Explain Transmitters and Receivers

Your radio is able to give you the news or play music because it can receive signals that are transmitted by radio stations. Radio waves are electromagnetic waves of higher frequency than those that occur in the audio range. The waves will radiate over significant distances. The frequencies that are suitable for radio transmission are called radio frequencies (RF). These radio frequencies are used as the carriers for the audio frequencies that make up speech or music and video frequencies (VF). The longitudinal waves of sound are converted to electrical currents of the same frequencies with a microphone. The audio frequencies can be amplified using transistors. These voltages are then impressed on the RF carrier; the AF is said to modulate the carrier frequency.

There are two different ways to modulate the carrier with the audio frequency voltages. One is amplitude modulation (AM): the amplitude of the carrier is changed at the frequency of the AF. Commercial AM broadcasting uses this method with carrier frequencies from 550 to 1650 kilohertz (kHz).

 
The other method is frequency modulation (FM) where the frequency of the carrier is changed at the frequency of the AF. The greater the amplitude of the AF, the greater the change in the frequency of the carrier. Commercial FM broadcasting use carrier frequencies of 88 to 108 megahertz (MHz). An FM system can be designed to be relatively static-free as compared with an AM system.

An antenna at the radio station is used to radiate electromagnetic waves whose frequency and variation are the same as those of the modulated RF described. These waves travel at the speed of light, 3 x 108 m/s. The length of the antenna is often made about one-half that of one wavelength.

At the receiver, currents are induced in the antenna at the same frequency as the carrier. When you tune a receiver, you usually vary a capacitor until you establish resonance for the desired carrier frequency. In the detector, you use a diode to rectify the modulated RF and a filter circuit to recover the AF used to modulate the RF. The detector (your radio) demodulates. The recovered AF currents are used to operate a reproducer which converts the electrical currents back to longitudinal sound waves. Speakers or earphones can be the reproducers.

Television transmissions have two signals: the sound and the picture. FM is used for sound transmission and AM is used for picture transmission. At the TV station, the camera or video player is used to translate information about the picture into electrical signals whose range is up to about 4 MHz (video frequencies).

Video cameras, television sets, and computer monitors use the photoelectric effect. They produce electrons by shining light on some metals at room temperature to create the electron beam whose intensity (and the color of the screen pixel from white to shades of gray to black) is controlled by the charging of the grid in front of the cathode. The electron beam is then scanned to draw 525 horizontal lines every 1/30th of a second by the crossed electric fields described in Chapter 12. As a result, thirty complete pictures are drawn on the screen every second. Color television is done on a similar basis in theory, but each pixel must have a red, green, and blue component which add up to the desired color.

High Definition Television (HDTV) which is now being developed would more than double the number of lines in the current standard, producing images of nearly photographic quality.