Radio_receiver - Wikiparency

# Radio receiver A **radio receiver** is an electronic device that picks up [[Radio_wave|radio waves]] with an [[Antenna_(radio)|antenna]] and converts the information they carry into a usable form, typically sound, video, or digital data. The antenna turns an incoming wave into a tiny alternating current at the same frequency; the receiver then has to find the one signal it wants among everything else the antenna has also picked up, make it large enough to use, and recover whatever was impressed on it before the [[Radio|station]] ever transmitted it. A three.js sketch further down this page follows this chain for a superheterodyne receiver, tracking the wanted signal, its local oscillator, and the unwanted image frequency together as the receiver is tuned. Nearly every receiver, from a pocket [[AM_broadcasting|AM]] set to a cellphone to a [[Radar|radar]] set's own receive chain, performs the same three jobs in some order: filtering out everything but the desired slice of the [[Radio_spectrum|radio spectrum]], amplifying what is left, and demodulating it to recover the original information. How a particular design orders and implements those three jobs is what separates the earliest tuned radio-frequency receivers from the [[Superheterodyne_receiver|superheterodyne]] design that displaced them and still dominates today. ## Applications A receiver is rarely sold on its own; it is a component built into almost anything that uses radio. Broadcast tuners in a car or a kitchen radio are the most familiar example, but the same three jobs are performed inside a television set, a cellphone's incoming-call and data path, a Wi-Fi or Bluetooth chip, a GPS unit working out its own position from satellite timing signals, a garage-door opener or keyless car entry fob, and the receive side of a two-way radio, a weather station, or a [[Radar|radar]] set reading its own reflected pulses. [[Avionics|Avionics]] depend on several receivers at once, for navigation beacons, instrument landing systems and air-traffic communications, each tuned to its own dedicated part of the spectrum, and radio astronomy pushes the same three jobs to their most sensitive extreme, listening for signals many orders of magnitude weaker than any terrestrial broadcast. Whatever the application, a system designer treats the receiver's own sensitivity as one half of a larger [[Link_budget|link budget]]: the other half, set by the transmitter's power, the antennas at each end and the distance and obstacles between them, decides how strong a signal actually reaches the receiver, and the receiver's own noise performance decides how weak a signal that arriving strength must still exceed. A satellite ground station and a garage-door remote sit at opposite ends of the same trade-off, one built around an antenna and a front end sensitive enough to pull a signal out of near-total silence, the other built to a price and expected to work only across a house's own driveway. ## Principles ### Reception An antenna converts an incoming electromagnetic wave into a tiny alternating current or voltage at the wave's own frequency, and everything the receiver does afterward starts from that one small signal, mixed in with whatever else the antenna happened to intercept at the same time. How weak a signal a receiver can still make use of, its sensitivity, is set mainly by the noise added in its very first amplifying stage, since noise added there is amplified along with the signal at every stage that follows and can never be removed afterward; this is why the [[Signal-to-noise_ratio|signal-to-noise ratio]] out of the front end, not any later stage, usually decides how weak a station a receiver can pull in. ### Bandpass filtering Of everything the antenna picks up, only a narrow band around the wanted station is useful, and a bandpass filter, tuned electrically to that band, rejects the rest before it can add noise or overload later stages. How narrow that band can be made while still passing the wanted signal cleanly, and how well the filter rejects a strong neighbouring station just outside it, is the receiver's selectivity, and it trades directly against how sharply the filter itself must be built. ### Amplification The filtered signal is still far too small to drive a loudspeaker, a screen, or a digital decoder directly, so it passes through one or more amplifying stages built from [[Transistor|transistors]] or an [[Integrated_circuit|integrated circuit]], typically split between a stage that amplifies the signal at the frequency it arrived on and, in most modern designs, a further stage that amplifies it again after it has been shifted to a fixed intermediate frequency. Each added stage of amplification also adds its own share of [[Noise_(electronics)|noise]] and can introduce [[Distortion|distortion]] if driven too hard, so gain is budgeted across the chain rather than piled entirely into one stage. ### Demodulation Demodulation recovers the information that was impressed on the carrier before transmission, and the circuit needed depends entirely on how that information was encoded: an [[Envelope_detector|envelope detector]] recovers audio from an [[Amplitude_modulation|amplitude-modulated]] carrier by following the envelope of the wave, while a [[Frequency_modulation|frequency-modulated]] signal needs a discriminator that turns frequency changes into a varying voltage, and a digital signal needs a decision circuit that maps the demodulated waveform back onto the symbols it represents. ### Automatic gain control (AGC) Because a receiver must work equally well on a powerful nearby station and a faint distant one, without a listener having to ride the volume control between them, most receivers include a feedback loop, [[Automatic_gain_control|automatic gain control]], that measures the strength of the incoming signal after amplification and reduces the receiver's own gain automatically when that signal is already strong. The loop's job is to keep the output roughly constant over an input range that can otherwise span a factor of a million or more between the weakest and strongest stations a receiver ever tunes across. ## Designs ### Tuned radio frequency (TRF) receiver The earliest practical receivers amplified and filtered the incoming station directly at whatever frequency it arrived on, using two or three tuned stages connected together and adjusted, ideally in step with each other, by a single dial. Building capacitors that tracked one another accurately enough across an entire tuning band was difficult, and a set whose stages drifted even slightly out of alignment lost sensitivity and selectivity together, unevenly across the dial, which made a good tuned radio-frequency receiver expensive to build and fussy to use well. ### The superheterodyne design The [[Superheterodyne_receiver|superheterodyne]] design sidesteps that problem by converting every incoming station, whatever frequency it arrives on, down to one fixed intermediate frequency using a local oscillator and a mixer, so that only the front end needs to retune at all; the bulk of the amplification and nearly all of the sharp filtering happen afterward, at that one frequency, built once and never readjusted. That architecture has dominated receiver design since the 1930s, at the cost of a second, unwanted input frequency, the image, that a well-designed front-end filter must reject before it ever reaches the mixer. ## History The earliest radio receivers used a coherer, a tube of loose metal filings that clumped together and conducted when a radio-frequency current from a spark-gap transmitter passed through it, detecting little more than the presence or absence of a signal; [[Guglielmo_Marconi|Guglielmo Marconi]]'s wireless telegraphy demonstrations of the 1890s and his transatlantic reception of December 1901 both relied on receivers of this basic kind.[^cn-marconi] Reliable amplification arrived only once Lee de Forest's three-electrode Audion vacuum tube, patented in 1908, made it possible to strengthen a weak signal electrically instead of merely detecting that one was present.[^deforest1908] Edwin Armstrong's regenerative circuit, for which he applied on October 29, 1913 and was granted a patent almost a year later, on October 6, 1914, fed part of an amplified radio-frequency signal back into its own input to multiply the gain of a single vacuum tube many times over, and the same reasoning, extended further, led Armstrong to the superheterodyne architecture that had displaced tuned radio-frequency and regenerative designs alike by the 1930s.[^armstrong1914] The transistor, replacing the vacuum tube from the 1950s onward, let a full receiver run on a fraction of the power and fit in a pocket: Texas Instruments and Regency announced the Regency TR-1, the first commercially produced transistor radio, in Dallas on October 18, 1954, priced at $49.95.[^regency1954] More recent designs, the [[Direct-conversion_receiver|direct-conversion receiver]] and the [[Software-defined_radio|software-defined radio]], move demodulation or nearly all of the receiver's remaining analogue work into digital signal processing, carrying the same three jobs of filtering, amplifying and demodulating into software running on a general-purpose chip. ## Microsims A three.js companion elsewhere on this page renders the receiver chain in more depth, following a station's frequency, its image, and a variable preselector together as the receiver is tuned. The fuller working version of that same idea already lives on the [[Superheterodyne_receiver]] page, whose primary sketch shows a station's radio-frequency spectrum above and its converted, fixed-frequency spectrum below, with tuning, the intermediate frequency, and a preselector filter all adjustable, a switch between high-side and low-side local-oscillator injection, and a diagnostics readout that reports the resulting image rejection in decibels. Between the two sketches, the reader can see both halves of this article at once: the general principles of reception, filtering, amplification, demodulation and automatic gain control described above, and the one specific architecture, the superheterodyne, that arranges them into the design nearly every modern receiver still builds on. *Try:* on that page, raise the preselector's Q while leaving the tuning alone, and watch the unwanted image shrink in the top pane even though neither the wanted station nor the local oscillator has moved. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Radio_receiver) : [Wikitube](https://en.wikitube.io/wiki/Radio_receiver) Skeleton mirrored at revision 1373740124. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Superheterodyne_receiver]] - [[Crystal_radio]] - [[Antenna_(radio)]] - [[Automatic_gain_control]] - [[Software-defined_radio]] ## References [^cn-marconi]: Citation needed: a primary account (Marconi's own report or a contemporary Marconi Company record) of the December 1901 transatlantic coherer-receiver reception has not been pinned to a specific source in this pass. [^deforest1908]: De Forest, L. U.S. Patent 879,532 (three-electrode Audion vacuum tube), granted 1908. Engineering and Technology History Wiki (IEEE): https://ethw.org/De_Forest_Files_Audion_Patent . [^armstrong1914]: Armstrong v. De Forest Radio Telephone & Telegraph Co. (regenerative-circuit priority dispute), on Armstrong's application filed October 29, 1913, granted as a patent on October 6, 1914. https://www.courtlistener.com/opinion/8840229/armstrong-v-de-forest-radio-telephone-telegraph-co/ . [^regency1954]: "T.I. Unveils Transistor Radio," Engineering and Technology History Wiki (IEEE): the Regency TR-1, announced Dallas, Texas, October 18, 1954, priced at $49.95. https://ethw.org/T.I._Unveils_Transistor_Radio . ## Further reading - Steven Ellingson. *Radio Systems Engineering*, Revised First Edition (2023). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering - Christian Tiberius; Max Mulder. *Engineering Signal Analysis: From Fourier to Filtering: Theory* (2026). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/engineering-signal-analysis-from-fourier-to-filtering-theory - Don Johnson. *Fundamentals of Electrical Engineering I* (2014). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/fundamentals-of-electrical-engineering-1