Radio receiver

After the radio signal is filtered and amplified, the receiver must extract the information-bearing

• an AM receiver that receives an (amplitude modulated) radio signal uses an AM demodulator
• an FM receiver that receives a frequency modulated signal uses an FM demodulator
• an FSK receiver which receives frequency-shift keying (used to transmit digital data in wireless devices) uses an FSK demodulator

Many other types of modulation are also used for specialized purposes.

The modulation signal output by the demodulator is usually amplified to increase its strength, then the information is converted back to a human-usable form by some type of

The easiest type of demodulation to understand is AM demodulation, used in

sound waves by the radio's speaker. It is accomplished by a circuit called an envelope detector (see circuit), consisting of a diode (D) with a bypass capacitor

(C) across its output.

The

With other types of modulation like FM or FSK the amplitude of the modulation does not vary with the radio signal strength, but in all types the demodulator requires a certain range of signal amplitude to operate properly.[4]^[5] Insufficient signal amplitude will cause an increase of noise in the demodulator, while excessive signal amplitude will cause amplifier stages to overload (saturate), causing distortion (clipping) of the signal.

Therefore, almost all modern receivers include a

In the simplest type of radio receiver, called a

Although the TRF receiver is used in a few applications, it has practical disadvantages which make it inferior to the superheterodyne receiver below, which is used in most applications.^[4] The drawbacks stem from the fact that in the TRF the filtering, amplification, and demodulation are done at the high frequency of the incoming radio signal. The bandwidth of a filter increases with its center frequency, so as the TRF receiver is tuned to different frequencies its bandwidth varies. Most important, the increasing congestion of the radio spectrum requires that radio channels be spaced very close together in frequency. It is extremely difficult to build filters operating at radio frequencies that have a narrow enough bandwidth to separate closely spaced radio stations. TRF receivers typically must have many cascaded tuning stages to achieve adequate selectivity. The Advantages section below describes how the superheterodyne receiver overcomes these problems.

The

To achieve both good image rejection and selectivity, many modern superhet receivers use two intermediate frequencies; this is called a

.

At the cost of the extra stages, the superheterodyne receiver provides the advantage of greater selectivity than can be achieved with a TRF design. Where very high frequencies are in use, only the initial stage of the receiver needs to operate at the highest frequencies; the remaining stages can provide much of the receiver gain at lower frequencies which may be easier to manage. Tuning is simplified compared to a multi-stage TRF design, and only two stages need to track over the tuning range. The total amplification of the receiver is divided between three amplifiers at different frequencies; the RF, IF, and audio amplifier. This reduces problems with feedback and parasitic oscillations that are encountered in receivers where most of the amplifier stages operate at the same frequency, as in the TRF receiver.^[10]

The most important advantage is that better selectivity can be achieved by doing the filtering at the lower intermediate frequency.^[4][8][10] One of the most important parameters of a receiver is its bandwidth, the band of frequencies it accepts. In order to reject nearby interfering stations or noise, a narrow bandwidth is required. In all known filtering techniques, the bandwidth of the filter increases in proportion with the frequency, so by performing the filtering at the lower ${\displaystyle f_{\text{IF}}}$, rather than the frequency of the original radio signal ${\displaystyle f_{\text{RF}}}$, a narrower bandwidth can be achieved. Modern FM and television broadcasting, cellphones and other communications services, with their narrow channel widths, would be impossible without the superheterodyne.^[8]

History

This section is an excerpt from History of radio receivers.[edit]

electromagnetic theory. Hertz used spark-excited dipole antennas to generate the waves and micrometer spark gaps attached to dipole and loop antennas to detect them.^[16][17][18]

These precursor radio receivers were primitive devices, more accurately described as radio wave "sensors" or "detectors", as they could only receive radio waves within about 100 feet of the transmitter, and were not used for communication but instead as laboratory instruments in scientific experiments and engineering demonstrations.

• Television receive-only

See also

Wikimedia Commons has media related to radio receivers.

• Batteryless radio
• Dielectric wireless receiver
• Digital Audio Broadcast
  (DAB)
• Direct conversion receiver
• List of radios – List of specific models of radios
• Minimum detectable signal
• Radio transmitter design
• Radio receiver design
• Radiogram (furniture)
• Receiver (information theory)
• Distortion
• Telecommunication
• Tuner (radio)

References

1. ^ Radio-Electronics, Radio Receiver Technology
2. ISBN 978-8120351240
   .
3. ^
   ISBN 978-1596933101
   .
4. ^
   ISBN 978-1118647844
   . Chapter 1
5. ^
   ISBN 978-0521553568
   .
6. ^ Armstrong, Edwin H. (February 1921). "A new system of radio frequency amplification". Proceedings of the Institute of Radio Engineers. 9 (1): 3–11. Retrieved December 23, 2015.
7. ^ Lee, Thomas H. (2004) The Design of CMOS Radio Frequency Integrated Circuits, 2nd Ed., p. 14-15
8. ^
   ISBN 978-0824701611
   .
9. ^ ^{a b c} Williams, Lyle Russell (2006) The New Radio Receiver Building Handbook, p. 28-30
10. ^ ^{a b c} Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers, 1952, p. 195-197
11. ^ McNicol, Donald (1946) Radio's Conquest of Space, p. 272-278
12. ^ Terman, Frederick E. (1943) Radio Engineers' Handbook, p. 636-638
13. ^
    ISBN 978-0750673198
    .
14. ISBN 978-0387981000
    .
15. ^ Terman, Frederick E. (1943) Radio Engineers' Handbook, p. 645
16. ISBN 978-0521835398
    .
17. ^ Appleyard, Rollo (October 1927). "Pioneers of Electrical Communication part 5 - Heinrich Rudolph Hertz" (PDF). Electrical Communication. 6 (2): 67. Retrieved December 19, 2015.
18. ISBN 978-0906048245
    .

Further reading

Wikimedia Commons has media related to Radio receivers.

• Communications Receivers, Third Edition, Ulrich L. Rohde, Jerry Whitaker, McGraw Hill, New York, 2001,
  ISBN 0-07-136121-9
• Buga, N.; Falko A.; Chistyakov N.I. (1990). Chistyakov N.I. (ed.). Radio Receiver Theory. Translated from the Russian by Boris V. Kuznetsov.
  ISBN 978-5-03-001321-3First published in Russian as «Радиоприёмные устройства»{{cite book}}: CS1 maint: postscript (link
  )