Editing Heterodyne (section)

==History==
[[File:Heterodyne radio receiver circuit 1920.png|thumb|Fessenden's heterodyne radio receiver circuit. The incoming radio frequency and local oscillator frequency mix in the crystal diode detector.]]

In 1901, [[Reginald Fessenden]] demonstrated a [[direct-conversion receiver]] or [[beat receiver]] as a method of making [[continuous wave]] [[radiotelegraphy]] signals audible.<ref>[http://www.nonstopsystems.com/radio/pdf-hell/article-IRE-7-1959.pdf Discussion of A History of Some Foundations of Modern Radio-Electronic Technology], Comments by Lloyd Espenschied, Proceedings of the IRE, July, 1959 (Vol. 47, No. 7), pp. 1254, 1256. ''Critique.'' ". . . the roots of our modern technology trace back generally to sources other than the Hammond Laboratory."  ''Comment.'' Many of the roots that nourished the work of the Hammond group and its contemporaries were recorded in our paper: the pioneering work of Wilson and Evans, Tesla, Shoemaker, in basic radiodynamics; . . . of Tesla and Fessenden leading to the development of basic intermediate frequency circuitry.</ref> Fessenden's receiver did not see much application because of its local oscillator's stability problem.   A stable yet inexpensive local oscillator was not available until [[Lee de Forest]] invented the [[triode|triode vacuum tube]] oscillator.<ref name=nahin1>{{Harvnb|Nahin|2001|p=91}}, stating "Fessenden's circuit was ahead of its time, however, as there simply was no technology available then with which to build the required local oscillator with the necessary frequency stability." Figure 7.10 shows a simplified 1907 heterodyne detector.</ref> In a 1905 patent, Fessenden stated that the frequency stability of his local oscillator was one part per thousand.<ref>{{Harvnb|Fessenden|1905|p=4}}</ref>

In radio telegraphy, the characters of text messages are translated into the short duration dots and long duration dashes of [[Morse code]] that are broadcast as radio signals. [[Radio telegraphy]] was much like ordinary [[telegraphy]]. One of the problems was building high power transmitters with the technology of the day. Early transmitters were [[spark gap transmitter]]s. A mechanical device would make sparks at a fixed but audible rate; the sparks would put energy into a resonant circuit that would then ring at the desired transmission frequency (which might be 100&nbsp;kHz). This ringing would quickly decay, so the output of the transmitter would be a succession of [[damped wave]]s. When these damped waves were received by a simple detector, the operator would hear an audible buzzing sound that could be transcribed back into alpha-numeric characters.<!-- spark gap transmitters had a lot of splatter. Californian's operator was asked to shut down early so Titanic could work Cape Race. Consequently, California's radio room was not manned when Titanic sent out its distress signal. http://www.environmentalhistory.org/revcomm/features/radio-and-the-titanic/-->

With the development of the [[arc converter]] radio transmitter in 1904, [[continuous wave]] (CW) modulation began to be used for radiotelegraphy.  CW Morse code signals are not amplitude modulated, but rather consist of bursts of sinusoidal carrier frequency.  When CW signals are received by an AM receiver, the operator does not hear a sound. The direct-conversion (heterodyne) detector was invented to make continuous wave radio-frequency signals audible.<ref>{{cite book |last1=Ashley |first1=Charles Grinnell |last2=Heyward |first2=Charles Brian |title=Wireless Telegraphy and Wireless Telephony |publisher=American School of Correspondence |year=1912 |location=Chicago |pages=103/15–104/16 |url=https://archive.org/details/wirelesstelegra00haywgoog }}</ref>

The "heterodyne" or "beat" receiver has a [[local oscillator]] that produces a radio signal adjusted to be close in frequency to the incoming signal being received.  When the two signals are mixed, a "beat" frequency equal to the difference between the two frequencies is created.  Adjusting the local oscillator frequency correctly puts the beat frequency in the [[audio signal|audio]] range, where it can be heard as a tone in the receiver's [[earphone]]s whenever the transmitter signal is present. Thus the Morse code "dots" and "dashes" are audible as beeping sounds. This technique is still used in radio telegraphy, the local oscillator now being called the [[beat frequency oscillator]] or BFO. Fessenden coined the word ''heterodyne'' from the Greek roots ''hetero-'' "different", and ''dyn-'' "power" (cf. [[Wiktionary:δύναμις|δύναμις or dunamis]]).<ref>[https://books.google.com/books?id=49jgQvbrvCUC&dq=heterodyne+Fessenden+coined&pg=PA370 Tapan K. Sarkar, History of wireless, page 372]</ref>

===Superheterodyne receiver===
[[Image:Superheterodyne receiver block diagram 2.svg|thumb|upright=1.6|Block diagram of a typical superheterodyne receiver.  <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.]]

An important and widely used application of the heterodyne technique is in the [[superheterodyne receiver]] (superhet). In the typical superhet, the incoming [[radio frequency]] signal from the antenna is mixed (heterodyned) with a signal from a local oscillator (LO) to produce a lower fixed frequency signal called the [[intermediate frequency]] (IF) signal. The IF signal is amplified and filtered and then applied to a [[detector (radio)|detector]] that extracts the audio signal; the audio is ultimately sent to the receiver's loudspeaker.

The superheterodyne receiver has several advantages over previous receiver designs. One advantage is easier tuning; only the RF filter and the LO are tuned by the operator; the fixed-frequency IF is tuned ("aligned") at the factory and is not adjusted. In older designs such as the [[tuned radio frequency receiver]] (TRF), all of the receiver stages had to be simultaneously tuned. In addition, since the IF filters are fixed-tuned, the receiver's selectivity is the same across the receiver's entire frequency band. Another advantage is that the IF signal can be at a much lower frequency than the incoming radio signal, and that allows each stage of the IF amplifier to provide more gain. To first order, an amplifying device has a fixed [[gain-bandwidth product]]. If the device has a gain-bandwidth product of 60&nbsp;MHz, then it can provide a voltage gain of 3 at an RF of 20&nbsp;MHz or a voltage gain of 30 at an IF of 2&nbsp;MHz. At a lower IF, it would take fewer gain devices to achieve the same gain. The [[regenerative radio receiver]] obtained more gain out of one gain device by using positive feedback, but it required careful adjustment by the operator; that adjustment also changed the selectivity of the regenerative receiver. The superheterodyne provides a large, stable gain and constant selectivity without troublesome adjustment.

The superior superheterodyne system replaced the earlier TRF and regenerative receiver designs, and since the 1930s most commercial radio receivers have been superheterodynes.