Editing Intermediate frequency

{{Short description|Frequency to which a carrier wave is shifted during transmission or reception}}
{{use dmy dates|date=July 2021|cs1-dates=y}}
[[File:19K1 IF stages.jpg|thumb|The IF stage from a Motorola 19K1 television set ''circa'' 1949]]
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 (telecommunications)|transmission]] or reception.<ref name="Langford-Smith_1941"/> 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 receiver|superheterodyne radio receivers]], in which an incoming signal is shifted to an IF for [[Amplifier|amplification]] before final [[Detector (radio)|detection]] is done.

Conversion to an intermediate frequency is useful for several reasons. When several stages of filters are used, they can all be set to a fixed frequency, which makes them easier to build and to tune. Lower frequency transistors generally have higher gains so fewer stages are required. It's easier to make sharply selective filters at lower fixed frequencies.

There may be several such stages of intermediate frequency in a superheterodyne receiver; two or three stages are called ''[[double conversion (superhet)|double]]'' (alternatively, ''dual'') or ''[[triple conversion (superhet)|triple conversion]]'', respectively.

==Justification==
Intermediate frequencies are used for three general reasons.<ref name="Army_1952"/><ref name="Rembovsky_2009"/> At very high ([[gigahertz]]) frequencies, signal processing circuitry performs poorly. Active devices such as [[transistor]]s cannot deliver much amplification ([[Gain (electronics)|gain]]).<ref name="Langford-Smith_1941"/>  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 more convenient processing. For example, in [[satellite dish]]es, the microwave downlink signal received by the dish is converted to a much lower IF at the dish so that a relatively inexpensive [[coaxial cable]] can carry the signal to the receiver inside the building. Bringing the signal in at the original microwave frequency would require an expensive [[waveguide]].

In receivers that can be tuned to different frequencies, a second reason is to convert the various different frequencies of the stations to a common frequency for processing. It is difficult to build multistage [[amplifier]]s, [[Electronic filter|filters]], and [[detector (radio)|detector]]s that can have all stages track the tuning of different frequencies, but it is comparatively easy to build tunable [[Electronic oscillator|oscillators]]. Superheterodyne receivers tune in different frequencies by adjusting the frequency of the local oscillator on the input stage, and all processing after that is done at the same fixed frequency: the IF. Without using an IF, all the complicated filters and detectors in a radio or television would have to be tuned in unison each time the frequency was changed as was necessary in the early [[tuned radio frequency receiver]]s (TRF). A more important advantage is that it gives the receiver a constant bandwidth over its tuning range. The bandwidth of a filter is proportional to its center frequency. In receivers like the TRF in which the filtering is done at the incoming RF frequency, as the receiver is tuned to higher frequencies, its bandwidth increases.

The main reason for using an intermediate frequency is to improve frequency [[selectivity (radio)|selectivity]].<ref name="Langford-Smith_1941"/> 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 [[Filter (signal processing)|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 (signal processing)|bandwidth]] increases proportionately with the frequency. So a narrower bandwidth and more selectivity can be achieved by converting the signal to a lower IF and performing the filtering at that frequency. [[FM broadcasting|FM]] and [[television broadcasting]] with their narrow channel widths, as well as more modern telecommunications services such as [[cell phone]]s and [[cable television]], would be impossible without using frequency conversion.<ref name="Dixon_1998"/>

==Uses==
Perhaps the most commonly used intermediate frequencies for broadcast receivers are around 455&nbsp;kHz for AM receivers and 10.7&nbsp;MHz for FM receivers. In special purpose receivers other frequencies can be used. A dual-conversion receiver may have two intermediate frequencies, a higher one to improve image rejection and a second, lower one, for desired selectivity. A first intermediate frequency may even be higher than the input signal, so that all undesired responses can be easily filtered out by a fixed-tuned RF stage.<ref name="Hayward_1977"/>

In a digital receiver, the [[analog-to-digital converter]] (ADC) operates at low sampling rates, so input RF must be mixed down to IF to be processed. Intermediate frequency tends to be lower frequency range compared to the transmitted RF frequency. However, the choices for the IF are most dependent on the available components such as [[Frequency_mixer|mixer]], filters, amplifiers and others that can operate at lower frequency. There are other factors involved in deciding the IF, because lower IF is susceptible to noise and higher IF can cause clock jitters.

Modern [[satellite television]] receivers use several intermediate frequencies.<ref name="Lundstrom_2006"/> The 500 television channels of a typical system are transmitted from the satellite to subscribers in the [[Ku band|Ku]] microwave band, in two subbands of 10.7–11.7 and 11.7–12.75&nbsp;GHz. The downlink signal is received by a [[satellite dish]]. In the box at the focus of the dish, called a [[low-noise block downconverter]] (LNB), each block of frequencies is converted to the IF range of 950–2150&nbsp;MHz by two fixed frequency local oscillators at 9.75 and 10.6&nbsp;GHz. One of the two blocks is selected by a control signal from the set top box inside, which switches on one of the local oscillators. This IF is carried into the building to the television receiver on a coaxial cable. At the cable company's [[set top box]], the signal is converted to a lower IF of 480&nbsp;MHz for filtering, by a variable frequency oscillator.<ref name="Lundstrom_2006"/> This is sent through a 30&nbsp;MHz bandpass filter, which selects the signal from one of the [[transponder]]s on the satellite, which carries several channels. Further processing selects the channel desired, demodulates it and sends the signal to the television.

==History==
An intermediate frequency was first used in the superheterodyne radio receiver, invented by American scientist Major [[Edwin Armstrong]] in 1918, during [[World War I]].<ref name="Redford_1996"/><ref name="Wiccanpiper_2004"/> A member of the [[Signal Corps (United States Army)|Signal Corps]], Armstrong was building radio [[direction finding]] equipment to track German military signals at the then-very high frequencies of 500 to 3500&nbsp;kHz. The [[triode vacuum tube]] amplifiers of the day would not amplify stably above 500&nbsp;kHz; however, it was easy to get them to [[Electronic oscillator|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&nbsp;kHz the local oscillator would be tuned to 1450&nbsp;kHz. Mixing the two created an intermediate frequency of 50&nbsp;kHz, which was well within the capability of the tubes. The name ''superheterodyne'' was a contraction of ''supersonic heterodyne'', to distinguish it from receivers in which the heterodyne frequency was low enough to be directly audible, and which were used for receiving [[continuous wave]] (CW) [[Morse code]] transmissions (not speech or music).

After the war, in 1920, Armstrong sold the patent for the superheterodyne to [[Westinghouse Electric (1886)|Westinghouse]], who subsequently sold it to [[RCA]]. The increased complexity of the superheterodyne circuit compared to earlier [[Regenerative receiver|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.

==Examples==
[[File:Radiola AR-812 superheterodyne ad.jpg|thumb|upright=1.3|The RCA Radiola AR-812<ref name="Malanowski_2011"/> used 6 triodes: a mixer, local oscillator, two IF and two audio amplifier stages, with an IF of 45&nbsp;kHz.]]
* down to c. 20&nbsp;kHz{{cn|date=July 2021|reason=The article [[Superheterodyne receiver]] originally stated this from 2006 to 2021<!-- https://en.wikipedia.org/w/index.php?title=Superheterodyne_receiver&type=revision&diff=89426788&oldid=89131394 -->, but without source}}, 30&nbsp;kHz (A. L. M. Sowerby and H. B. Dent),<ref name="Bussey_1990"/> 45&nbsp;kHz (first commercial superheterodyne receiver: RCA Radiola AR-812 of 1923/1924),<ref name="Malanowski_2011"/> c. 50&nbsp;kHz,<ref name="Bussey_1990"/> c. 100&nbsp;kHz,<ref name="Bussey_1990"/> c. 120&nbsp;kHz<ref name="Bussey_1990"/>
* 110&nbsp;kHz was used in European AM [[longwave]] broadcast receivers.<ref name="Langford-Smith_1941"/><ref name="Langford-Smith_1953"/>
* 175&nbsp;kHz (early wide band and communications receivers before introduction of powdered iron cores)<ref name="Langford-Smith_1941"/><ref name="Langford-Smith_1953"/><ref name="Bussey_1990"/>
* 260&nbsp;kHz (early standard broadcast receivers),<ref name="Langford-Smith_1953"/> 250–270&nbsp;kHz<ref name="Langford-Smith_1941"/>
* Copenhagen Frequency Allocations: 415–490&nbsp;kHz, 510–525&nbsp;kHz<ref name="Langford-Smith_1953"/>
* [[AM radio]] receivers: 450&nbsp;kHz, 455&nbsp;kHz (most common),<ref name="Langford-Smith_1953"/> 460&nbsp;kHz, 465&nbsp;kHz,<ref name="Bussey_1990"/> 467&nbsp;kHz, 470&nbsp;kHz, 475&nbsp;kHz, and 480&nbsp;kHz.<ref name="Ravalico_1992"/>
* [[FM radio]] receivers: 262&nbsp;kHz (old car radios),<ref name="Wiccanpiper_2004"/> 455&nbsp;kHz, 1.6&nbsp;MHz, 5.5&nbsp;MHz, 10.7&nbsp;MHz (most common),<ref name="Langford-Smith_1953"/> 10.8&nbsp;MHz,<ref name="Electra_Bearcat"/> 11.2&nbsp;MHz, 11.7&nbsp;MHz, 11.8&nbsp;MHz, 13.45&nbsp;MHz,<ref name="Pioneer_1987"/> 21.4&nbsp;MHz, 75&nbsp;MHz and 98&nbsp;MHz. In double-conversion superheterodyne receivers, a first intermediate frequency of 10.7&nbsp;MHz is often used, followed by a second intermediate frequency of 470&nbsp;kHz (or 700&nbsp;kHz with [[DYNAS]]<ref name="Telefunken_1996"/>). There are triple conversion designs used in police scanner receivers, high-end communications receivers, and many point-to-point microwave systems. Modern DSP chip consumer radios often use a '[[low IF receiver|low-IF]]' of 128&nbsp;kHz for FM.
* [[Narrowband FM]] receivers: 455&nbsp;kHz (most common),<ref name="Langford-Smith_1953"/><ref name="Hansen_ICS"/> 470&nbsp;kHz<ref name="Hansen_ICS"/>
* Shortwave receivers: 1.6&nbsp;MHz,<ref name="Langford-Smith_1953"/> 1.6–3.0&nbsp;MHz,<ref name="Langford-Smith_1941"/> 4.3&nbsp;MHz (for 40–50 MHz-only receivers).<ref name="Langford-Smith_1953"/> In double-conversion superheterodyne receivers, a first intermediate frequency of 3.0&nbsp;MHz is sometimes combined with a second IF of 465&nbsp;kHz.<ref name="Langford-Smith_1941"/>
* [[Analog transmission|Analogue]] television receivers using system M: 41.25&nbsp;MHz (audio) and 45.75&nbsp;MHz (video). Note, the channel is flipped over in the conversion process in an [[intercarrier method|intercarrier]] system, so the audio IF is lower than the video IF. Also, there is no audio local oscillator; the injected video carrier serves that purpose.
* [[Analog transmission|Analogue]] television receivers using system B and similar systems: 33.4&nbsp;MHz for the aural and 38.9&nbsp;MHz for the visual signal. (The discussion about the frequency conversion is the same as in system M.)
* Satellite [[uplink]]-[[downlink]] equipment: 70&nbsp;MHz, 950–1450&nbsp;MHz (L-band) downlink first IF.
* Terrestrial [[microwave]] equipment: 250&nbsp;MHz, 70&nbsp;MHz or 75&nbsp;MHz.
* [[Radar]]: 30&nbsp;MHz.
* [[Radio frequency|RF]] test equipment: 310.7&nbsp;MHz, 160&nbsp;MHz, and 21.4&nbsp;MHz.

==See also==
* [[Low-IF receiver]]
* [[Mechanical filter]]
* [[Zero-IF receiver]]

==References==
{{reflist|refs=
<ref name="Malanowski_2011">{{cite book |author-last=Malanowski |author-first=Gregory |title=The Race for Wireless: How Radio Was Invented (or Discovered?) |publisher=Authorhouse |date=2011 |page=69 |url=https://books.google.com/books?id=IAjtEeVtXqAC&q=superheterodyne&pg=PA69 |isbn=978-1-46343750-3}}</ref>

<ref name="Pioneer_1987">{{cite book |title=F-91 FM/AM Digital Synthesizer Tuner - Service Manual |chapter=11. Circuit description - 11.1. New IF system principle |language=en, fr, es |publisher=[[Pioneer Electronic Corporation]] |publication-place=Tokyo, Japan / Long Beach, USA |id=Order No. ARP1465 |date=August 1987 |pages=35–38 [37–38] |url=https://fmtunerinfo.com/F-91service.pdf |access-date=2021-06-10 |url-status=live |archive-url=https://web.archive.org/web/20210411023352/https://fmtunerinfo.com/F-91service.pdf |archive-date=2021-04-11 |quote-page=37 |quote=[…] Mixer […] perform frequency change so that multiply input FM signal by [[Voltage-controlled oscillator|VCO]] output. F-91 introduce the secondary IF as 13.45&nbsp;MHz. Band-pass filter […] has the same narrow bandwidth characteristic as the band-pass filter […] Input signal […] passed through the band-pass filter […] is multiplied by VCO output at mixer […] then change[d] to the original frequency. Original signal is detected by FM detector […] audio output is obtained. […] in spite of use the filter of fixed the center frequency, F-91 operate to the variable filter so that center frequency follow the input signal as equivalent. […]}}[https://web.archive.org/web/20210411023407/https://www.fmtunerinfo.com/F-91ARTS.pdf][https://web.archive.org/web/20200114001025/http://nice.kaze.com/av/f-91_svm.pdf] (4 of 40 pages) (NB. The ''Pioneer Elite F-91'' and the very similar ''Pioneer Reference Digital Synthesizer Tuner F-717'' (as sold in Japan) supported [[Active Real-time Tracing System]] (ARTS) in 1987, whereas the completely different but almost identically named ''Pioneer Digital Synthesizer Tuner F-717'' and ''F-717L'' (as sold internationally in 1987) were based on the F-77 and did not support ARTS.)</ref>

<ref name="Telefunken_1996">{{cite web |title=U4292B - FM-IF IC for the DYNAS System |type=datasheet |edition=preliminary |publisher={{ill|Telefunken Semiconductors|de}} / {{ill|TEMIC{{!}}TEMIC TELEFUNKEN microelectronic GmbH|de|TEMIC}} |location=Heilbronn, Germany |date=1996-08-19 |version=A1 |url=http://pdf.datasheetcatalog.com/datasheet/Temic/mXyzurwy.pdf |access-date=2021-06-07 |url-status=live |archive-url=https://web.archive.org/web/20200315134551/http://datasheetcatalog.com/datasheets_pdf/U/4/2/9/U4292B.shtml |archive-date=2020-03-15 |quote-page=1 |quote=[…] [[DYNAS]] system […] for car radio and home receiver applications […] system of [[frequency modulation|FM]]-IF processing […] [[bandpass filter]]s with a [[frequency bandwidth|bandwidth]] down to about 20&nbsp;kHz compared to 160&nbsp;kHz for a conventional […] filter […] tracks the [[resonant frequency]] to the actual frequency […]}} [https://web.archive.org/web/20210611182301/https://www.circuitsonline.net/forum/file/53516] (13+1 pages)</ref>

<ref name="Hansen_ICS">{{cite book |title=ICS - In-Channel-Select - das Empfangssystem der Zukunft / ICS-Restsignalverstärker |language=de |type=product flyer and manual |publisher=H.u.C. Elektronik / Hansen & Co. |date= |publication-place=Berlin, Germany |url=https://www.circuitsonline.net/forum/file/53580 |access-date=2021-06-11 |url-status=live |archive-url=https://web.archive.org/web/20210611182422/https://www.circuitsonline.net/forum/file/53580 |archive-date=2021-06-11}} (3+7 pages, page 6 missing)</ref>

<ref name="Electra_Bearcat">Electra Bearcat scanner radios</ref>

<ref name="Army_1952">{{cite book |title=Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers |publisher=[[US Department of the Army]] |date=1952 |pages=195–197 |url=https://books.google.com/books?id=f9QXAAAAYAAJ&pg=PA195}}</ref>

<ref name="Rembovsky_2009">{{cite book |author-last1=Rembovsky |author-first1=Anatoly |author-last2=Ashikhmin |author-first2=Alexander |author-last3=Kozmin |author-first3=Vladimir |title=Radio Monitoring: Problems, Methods and Equipment |publisher=Springer Science and Business Media |date=2009 |pages=26 |url=https://books.google.com/books?id=2ra1lg9MCLgC&q=selectivity+sensitivity+image&pg=PA26 |isbn=978-0387981000 |display-authors=etal}}</ref>

<ref name="Langford-Smith_1941">{{cite book |title=Radiotron Designer's Handbook |chapter=Chapter 15. Frequency conversion: The principle of the Superheterodyne / Chapter 17. Intermediate Frequency Amplifiers: Choice of Frequency |editor-first=Fritz |editor-last=Langford-Smith |editor-link=Fritz Langford-Smith |edition=4th impression, 3rd |publisher=[[Wireless Press]] for [[AWA Technology Services|AWA]] / [[RCA]] |publication-place=Sydney, Australia / Harrison, New Jersey, USA |date=November 1941 |orig-date=1940 |pages=90, 99–100, 104, 158–159 [100, 159] |url=https://pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_16/Sec_51/4394_Radiotron_Designers_Handbook_3rd_Ed.pdf |access-date=2021-07-10 |url-status=live |archive-url=https://web.archive.org/web/20210203003154/https://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_16/Sec_51/4394_Radiotron_Designers_Handbook_3rd_Ed.pdf |archive-date=2021-02-03 |quote-pages=100, 158–159 |quote=[…] it can be assumed that the desired intermediate frequency is 465&nbsp;[[kilohertz|Kc/s]] […] for this reason frequencies in the region of 450–465&nbsp;Kc/s are very widely used […] [[Superheterodyne receiver]]s, designed specifically for short-wave communication work, usually have a higher frequency for the I.F., from about 1,600 to 3,000&nbsp;Kc/s, and may also incorporate double frequency changing. For example the receiver may change the incoming signal first to 3,000&nbsp;Kc/s and then to 465&nbsp;Kc/s or lower. […] Various frequencies are used for the I.F. amplifiers of radio receivers. A frequency of 110&nbsp;Kc/s. has been used widely in Europe where the [[long wave band]] is in use. This gives extremely good selectivity but serious side band cutting. A frequency of 175&nbsp;Kc/s. has been used for broadcast band reception both in America and Australia for a number of years but its use on the [[short-wave band]] is not very satisfactory. A frequency in the region on 250–270&nbsp;Kc/s. has also been used to a limited extent as a compromise between 175 and 465&nbsp;Kc/s. The most common frequencies for dual wave receivers are between 450 and 465&nbsp;Kcs.[…] and, particularly if iron cored I.F. transformers are used, this frequency band is a very good compromise. For short-wave receivers which are not intended for operation at lower frequencies, an intermediate frequency of 1,600&nbsp;Kc/s. or higher may be used. […] A frequency of 455&nbsp;Kc/s. is receiving universal acceptance as a standard frequency, and efforts are being made to maintain this frequency free from radio interference. […]}} (See also: [[Radiotron Designer's Handbook]])</ref>

<ref name="Langford-Smith_1953">{{cite book |editor-first=Fritz |editor-last=Langford-Smith |editor-link=Fritz Langford-Smith |author-first1=Bill |author-last1=Sandel<!-- (1917-) --> |author-first2=Ian C. |author-last2=Hansen |display-authors=etal |title=Radiotron Designer's Handbook |chapter=Chapter 26. Intermediate Frequency Amplifiers. Section 1. Choice of Frequency (ii) Commonly accepted intermediate frequencies / Section 2: Number of stages / Chapter 34. Types of A-M Receivers. Section 2: The Superheterodyne / Chapter 38. Tables, Charts and Sundry Data. Section 4. Standard Frequencies (iii) Standard Intermediate Frequencies |edition=4 |publisher=[[Wireless Press]] for [[AWA Technology Services|AWA]] / [[RCA]], Electron Tube Division |publication-place=Sydney, Australia / Harrison, New Jersey, USA |date=January 1960 |orig-date=1953, 1952, 1940, 1935, 1934 |pages=1021–1022, 1226, 1293–1295, 1361 |url=http://www.tubebooks.org/books/rdh4.pdf |access-date=2021-07-09 |url-status=live |archive-url=https://web.archive.org/web/20210708135027/http://www.tubebooks.org/books/rdh4.pdf |archive-date=2021-07-08 |quote-pages=1021–1022, 1226, 1361 |quote=[…] As a result of the experience gained over a number of years in addition to the considerations stated previously the values selected for the intermediate frequencies of most commercial receivers have become fairly well standardized. For the majority of broadcast receivers tuning the bands 540–1600&nbsp;[[kilohertz|Kc/s]] and 6–18&nbsp;[[megahertz|Mc/s]], an i-f of about 455&nbsp;Kc/s is usual. A frequency of 110&nbsp;Kc/s has been extensively used in Europe where the [[long wave band]] of 150–350&nbsp;Kc/s is in operation. Receivers for use only on the [[short wave band]] commonly the 40–50&nbsp;Mc/s band generally use a 4.3&nbsp;Mc/s i-f, and for the 88–108&nbsp;Mc/s band they use 10.7&nbsp;Mc/s. This latter value has been adopted as standard in U.S.A., and some other countries, for [[Very high frequency|v-h-f]] receivers. […] Short wave receivers using 1600&nbsp;Kc/s i-f transformers commonly employ two stages (3 transformers) although one stage is often used […] In wide band and communication receivers, two or more stages are commonly used. The intermediate frequency in general use is 455&nbsp;Kc/s. Earlier receivers used 175&nbsp;Kc/s but with the appearance of powdered iron cores and the development of high slope amplifier valves, the previous objection to the use of higher intermediate frequencies, i.e. lower gain, was nullified. […] It is recommended that [[superheterodyne receiver]]s operating in the [[medium frequency]] [[broadcast band]] use an intermediate frequency of 455&nbsp;Kc/s. This frequency is reserved as a clear channel for the purpose in most countries of the world. […] The European "[[Copenhagen Frequency Plan of 1948|Copenhagen Frequency Allocations]]" provide the following two intermediate frequency bands: 415–490&nbsp;Kc/s and 510–525&nbsp;Kc/s. […] An intermediate frequency of 175&nbsp;Kc/s is also used. […] The American [[Radio Television Manufacturers Association|RTMA]] has standardized the following intermediate frequencies (REC-109-B, March 1950): Standard broadcast receivers&mdash;either 260 or 455&nbsp;Kc/s. V-H-F broadcast receivers&mdash;10.7 Mc/s.}} [https://web.archive.org/web/20210709230051/https://worldradiohistory.com/BOOKSHELF-ARH/Handbooks/Radiotron-Designer%27s-Handbook-4th-Edition.pdf][https://archive.org/details/bitsavers_rcaRadiotr1954_94958503] (See also: [[Radiotron Designer's Handbook]])</ref>

<ref name="Dixon_1998">{{cite book |author-last=Dixon |author-first=Robert |title=Radio Receiver Design |publisher=[[CRC Press]] |date=1998 |pages=57–61 |url=https://books.google.com/books?id=hqkKAV1KsrQC&pg=PA57 |isbn=0-82470161-5}}</ref>
<ref name="Hayward_1977">{{cite book |author-first=Wes |author-last=Hayward |editor-first=Doug |editor-last=De Maw |title=Solid state design for the radio amateur |publisher=[[American Radio Relay League]] |date=1977 |pages=82–87}}</ref>
<ref name="Lundstrom_2006">{{cite book |author-last=Lundstrom |author-first=Lars-Ingemar |title=Understanding Digital Television: An Introduction to DVB Systems with Satellite, Cable, Broadband and Terrestrial |publisher=[[Taylor & Francis]] |date=2006 |location=USA |pages=81–83 |url=https://books.google.com/books?id=IW-iqhtqYGMC&q=%22satellite+receiver%22+LNB+%22intermediate+frequency%22&pg=PA81 |isbn=0-24080906-8}}</ref>

<ref name="Redford_1996">{{cite web |author-last=Redford |author-first=John |title=Edwin Howard Armstrong |date=February 1996 |work=Doomed Engineers |publisher=John Redford's personal website |url=http://world.std.com/~jlr/doom/armstrng.htm |access-date=2008-05-10 |url-status=dead |archive-url=https://web.archive.org/web/20080509070320/http://world.std.com/~jlr/doom/armstrng.htm |archive-date=2008-05-09}}</ref>

<ref name="Wiccanpiper_2004">{{cite web |author=Wiccanpiper<!-- alisdair --> |title=Superheterodyne |date=2004-01-08 |work=everything.com |url=http://everything2.com/index.pl?node_id=1356743 |access-date=2008-05-10 |url-status=live |archive-url=https://web.archive.org/web/20210709224144/https://everything2.com/index.pl?node_id=1356743 |archive-date=2021-07-09}}</ref>

<ref name="Ravalico_1992">{{cite book |author-last=Ravalico |author-first=Domenico E. |title=Radioelementi |language=it |location=Milan, Italy |publisher=Hoepli |date=1992}}</ref>

<ref name="Bussey_1990">{{cite book |author-last=Bussey |author-first=Gorden |date=1990 |title=Wireless: the crucial decade - History of the British wireless industry 1924–34 |publisher=Peter Peregrinus Ltd. / [[Institution of Electrical Engineers]] |publication-place=London, UK |volume=13 |series=IEE History of Technology Series |isbn=0-86341-188-6 |id={{ISBN|978-0-86341-188-5}} |pages=18–19, 78 |url=https://books.google.com/books?id=QJzDsSuaqU4C&pg=PA78 |access-date=2021-07-11 |url-status=live |archive-url=https://web.archive.org/web/20210711072729/https://books.google.de/books?id=QJzDsSuaqU4C&pg=PA78&lpg=PA78&ots=D8X5i8YOP-&focus=viewport&vq=30+kc%2Fs&dq=Wireless+-+the+crucial+decade+1924%25E2%2580%25931934&hl=de |archive-date=2021-07-11}} (136 pages)</ref>
}}

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