Tuning a Radio Wave Receiver
Variable capacitors are used in conjunction with inductor coils in tuning circuits of radios, television sets, and a number of other devices that must isolate electromagnetic radiation of selected frequencies in the radio wave region. This interactive tutorial explores how a variable capacitor is coupled to a simple antenna transformer circuit to tune a radiofrequency spectrum.
The tutorial initializes with the variable capacitor set to a value of 50 picofarads, which enables the tuner to receive radio frequencies of 107.0 megahertz. In order to operate the tutorial, use the mouse cursor to translate the Capacitance slider to the right, between a range of 50 and 450 picofarads, corresponding to a radiofrequency range of 87 to 107 megahertz (the FM band). As the slider is translated, the variable capacitor plates rotate to simulate increased or decreased overlap, and the sine wave on the virtual oscilloscope changes wavelength.
Transmitted radio waves produce an induced current to flow in the antenna through the primary inductor coil of the transformer, directly to ground (the negative pole). A secondary current in the opposite direction is simultaneously induced in the secondary inductor coil of the transformer, sending an electron flow to the capacitor. The induced current flow in the secondary coil, and to the capacitor, induce counter electromotive forces termed reactance. The variable capacitor is used to equalize the inductive and capacitive reactance.
The condition in which the reactances are equalized is termed resonance, and the particular frequency that is isolated by the equalized reactance is called the resonant frequency. Therefore, the radio circuit in the tutorial is tuned by adjusting the capacitance of the variable capacitor to equalize the inductive and capacitive reactance for the desired resonant frequency, or in other words, to tune in the desired radio frequency.
The expansive radiofrequency portion of the electromagnetic spectrum includes wavelengths from about 30 centimeters to thousands of kilometers. Radiation in this range contains very little energy, and the upper frequency limit (about 1 gigahertz) occurs at the end of the band where radio and television broadcasting is confined. At such low frequencies, the photon (granular) character of the radiation is not apparent, and the waves appear to transfer energy in a smooth, continuous fashion. There is no theoretical upper limit to the wavelength of radiofrequency radiation. The low-frequency (60 hertz) alternating current carried by power lines, as an example, has a wavelength of about five million meters (or approximately 3,000 miles). Radio waves used for communication are modulated in one of two transmission specifications: amplitude modulated (AM) waves that vary in the amplitude of the wavelengths, and frequency modulated (FM; see Figure 8) waves that vary in wavelength frequency. Radio waves play important roles in industry, communications, medicine, and magnetic resonance imaging (MRI).
The sound and video portion of television is transported through the atmosphere by shorter radio waves having wavelengths less than a meter, which are modulated for broadcast much like FM radio. Radio waves are also produced by stars in distant galaxies, and can be detected by astronomers using specialized radiotelescopes. Long waves, several million miles in length, have been detected radiating toward the Earth from deep in space. Because the signals are so weak, radiotelescopes are often banded together in parallel arrays containing large numbers of enormous antenna-based receivers.
Matthew J. Parry-Hill and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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