Radio Receivers, from crystal set to stereo
Author: Miomir Filipovic">
Author: Miomir Filipovic
Radio communication is created by means of electromagnetic waves, of which the existence and features were theoretically described and predicted by James Maxwell, in 1864. First experimental proof of this theory was given by Heinrich Hertz in 1888, ten years after Maxwell's death. It was already known at that time that electric current exists in oscillatory circuits made of a capacitor of capacity C and coil of inductance L. It was Thomson, back in 1853 that determined the frequency of this arrangement to be:
Hertz used an oscillatory circuit with a capacitor made of two bowls, K1 and K2 (Pic. 1.1), and the "coil" was made of two straight conductors. The bowls could be moved along the conductors. In this way the capacitance of the circuit could be altered, and also its resonance frequency. With every interruption from the battery, a high voltage was produced at the output of the inductor, creating a spark between the narrow placed balls k1 and k2. According to Maxwell's theory, as long as there was a spark, i.e. alternating current in the circuitry, there was an electromagnetic field surrounding the conductors, spreading itself through the surrounding space. A few metres away from this device Hertz placed a bent conductor with metal balls k3, k4 placed on the ends, positioned very close to each other. This also was an oscillatory circuit, called the resonator. According to Maxwell's theory, voltage induced by the electromagnetic waves should be created in the resonator. Voltage existence would be shown by a spark between the balls k3 and k4. And that's the way it was: Whenever there was a spark in the oscillator between the balls k1 and k2, a spark would also be produced by the resonator, between balls k3 and k4. With various forms of the arrangement in Pic. 1.1, Hertz proved that electromagnetic waves behave as light since they could also be reflected and refracted. It was also shown that light is of electromagnetic nature, as stated by Maxwell. Hertz, however, did not believe in the practical value of his electromagnetic waves experiments. The range of the link was no further than a few meters. The transmitted signal was very weak, therefore the signal in the receiver had a very small amplitude and it wasn't possible to detect it at a greater distance. The possibility of amplifying the signal in the receiver did not exist at the time. Besides the short range, another shortcoming of the link was noted: If another similar transmitter was working nearby, a receiver detected all the signals at the same time. It did not have the ability of isolation. However crude and simple these experiments were at the time, they represented the birth of a new scientific branch - Radio Engineering.The pioneers of radio were Popov and Marconi, but the place of honor belongs to Nikola Tesla, who demonstrated wireless broadcasting in 1893, at the Franklin Institute. Pic.1.2 shows the arrangement of this broadcast system. Tesla's idea was to produce electromagnetic waves by means of oscillatory circuits and transmit them over an antenna. A receiver would then receive the waves with another antenna and oscillatory circuit being in resonance with the oscillatory circuit of the transmitter. This represented the groundwork of today's radio communications.In 1904 John Flemming created the diode, and in 1907 Lee De Forest invented the triode. That year can be considered the birth of electronics, with the triode being the first electronic component used in a circuit for signal amplification.
Rapid development of radio engineering over the ensuing years produced many innovations and after the First World War a huge number of radio stations emerged. At that time TRF (Tuned Radio Frequency) receivers were used. Compared to modern receivers they had both poor selectivity and sensitivity, but back then they fulfilled the demands. The number of radio stations was much less than today and their transmitting power was much smaller. The majority of listeners were satisfied with the reception of only local stations. However as the number of stations increased, as well as their transmitting power, the problem of selecting one station out of the jumble of stations, was becoming increasingly more difficult.
It was partially solved with an increase in the number of oscillatory circuits in the receiver and the introduction of positive feedback, but the true solution was the invention of the superheterodyne receiver. This was accomplished by Lewy (1917), and improved by E.H. Armstrong (1918).An enormous impact on the world of radio was the invention of the transistor by Bardeen, Bretten & Schockley, in 1948. This reduced the size of the radio receiver and made truly portable sets a reality. This was followed by the introduction of the integrated circuit, enabling the construction of devices that not only proved better in every way than those using values, but also new designs.Radio amateurs' contribution to radio engineering should also be emphasized. In the beginning, radio communication was being conducted in the LW and MW bands. But achieving long-distance reception required very powerful transmitters. The SW band was considered to be useless for radio broadcast on long distances and was given to radio amateurs.The were banned from using LW and MW bands by commercial radio stations. However, something unexpected happened: Amateurs were able to accomplish extremely long distance transmissions (thousands of kilometres), by using very low-power transmitters. This was later explained by the influence of the ionosphere layer, the existence of which was also predicted by Tesla. Modern radio receivers differ greatly from the "classical" types, however the working principles are the same.
The only significant difference is in the way the receiver is tuned to a station. Classical devices used a variable capacitor, coil or varicap diode, with the frequency read from a scale with movable pointer. In modern devices, the adjustment is done with a frequency synthesizer controlled by a microprocessor and the reading is displayed on an optical readout.
The inclusion of a microprocessor enables any one of a large number of pre-tuned stations to be selected and displayed and the use of a remote control makes the receiver even more user friendly.