Editing Superheterodyne receiver (section)

==Principle of operation==
[[File:Superheterodyne receiver block diagram 2.svg|thumb|upright=2.0|Block diagram of a typical single-conversion superheterodyne receiver. The diagram has blocks that are common to superheterodyne receivers,<ref name="Carr_2002"/> with only the RF amplifier being optional. <span style="color:red;">Red</span> parts are those that handle the incoming radio frequency (RF) signal; <span style="color:green;">green</span> are parts that operate at the intermediate frequency (IF), while <span style="color:blue;">blue</span> parts operate at the modulation (audio) frequency. The dotted line indicates that the local oscillator and RF filter must be tuned in tandem.]]
[[File:How superheterodyne receiver works.svg|thumb|upright=1.6|How a superheterodyne radio works. The horizontal axes are frequency ''f''. The blue graphs show the voltage spectra of the radio signals at various points in the circuit. The red graphs show the [[transfer function]]s of the filters in the circuit; the thickness of the red bands shows the fraction of signal from the previous graph that passes through the filter at each frequency. The incoming radio signal from the antenna ''(top graph)'' consists of the desired radio signal ''S1'' plus others at different frequencies. The RF filter ''(2nd graph)'' removes any signal such as ''S2'' at the [[image frequency]] ''LO''&nbsp;-&nbsp;''IF'', which would otherwise pass through the IF filter and interfere. The remaining composite signal is applied to the mixer along with a local oscillator signal (''LO'') ''(3rd graph)''. In the mixer the signal ''S1'' combines with the LO frequency to create a heterodyne at the difference between these frequencies, the intermediate frequency (IF), at the mixer output ''(4th graph)''. This passes through the IF bandpass filter ''(5th graph)'' and then is amplified and demodulated (demodulation is not shown).   The unwanted signals create heterodynes at other frequencies ''(4th graph)'', which are filtered out by the IF filter .]]

The [[Antenna (radio)|antenna]] collects the radio signal. The tuned RF stage with optional RF amplifier provides some initial selectivity; it is necessary to suppress the ''[[#Image frequency|image frequency]]'', and may also serve to prevent strong out-of-passband signals from saturating the initial amplifier. A [[local oscillator]] provides the mixing frequency; it is usually a variable frequency oscillator which is used to tune the receiver to different stations. The [[frequency mixer]] does the actual [[heterodyning]] that gives the superheterodyne its name; it changes the incoming radio frequency signal to a higher or lower, fixed, [[intermediate frequency]] (IF). The IF [[band-pass filter]] and amplifier supply most of the gain and the narrowband filtering for the radio. The [[demodulator]] extracts the audio or other [[modulation]] 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 (radio)|antenna]] is required. The output of the antenna may be very small, often only a few [[microvolt]]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]]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]], or [[varicap diode]].<ref name="Hagen_1996"/> 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]] (LO). The mixer uses a non-linear component to produce both sum and difference [[Beat (acoustics)#Mathematics and physics of beat tones|beat frequency]] signals,<ref name="AOE_2006"/> each one containing the [[modulation]] in the desired signal. The output of the mixer may include the original RF signal at ''f''<sub>RF</sub>, the local oscillator signal at ''f''<sub>LO</sub>, and the two new heterodyne frequencies ''f''<sub>RF</sub>&nbsp;+&nbsp;''f''<sub>LO</sub> and ''f''<sub>RF</sub>&nbsp;&minus;&nbsp;''f''<sub>LO</sub>. The mixer may inadvertently produce additional frequencies such as third- and higher-order intermodulation products. Ideally, the IF [[bandpass filter]] removes all but the desired IF signal at ''f''<sub>IF</sub>. The IF signal contains the original modulation (transmitted information) that the received radio signal had at ''f''<sub>RF</sub>.

The frequency of the local oscillator ''f''<sub>LO</sub> is set so the desired reception radio frequency ''f''<sub>RF</sub> mixes to ''f''<sub>IF</sub>. There are two choices for the local oscillator frequency because of the correspondence between positive and negative frequencies. If the local oscillator frequency is less than the desired reception frequency, it is called ''low-side injection'' (''f''<sub>IF</sub> = ''f''<sub>RF</sub> − ''f''<sub>LO</sub>); if the local oscillator is higher, then it is called ''high-side injection'' (''f''<sub>IF</sub> = ''f''<sub>LO</sub> − ''f''<sub>RF</sub>).<!-- only cases for difference mixing given -->

The mixer will process not only the desired input signal at f<sub>RF</sub>, but also all signals present at its inputs. There will be many mixer products (heterodynes).<!-- ignoring higher order products --> Most other signals produced by the mixer (such as due to stations at nearby frequencies) can be [[Filter (signal processing)|filtered]] out in the IF [[tuned amplifier]];<!-- dynamic range issues --> that gives the superheterodyne receiver its superior performance. However, if ''f''<sub>LO</sub> is set to ''f''<sub>RF</sub>&nbsp;+&nbsp;''f''<sub>IF</sub>, then an incoming radio signal at ''f''<sub>LO</sub>&nbsp;+&nbsp;''f''<sub>IF</sub> will ''also'' produce a heterodyne at ''f''<sub>IF</sub>; the frequency  ''f''<sub>LO</sub>&nbsp;+&nbsp;''f''<sub>IF</sub> is called the ''image frequency'' and must be rejected by the tuned circuits in the RF stage. The image frequency is 2&nbsp;''f''<sub>IF</sub> higher (or lower) than the desired frequency ''f''<sub>RF</sub>, so employing a higher IF frequency ''f''<sub>IF</sub> 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&nbsp;kHz to 1655&nbsp;kHz (a roughly 1160&nbsp;kHz input band) with a 455&nbsp;kHz IF frequency; an FM broadcast band receiver covers 88&nbsp;MHz to 108&nbsp;MHz band with a 10.7&nbsp;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&nbsp;kHz, it would see stations at both 745&nbsp;kHz (1200&minus;455&nbsp;kHz) and 1655&nbsp;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]]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.<ref name="Terman_1943"/><ref name="Rohde-Bucher_1988"/>

In the days of [[vacuum tube|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]] tube would oscillate and also provide signal amplification as well as frequency mixing.<ref name="Langford-Smith_1940"/>

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''.<ref>{{cite book|first=Ralph S.|last=Carson|title=Radio Communications Concepts: Analog|publisher=Wiley|location=New York|year=1990|isbn=978-0-47162-169-0|page=326}}</ref>  In a dual-conversion superhet there are two mixers, so the demodulator is called the ''third detector''.

===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.<ref name="Carr_2002"/> The IF amplifier is selective around its center frequency ''f''<sub>IF</sub>. 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''<sub>IF</sub> is chosen to be less than the range of desired reception frequencies ''f''<sub>RF</sub>. 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 factor|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  [[#Multiple conversion|double conversion]] configuration. For instance, the Rohde & Schwarz EK-070 VLF/HF receiver covers 10&nbsp;kHz to 30&nbsp;MHz.<ref name="Rohde-Bucher_1988"/> It has a band switched RF filter and mixes the input to a first IF of 81.4&nbsp;MHz and a second IF frequency of 1.4&nbsp;MHz. The first LO frequency is 81.4 to 111.4&nbsp;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&nbsp;MHz intermediate frequency, the LO frequency would need to cover 1.4-31.4&nbsp;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&nbsp;kHz bandwidth. There is a second frequency conversion (making a triple-conversion receiver) that mixes the 81.4&nbsp;MHz first IF with 80&nbsp;MHz to create a 1.4&nbsp;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&nbsp;MHz.<!-- easy to place zero at image frequency. -->

To avoid interference to receivers, licensing authorities will avoid assigning common IF frequencies to transmitting stations. Standard intermediate frequencies used are 455&nbsp;kHz for [[medium-wave]] AM radio, 10.7&nbsp;MHz for broadcast FM receivers, 38.9&nbsp;MHz (Europe) or 45&nbsp;MHz (US) for television, and 70&nbsp;MHz for satellite and terrestrial microwave equipment. To avoid [[Machine tool|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.<ref name="TT"/> This is also called a [[Q multiplier]], 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 (radio)|selectivity]]. 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]], or a multipole [[ceramic resonator|ceramic crystal filter]].<ref name="QSL"/>

In the case of television receivers, no other technique was able to produce the precise [[bandpass]] characteristic needed for [[vestigial sideband]] reception, such as that used in the [[NTSC]] 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]] (SAW) [[Electronic filter|filters]]. 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]] stage where the audio signal (or other [[baseband]] signal) is recovered and then further amplified. AM demodulation requires [[envelope detector|envelope detection]], which can be achieved by means of [[rectification (electricity)|rectification]] and a [[low-pass filter]] (which can be as simple as an [[RC circuit]]) to remove remnants of the intermediate frequency.<ref name="AMDEM"/> FM signals may be detected using a discriminator, [[Detector (radio)#Ratio detector|ratio detector]], or [[phase-locked loop]]. [[Continuous wave]] and [[single sideband]] signals require a [[product detector]] using a so-called [[beat frequency oscillator]], and there are other techniques used for different types of [[modulation]].<ref name="TSCM"/> 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]].