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Radio receiver
The radio for many decades it has become customary, but not everyone knows how it works.
By definition, a radio receiver is that equipment that allows receiving an electromagnetic
signal (radio waves) emitted by a transmitter. In addition to the normal transmission intended
for peoples (broadcasting), there are special service broadcasting for specialized sectors or
areas of interest, as: marine navigation or air, radio bridges, radar, radio amateurs, etc.
A little bit of hystory
As about every invention, many are the fathers to ascribe credit for the realization of
the radio radio. In fact, without the discoveries of Faraday, Maxwell, Hertz and Branly would
had been difficult for Italian Marconi and Russian Popov, realize the first distance transmissions.
Historical that of Marconi who in 1901 made the first transoceanic transmission, from Poldhu,
western end of the peninsula of Cornwall (England), to Newfoundland Island, Nova Scotia, Canada.
It was a simlpy telegraphic signal in Morse code, a rhythmic sequence of three points, identifying
the letter "S". It was about 12:30 local time on 12th of December, 1901 when the signal was received
at wavelength of approximately 1800 m, transmitted by a distance of about 3,000 kilometers.
The first radio broadcast is dated on 24th of December, 1906 by Reginald Fessenden's work,
but the birth of the first radio station with programs devoted to "public audience" is dated
in 1919, by a Westinghouse engineer Frank Conrad, who began a series of broadcasts from his garage in Pittsburgh.
In 1929, a general store in the city, selling some rudimentary receivers, quickly sold out all
the equipment available. As happens in these cases, the phenomenon interested in the industry, so much
that the Vice-President of Westinghouse decided to build radio receivers for home use in a part of their
assembly factory, which during the war served to manufacture equipment for the military.
At the same time gave to Conrad, and his assistant Donald Little, the management to produce
a radio station inside the Westinghouse plants, so be able to begin regular broadcasts: the first
radio station was born (KDKA).
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Working principles
An elemental radio receiver consists of a tuner circuit, a detector and a sound transducer.
The tuning circuit consists of a solenoid and a capacitor (leaving aside the inherent electrical
resistance in the conductor by which the solenoid is formed and connection conductors of elements),
these components according to the inductance, resistance and capacitance sizes, resonate at a certain frequency.
That frequency is the frequency of the signal you want to receive, from the transmitter. |
circuitry of an elemental radio receiver
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In order to receive a band of frequencies, as well as to address the critical issues of tuning,
a variable capacitor is used, which by varying the value of its capacity, varies consequently
the resonant frequency. You have guessed that when we act on the tuning knob of our radio, in fact,
you vary the capacitance of the circuit tune. The detector circuit, consisting basically of a diode
with a capacitor to filter the RF, discriminates the signal component that actually we want to listen,
while the acoustic transducer (earphone, headphone, speaker) converts the electrical signal in the
sound wave.
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Period, Frequency, Wave length
The radio signal emitted from the transmitter is a high frequency signal, but what
does frequency mean? The frequency of a signal is the number of oscillations that is
accomplished in a second. A complete oscillation, or period, represented graphically,
has the shape visible in the figure. As you can see, starting from time zero of an axes
Cartesian it reaches a maximum positive then decline passing again to zero to reach a
maximum negative for returning to the zero point. This is a complete oscillation that
can be defined cycle or period as well. The frequency, in Hertz (Hz) or cycles per second
(c/s), is the number of oscillations that the signal does in every second.
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Putting that in matematical formulas, we have:
knowing the frequency we can calculate the period time
P = 1 / ƒ express in seconds
where
knowing the Period, we can calculate the Frequency
ƒ = 1/P express in Hz (cycles per second)
where
- ƒ = Frequency
- 1 = Time unit (second)
- P = Period
The wave length can be defined as the space in which stays a complete cycle.
Knowing that electromagnetic wave travel at light speed, in matematic terms we have:
λ = c/F express in meters
where
- λ = Wave length
- c = light speed (300.000 Km/s)
- F = Frequency (in KHz)
Someone of you is asking he/herself, what about if none of parameters are known, how can we
find the period time or a signal frequency?
The answer is: by having adequate measuring instruments. To reach the goal, frequency meter or better
an oscilloscope can be good for the purpose. The last one, in addition of measuring, permits us to see
the signal on the screen.
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period or cycle
frequency
wave length
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Radio Wave Classification
Now that we got acquainted with some of the physical units involved and their
relationships, let see how radio waves have been classified. Given that radio
comes from radiation and radio waves are part of the broader category of
electromagnetic waves, all the various frequencies of radio and television broadcasts
interest have been classified in an international assembly of Radio and Telecommunications
held in Atlantic City (USA) in 1947.
The following table shows the division of radio waves in the various bands of frequency,
with their names according to the IEEE (Institute of Electrical and Electronic Engineers -
US Organization), adopted by the ITU (International Telecommunication Union - United Nations Agency)
- the same year.
| Frequency Range | Abbreviation | Wave length range | Wave Definition |
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| from 3 KHz to 30 KHz | VLF (Very Low Frequencies) | from 100 Km to 10 Km | myriameters |
| from 30 KHz to 300 KHz | LF (Low Frequencies) | from 10 Km to 1 Km | kilometers |
| from 300 KHz to 3000 KHz | MF (Medium Frequencies) | from 1 Km to 0,1 Km | hectometers |
| from 3 MHz to 30 MHz | HF (High Frequencies) | from 100 m to 10 m | tenmeters |
| from 30 MHz to 300 MHz | VHF (Very High Frequencies) | from 10 m to 1 m | meters |
| from 300 MHz to 3000 MHz | UHF (Ultra High Frequencies) | from 100 cm to 10 cm | tenthmeters |
| from 3 GHz to 30 GHz | SHF (Super High Frequencies) | from 10 cm to 1 cm | centimeters |
| from 30 GHz to 300 GHz | EHF (Extra High Frequencies) | from 10 mm to 1 mm | millimeters |
| from 300 GHz to 3000 GHz | microonde | from 1 mm to 0,1 mm | tenthmillimeters |
Within the above classification, in Europe, so in Italy, for radio-television broadcast some frequency bands
had been reserved. In the following table, those more used, are reported.
| Frequency Range | Abbreviation | Use |
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| from 153 KHz to 279 KHz | Long Wave | naval navigation transmissions |
| from 531 KHz to 1602 KHz | Middle Wave | AM Radio broadcasting trasmissions |
| from 3 MHz to 26,100 MHz | Short Wave | Long Distance Radio Broadcasting Transmissions (*) |
| from 26,865 MHz to 27,275 MHz | Citizen Band (CB) | Radio-amateurs Transmissions (Used by Track Driver) |
| from 87,500 MHz to 108 MHz | FM Radio | FM Radio Broadcasting Transmissions (mostly in stereo mode) |
| from 47 MHz to 300 MHz | VHF (Very High Frequencies) | RAI 1 - Television Transmissions |
| from 300 MHz to 870 MHz | UHF (Ultra High Frequencies) | RAI (2°- 3° canale) and Private Television Transmissions |
| from 10,700 GHz to 12,750 GHz | Microwave | Satellite Data and Television Broadcasting Transmissions, analog and digital |
(*) = In ionosphere layers by refraction phenomenon
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Carrier, Information, Modulation
To this point, to proceed in transmission technique awareness, it is necessary introducing the modulation concept and how the radio signal is made.
Let's start by saying that the signal irradiated by the transmitted antenna is a composite signal, that contains two signal: carrier and information.
The information we want to broadcast, either music or voice, is a low frequency signal. The uman ear, in the best hypothesis, can sense frequencies in the range
from 20 to 20.000 Hz. The information is then a set of such frequencies that we obtain by different devices (microphone, CD/DVD driver, etc.) in electrical signals.
Those low frequencies electric signals are used to modulating the high frequency signal called carrier because is used as a transportation mean.
Premised that diverse kind of modulation exist, those more used by the radio reciever by the general people, are two: Amplitude Modulation or AM; Frequency Modulation or FM.
Amplitude Modulation
In the AM transmission we irradiate a signal at a fixed frequency modulated in amplitude according to the low frequency signal.
Frequency Modulation
In the FM transmission we irradiate a signal at a fixed amplitude but at variable frequency (more or less the central frequency, carrier), according to the low frequency signal.
Detection or De-modulation
The riceiver applys the inverse procedure, by detection a semi-wave of the low frequency signal (by the detector circuit), and filtering the high frequency part.
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carrier (RF signal)
information (BF signal)
amplitude modulation signal
frequency modulation signal
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Why private radios chose the FM band instead of AM
Without entering in deep technical details, the FM transmission takes advantage of the following benefits:
- Better sound quality
- Less power to transmit to the same distance
- The signal isn't influenced by electric atmosphere turbulence
By FM we can transmit music in Hi-Fi quality (20 - 15.000 Hz), while by AM we have a very less width BF band.
In addition, by multiplexing the Right and Left channels, we can transmit in stereophonic mode.
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