amplifier" or "IF strip") are tuned to a fixed frequency that does not change as the receiving frequency changes. The fixed frequency simplifies optimization of the IF amplifier.
The IF amplifier is selective around its center frequency ''f''
IF. The fixed center frequency allows the stages of the IF amplifier to be carefully tuned for best performance (this tuning is called "aligning" the IF amplifier). If the center frequency changed with the receiving frequency, then the IF stages would have had to track their tuning. That is not the case with the superheterodyne.
Normally, the IF center frequency ''f''
IF is chosen to be less than the range of desired reception frequencies ''f''
RF. That is because it is easier and less expensive to get high selectivity at a lower frequency using tuned circuits. The bandwidth of a tuned circuit with a certain
Q is proportional to the frequency itself (and what's more, a higher Q is achievable at lower frequencies), so fewer IF filter stages are required to achieve the same selectivity. Also, it is easier and less expensive to get high gain at a lower frequencies.
However, in many modern receivers designed for reception over a wide frequency range (e.g. scanners and spectrum analyzers) a first IF frequency ''higher'' than the reception frequency is employed in a
double conversion configuration. For instance, the Rohde & Schwarz EK-070 VLF/HF receiver covers 10 kHz to 30 MHz.
It has a band switched RF filter and mixes the input to a first IF of 81.4 MHz and a second IF frequency of 1.4 MHz. The first LO frequency is 81.4 to 111.4 MHz, a reasonable range for an oscillator. But if the original RF range of the receiver were to be converted ''directly'' to the 1.4 MHz intermediate frequency, the LO frequency would need to cover 1.4-31.4 MHz which cannot be accomplished using tuned circuits (a variable capacitor with a fixed inductor would need a capacitance range of 500:1). Image rejection is never an issue with such a high IF frequency. The first IF stage uses a crystal filter with a 12 kHz bandwidth. There is a second frequency conversion (making a triple-conversion receiver) that mixes the 81.4 MHz first IF with 80 MHz to create a 1.4 MHz second IF. Image rejection for the second IF is not an issue as the first IF has a bandwidth of much less than 2.8 MHz.
To avoid interference to receivers, licensing authorities will avoid assigning common IF frequencies to transmitting stations. Standard intermediate frequencies used are 455 kHz for
medium-wave
Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the daytime ...
AM radio, 10.7 MHz for broadcast FM receivers, 38.9 MHz (Europe) or 45 MHz (US) for television, and 70 MHz for satellite and terrestrial microwave equipment. To avoid
tooling costs associated with these components, most manufacturers then tended to design their receivers around a fixed range of frequencies offered, which resulted in a worldwide ''de facto'' standardization of intermediate frequencies.
In early superhets, the IF stage was often a regenerative stage providing the sensitivity and selectivity with fewer components. Such superhets were called super-gainers or regenerodynes.
This is also called a
Q multiplier In electronics, a Q multiplier is a circuit added to a radio receiver to improve its selectivity and sensitivity. It is a regenerative amplifier adjusted to provide positive feedback within the receiver. This has the effect of narrowing the receiv ...
, involving a small modification to an existing receiver especially for the purpose of increasing selectivity.
IF bandpass filter
The IF stage includes a filter and/or multiple tuned circuits to achieve the desired
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 ...
. This filtering must have a band pass equal to or less than the frequency spacing between adjacent broadcast channels. Ideally a filter would have a high attenuation to adjacent channels, but maintain a flat response across the desired signal spectrum in order to retain the quality of the received signal. This may be obtained using one or more dual tuned IF transformers, a quartz
crystal filter
A crystal filter allows some frequencies to 'pass' through an electrical circuit while attenuating undesired frequencies. An electronic filter can use quartz crystals as resonator components of a filter circuit. Quartz crystals are piezoelectric ...
, or a multipole
ceramic crystal filter.
In the case of television receivers, no other technique was able to produce the precise
bandpass
A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.
Description
In electronics and signal processing, a filter is usually a two-port ...
characteristic needed for
vestigial sideband
In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude modul ...
reception, such as that used in the
NTSC
The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
system first approved by the US in 1941. By the 1980s, multi-component capacitor-inductor filters had been replaced with precision electromechanical
surface acoustic wave
A surface acoustic wave (SAW) is an acoustic wave traveling along the surface of a material exhibiting elasticity, with an amplitude that typically decays exponentially with depth into the material, such that they are confined to a depth of abou ...
(SAW)
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 ...
. Fabricated by precision laser milling techniques, SAW filters are cheaper to produce, can be made to extremely close tolerances, and are very stable in operation.
Demodulator
The received signal is now processed by the
demodulator
Demodulation is extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated ...
stage where the audio signal (or other
baseband
In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable into a ...
signal) is recovered and then further amplified. AM demodulation requires
envelope detection, which can be achieved by means of
rectification
Rectification has the following technical meanings:
Mathematics
* Rectification (geometry), truncating a polytope by marking the midpoints of all its edges, and cutting off its vertices at those points
* Rectifiable curve, in mathematics
* Recti ...
and a
low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter des ...
(which can be as simple as an
RC circuit
A resistor–capacitor circuit (RC circuit), or RC filter or RC network, is an electric circuit composed of resistors and capacitors. It may be driven by a voltage or current source and these will produce different responses. A first order RC ci ...
) to remove remnants of the intermediate frequency.
FM signals may be detected using a discriminator,
ratio detector
The ratio detector is a type of detector circuit, commonly used in radio receivers for demodulating frequency modulated (FM) signal.
The ratio detector is a variant of the Foster–Seeley discriminator, but one diode conducts in an opposite d ...
, or
phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
.
Continuous wave
A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration. It may refer to e.g. a laser or particle ...
and
single sideband
In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude modul ...
signals require a
product detector
A product detector is a type of demodulator used for AM and SSB signals. Rather than converting the envelope of the signal into the decoded waveform like an envelope detector, the product detector takes the product of the modulated signal and ...
using a so-called
beat frequency oscillator
In a radio receiver, a beat frequency oscillator or BFO is a dedicated oscillator used to create an audio frequency signal from Morse code radiotelegraphy ( CW) transmissions to make them audible. The signal from the BFO is mixed with the recei ...
, and there are other techniques used for different types of
modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
.
The resulting audio signal (for instance) is then amplified and drives a loudspeaker.
When so-called high-side injection has been used, where the local oscillator is at a ''higher'' frequency than the received signal (as is common), then the frequency spectrum of the original signal will be reversed. This must be taken into account by the demodulator (and in the IF filtering) in the case of certain types of modulation such as
single sideband
In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude modul ...
.
Multiple conversion
![Double-conversion superheterodyne receiver block diagram](https://upload.wikimedia.org/wikipedia/commons/5/5c/Double-conversion_superheterodyne_receiver_block_diagram.svg)
To overcome obstacles such as
image response
Image response (or more correctly, image response rejection ratio, or IMRR) is a measure of performance of a radio receiver that operates on the Superheterodyne receiver, superheterodyne principle.
In such a radio receiver, a local oscillator ( ...
, some receivers use multiple successive stages of frequency conversion and multiple IFs of different values. A receiver with two frequency conversions and IFs is called a ''dual conversion superheterodyne'', and one with three IFs is called a ''triple conversion superheterodyne''.
The main reason that this is done is that with a single IF there is a tradeoff between low
image response
Image response (or more correctly, image response rejection ratio, or IMRR) is a measure of performance of a radio receiver that operates on the Superheterodyne receiver, superheterodyne principle.
In such a radio receiver, a local oscillator ( ...
and selectivity. The separation between the received frequency and the
image frequency
A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carri ...
is equal to twice the IF frequency, so the higher the IF, the easier it is to design an RF filter to remove the image frequency from the input and achieve low
image response
Image response (or more correctly, image response rejection ratio, or IMRR) is a measure of performance of a radio receiver that operates on the Superheterodyne receiver, superheterodyne principle.
In such a radio receiver, a local oscillator ( ...
. However, the higher the IF, the more difficult it is to achieve high selectivity in the IF filter. At
shortwave
Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
frequencies and above, the difficulty in obtaining sufficient selectivity in the tuning with the high IFs needed for low image response impacts performance. To solve this problem two IF frequencies can be used, first converting the input frequency to a high IF to achieve low image response, and then converting this frequency to a low IF to achieve good selectivity in the second IF filter. To improve tuning, a third IF can be used.
For example, for a receiver that can tune from 500 kHz to 30 MHz, three frequency converters might be used.
With a 455 kHz IF it is easy to get adequate front end selectivity with broadcast band (under 1600 kHz) signals. For example, if the station being received is on 600 kHz, the local oscillator can be set to 1055 kHz, giving an image on (-600+1055=) 455 kHz. But a station on 1510 kHz could also potentially produce an image at (1510-1055=) 455 kHz and so cause image interference. However, because 600 kHz and 1510 kHz are so far apart, it is easy to design the front end tuning to reject the 1510 kHz frequency.
However at 30 MHz, things are different. The oscillator would be set to 30.455 MHz to produce a 455 kHz IF, but a station on 30.910 would also produce a 455 kHz beat, so both stations would be heard at the same time. But it is virtually impossible to design an RF tuned circuit that can adequately discriminate between 30 MHz and 30.91 MHz, so one approach is to "bulk downconvert" whole sections of the shortwave bands to a lower frequency, where adequate front-end tuning is easier to arrange.
For example, the ranges 29 MHz to 30 MHz; 28 MHz to 29 MHz etc. might be converted down to 2 MHz to 3 MHz, there they can be tuned more conveniently. This is often done by first converting each "block" up to a higher frequency (typically 40 MHz) and then using a second mixer to convert it down to the 2 MHz to 3 MHz range. The 2 MHz to 3 MHz "IF" is basically another self-contained superheterodyne receiver, most likely with a standard IF of 455 kHz.
Modern designs
Microprocessor technology allows replacing the superheterodyne receiver design by a
software-defined radio
Software-defined radio (SDR) is a radio communication system where components that have been traditionally implemented in analog hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by me ...
architecture, where the IF processing after the initial IF filter is implemented in software. This technique is already in use in certain designs, such as very low-cost FM radios incorporated into mobile phones, since the system already has the necessary
microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
.
Radio transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
s may also use a mixer stage to produce an output frequency, working more or less as the reverse of a superheterodyne receiver.
Advantages and disadvantages
Superheterodyne receivers have essentially replaced all previous receiver designs. The development of modern
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
electronics negated the advantages of designs (such as the
regenerative receiver
A regenerative circuit is an amplifier circuit that employs positive feedback (also known as regeneration or reaction). Some of the output of the amplifying device is applied back to its input so as to add to the input signal, increasing the am ...
) that used fewer vacuum tubes. The superheterodyne receiver offers superior sensitivity, frequency stability and selectivity. Compared with the
tuned radio frequency receiver
A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and usually an ...
(TRF) design, superhets offer better stability because a tuneable oscillator is more easily realized than a tuneable amplifier. Operating at a lower frequency, IF filters can give narrower passbands at the same
Q factor
In physics and engineering, the quality factor or ''Q'' factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is. It is defined as the ratio of the initial energy stored in the resonator to the energy los ...
than an equivalent RF filter. A fixed IF also allows the use of a
crystal filter
A crystal filter allows some frequencies to 'pass' through an electrical circuit while attenuating undesired frequencies. An electronic filter can use quartz crystals as resonator components of a filter circuit. Quartz crystals are piezoelectric ...
or similar technologies that cannot be tuned.
Regenerative and super-regenerative receivers offered a high sensitivity, but often suffer from stability problems making them difficult to operate.
Although the advantages of the superhet design are overwhelming, there are a few drawbacks that need to be tackled in practice.
Image frequency (''f''IMAGE)
![Superheterodyne image problem](https://upload.wikimedia.org/wikipedia/commons/b/bb/Superheterodyne_image_problem.svg)
One major disadvantage to the superheterodyne receiver is the problem of ''image frequency''. In heterodyne receivers, an image frequency is an undesired input frequency equal to the station frequency plus (or minus) twice the intermediate frequency. The image frequency results in two stations being received at the same time, thus producing interference. Reception at the image frequency can be combated through tuning (filtering) at the antenna and RF stage of the superheterodyne receiver.
:
For example, an AM broadcast station at 580 kHz is tuned on a receiver with a 455 kHz IF. The local oscillator is tuned to 1035 kHz. But a signal at 1490 kHz is also 455 kHz away from the local oscillator; so both the desired signal and the image, when mixed with the local oscillator, will appear at the intermediate frequency. This image frequency is within the AM broadcast band. Practical receivers have a tuning stage before the converter, to greatly reduce the amplitude of image frequency signals; additionally, broadcasting stations in the same area have their frequencies assigned to avoid such images.
The unwanted frequency is called the ''image'' of the wanted frequency, because it is the "mirror image" of the desired frequency reflected about
. A receiver with inadequate filtering at its input will pick up signals at two different frequencies simultaneously: the desired frequency and the image frequency. A radio reception which happens to be at the image frequency can interfere with reception of the desired signal, and noise (static) around the image frequency can decrease the receiver's signal-to-noise ratio (SNR) by up to 3dB.
Early
Autodyne
The autodyne circuit was an improvement to radio signal amplification using the Lee De Forest, De Forest Audion vacuum tube amplifier. By allowing the tube to oscillate at a frequency slightly different from the desired signal, the sensitivity ov ...
receivers typically used IFs of only 150 kHz or so. As a consequence, most Autodyne receivers required greater front-end selectivity, often involving double-tuned coils, to avoid image interference. With the later development of tubes able to amplify well at higher frequencies, higher IF frequencies came into use, reducing the problem of image interference. Typical consumer radio receivers have only a single tuned circuit in the RF stage.
Sensitivity to the image frequency can be minimized only by (1) a filter that precedes the mixer or (2) a more complex mixer circuit
to suppress the image; this is rarely used. In most tunable receivers using a single IF frequency, the RF stage includes at least one tuned circuit in the
RF front end RF is an abbreviation for radio frequency.
Rf or RF may also mean:
Arts and entertainment
* ''Red Faction (series)'', a series of revolution video games
* Rinforzando, , in music notation
* '' RF Online'', an online RPG made by CCR
Businesses
* ...
whose tuning is performed in tandem with the local oscillator. In double (or triple) conversion receivers in which the first conversion uses a fixed local oscillator, this may rather be a fixed
bandpass filter
A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.
Description
In electronics and signal processing, a filter is usually a two-port ...
which accommodates the frequency range being mapped to the first IF frequency range.
Image rejection is an important factor in choosing the intermediate frequency of a receiver. The farther apart the bandpass frequency and the image frequency are, the more the bandpass filter will attenuate any interfering image signal. Since the frequency separation between the bandpass and the image frequency is
, a higher intermediate frequency improves image rejection. It may be possible to use a high enough first IF that a fixed-tuned RF stage can reject any image signals.
The ability of a receiver to reject interfering signals at the image frequency is measured by the
image rejection ratio
Image response (or more correctly, image response rejection ratio, or IMRR) is a measure of performance of a radio receiver that operates on the Superheterodyne receiver, superheterodyne principle.
In such a radio receiver, a local oscillator ( ...
. This is the ratio (in
decibel
The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a po ...
s) of the output of the receiver from a signal at the received frequency, to its output for an equal-strength signal at the image frequency.
Local oscillator radiation
It can be difficult to keep stray radiation from the local oscillator below the level that a nearby receiver can detect. If the receiver's local oscillator can reach the antenna it will act as a low-power
CW transmitter. Consequently, what is meant to be a receiver can itself create radio interference.
In intelligence operations, local oscillator radiation gives a means to detect a covert receiver and its operating frequency. The method was used by MI5 during
Operation RAFTER
RAFTER was a code name for the MI5 radio receiver detection technique, mostly used against clandestine Soviet agents and monitoring of domestic radio transmissions by foreign embassy personnel from the 1950s on.
Explanation
Most radio receivers ...
.
This same technique is also used in
radar detector detectors used by traffic police in jurisdictions where radar detectors are illegal.
Local oscillator radiation is most prominent in receivers in which the antenna signal is connected directly to the mixer (which itself receives the local oscillator signal) rather than from receivers in which an RF amplifier stage is used in between. Thus it is more of a problem with inexpensive receivers and with receivers at such high frequencies (especially microwave) where RF amplifying stages are difficult to implement.
Local oscillator sideband noise
Local oscillators typically generate a single frequency signal that has negligible
amplitude modulation
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...
but some random
phase modulation
Phase modulation (PM) is a modulation pattern for conditioning communication signals for transmission. It encodes a message signal as variations in the instantaneous phase of a carrier wave. Phase modulation is one of the two principal forms ...
which spreads some of the signal's energy into sideband frequencies. That causes a corresponding widening of the receiver's frequency response, which would defeat the aim to make a very narrow bandwidth receiver such as to receive low-rate digital signals. Care needs to be taken to minimize oscillator phase noise, usually by ensuring that the oscillator never enters a
non-linear
In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...
mode.
Terminology
; First detector, second detector: The mixer tube or transistor is sometimes called the ''first detector'', while the demodulator that extracts the modulation from the IF signal is called the ''second detector''. In a dual-conversion superhet there are two mixers, so the demodulator is called the ''third detector''.
;
RF front end RF is an abbreviation for radio frequency.
Rf or RF may also mean:
Arts and entertainment
* ''Red Faction (series)'', a series of revolution video games
* Rinforzando, , in music notation
* '' RF Online'', an online RPG made by CCR
Businesses
* ...
: Refers to all the components of the receiver up to and including the mixer; all the parts that process the signal at the original incoming radio frequency. In the block diagram above the RF front end components are colored red.
See also
*
H2X radar
H2X, officially known as the AN/APS-15, was an American ground scanning radar system used for blind bombing during World War II. It was a development of the British H2S radar, the first ground mapping radar to be used in combat. It was also known ...
*
Automatic gain control
Automatic gain control (AGC) is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the inpu ...
*
Demodulator
Demodulation is extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated ...
*
Direct conversion 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 iden ...
*
VFO
A variable frequency oscillator (VFO) in electronics is an oscillator whose frequency can be tuned (i.e., varied) over some range. It is a necessary component in any tunable radio transmitter or receiver that works by the superheterodyne princip ...
*
Single sideband modulation (demodulation)
*
Tuned radio frequency receiver
A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and usually an ...
*
Reflex receiver
A reflex radio receiver, occasionally called a reflectional receiver, is a radio receiver design in which the same amplifier is used to amplify the high-frequency radio signal (RF) and low-frequency audio (sound) signal (AF). It was first invente ...
*
Optical heterodyne detection Optical heterodyne detection is a method of extracting information encoded as modulation of the phase, frequency or both of electromagnetic radiation in the wavelength band of visible or infrared light. The light signal is compared with standard or ...
*
Superheterodyne transmitter
Superheterodyne transmitter is a radio or TV transmitter which uses an intermediate frequency signal in addition to radio frequency signal.
Types of transmitters
There are two types of transmitters. In some transmitters, the information ...
Notes
References
Further reading
*
*
*
*
*
External links
* http://ethw.org/Superheterodyne_Receiver
* . An article giving the history of the various inventors working on the superheterodyne method.
*
*
Raises Paul Laüt published six months before Lévy; Étienne published the memo.
*
* Describes English efforts.
* 29F(2d)953. Armstrong v. Lévy, decided Dec. 3, 1928 http://www.leagle.com/decision/192898229F2d953_1614/ARMSTRONG%20v.%20LEVY
An in-depth introduction to superheterodyne receiversSuperheterodyne receivers from microwaves101.comMultipage tutorial describing the superheterodyne receiver and its technology
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