CHAPTER 14
Modern Communications Applications
1. Facsimile or fax is an electronic communication technique for transmitting printed documents including text, line drawings, photos, and other graphical information via telephone lines or radio.
2. Fax machines scan the document to be transmitted using photo-optical techniques to create a baseband signal that modulates a carrier prior to transmission.
3. The receiving fax machine demodulates the carrier to recover the baseband signal which is sent to a printer where the original document is faithfully reproduced.
4. The quality of reproduction is a function of the scanning resolution in lines per inch. The greater the number of scan lines, the finer the definition.
5. The most widely used device to convert the scanned lines into an electrical signal is a light-sensitive semiconductor component known as a charge coupled device (CCD).
6. The CCD contains thousands of tiny capacitors that charge to a value proportional to the light intensity. The capacitors are sequentially sampled. Their charges are read out, creating an analog signal corresponding to the lines scanned.
7. The baseband signal modulates a carrier prior to transmission; AM, FM, and PSK are commonly used.
8. Facsimile standards for modulation, transmission speed, and other factors are set by the International Telegraph and Telephone Consultative Committee (CCITT). There are four basic standards designated group 1 through 4.
9. Group I fax machines are analog and use FM where black is 1500 Hz (or 1300 Hz) and, white is 2300 Hz (or 2100 Hz). Resolution is 96 lines per inch, and transmission, speed is 6 min per page. Group 1 machines are no longer used.
10. Group 2 machines are analog and use FM or vestigial sideband AM with a 21O0-Hz carrier. Resolution is 96 lines per inch, and transmission speed is 3 min per page or less.
11. Group 3 machines use digital techniques and PSK or QAM to achieve speeds of up to 9600 baud over the telephone lines. Resolution is 200 lines per inch, and transmission times are less than I min, with less than 30s being common. Most modern fax machines are of the group 3 type.
12. Group 4 fax machines are digital and are designed for wideband telephone lines. Digital transmission rates are 56 kbits/s. Resolution is 400 lines per inch. Few group 4 machines are in use yet.
13. Most fax printers are of the thermal type and use special heat-sensitive paper, although laser printers using xerography are used in some higher-priced machines.
14. Fax is widely used by newspapers, business, and the military. Radio fax is used for transmitting weather satellite photos to earth.
15. Cellular radios provide telephone service in vehicles and are used as portable telephone units.
16. The area served by cellular telephones is divided into small zones called cells.
17. Each cell is served by a repeater containing a low-power transmitter and a receiver.
18. The cells are connected by wire to a computer-controlled master station called the mobile telephone, switching office (MTSO). The MTSO links to the telephone system.
19. By operating at high frequencies in the spectrum and through the application of frequency reuse, many channels are available to users.
20. There are 666 full-duplex telephone channels in most service areas.
21. The receive frequencies are in the 870-to 890-MHz range.
22. The transmit frequencies are in the 825-to 845-MHz range.
23. Channel spacing is 30 kHz.
24. Spacing between the simultaneously used transmit and receive frequencies is 45 MHz.
25. Cellular radios use' FM with a maximum deviation, of 612 kHz.
26. Mobile cellular transmitters have a maximum output power of 3 W which can be decreased in steps under the control of the MTSO to minimize adjacent cell interference.
27. Mobile cellular receivers are of the dual conversion type with a 45- or 82.2-MHz first IF and a 10.7-MHz or 455 kHz second IF.
28. Both transmit and receive frequencies are determined by PLL frequency synthesizers that are set by the MTSO to a clear channel.
29. The MTSO monitors received cell signal strength [received signal strength indicator (RSSI) and transmitter power output and makes decisions about when to "hand off" the mobile unit to another cell to maintain optimum signal strength.
30. The MTSO controls transmitter power as well as transmit and receive frequencies. Serial digital data containing this information is transmitted to the mobile unit whose logic section interprets it and effects the changes.
31. Each cellular radio contains a PROM called the number assignment module (NAM) which stores the unit's telephone number referred to as the mobile identification number (MIN).
32. Most cellular radios contain two microprocessors, one for controlling the logic section and another for operating the displays and dialing circuits in the handset and control section.
33. Radar b the acronym for radio detection and ranging.
34. Radar uses a reflected radio signal from a target to determine its distance, azimuth, elevation, and speed.
35. Radio waves travel at a speed of 186,000, mi/s or 162,000 nmi/s. Knowing the speed of radio waves permits the distance (range) of a remote target to be determined.
36. Radio signals travel at a speed of 5.375mS/nmi or 6.18 mS/nmi.
37. The distance D in nautical miles to a target can be computed by knowing the total delay time T in microseconds for a signal to reach the target and return. (D = T/12.36).
38. The strength of the reflection is a function of the wavelength of the radar signal and its relationship to the size of the target. Optimum reflection is obtained when the size of the target is one-quarter wavelength or larger of the signal frequency. Most radars operate in the microwave part of the spectrum.
39. The bearing or azimuth of a target with respect to the radar set is determined by a highly directional antenna. A narrow beam-width antenna is rotated continuously over 360° and the detection of a reflection from a target at a given bearing gives the target's direction.
40. There are two basic types of radar: pulse and continuous wave (CW).
41. In pulse radar, the transmitter emits microwave energy in the form of repetitive sine wave bursts or pulses.
42. The pulse repetition time (PRT) is the time interval between the beginning of successive pulses.
43. The pulse repetition frequency (PRF) or the pulse repetition rate in pulses per second is the reciprocal of PRT (PRF = 1/PRT).
44. The ratio of the pulse burst duration to the PRT is known as the duty cycle and is usually expressed as a percentage.
45. The PRT and duty cycle set the range of a radar: a short PRT and pulse width for short range, and a long PRT and pulse width for long range.
46. The reflection from the target occurs in the interval between successively radiated pulses.
47. In CW radar, constant-amplitude-constant frequency signal is continuously radiated. If the target is stationary, the reflected signal contains no distance information.
48. Continuous wave radar relies on the Doppler effect to produce frequency modulation of the carrier.
49. The Doppler effect is the change in frequency that occurs as the result of relative motion between the transmitter and a target. The Doppler effect occurs with sound, radio, and light signals.
50. The relative speed between transmitter and target is directly proportional to the amount of Doppler frequency shift.
51. The CW Doppler radar is used for speed measurement. Police radars are an example.
52. By frequency modulating a CW radar with a sawtooth or triangular wave, the frequency difference between the transmitted and received signals can be used to compute the distance or range to a target.
53. Radar sets consist of a transmitter, a receiver, an antenna, a master timing section, and a display.
54. Most radar transmitters use a magnetron oscillator, although some high-power radars use klystrons or TWTs. Low-power radars use a Gunn diode oscillator.
55. Radar receivers are super heterodynes with diode mixers.
56. The radar antenna is usually a horn with a parabolic reflector that rotates over a 3608 angle.
57. A duplexer is a waveguide assembly that allows both transmitter and receiver to share the same antenna.
58. Spark-gap tubes called transmit-receive (TR) and anti-transmit-receive(ATR) tubes prevent high-power energy from the transmitter from getting into and damaging or desensitizing the receiver.
59. The radar display is normally a cathode ray tube (CRT) that is calibrated to read out the range, bearing, and other data.
60. The most common CRT readout is the plan position indicator (PPI) where the radar is at the center and a radius rotates to reveal reflected targets as blips.
61. Phased array radars use a matrix of dipoles or slot antennas with variable phase shifters to permit automatic, high-speed, electronic beam switching; beam width changes; and sweeping or scanning.
62. Television is the radio transmission of sound and pictures in the VHF and UHF ranges. The voice signal from a microphone frequency modulates a sound transmitter. A camera converts a picture or scene into an electrical signal called the video or luminance Y signal, which amplitude modulates a separate video transmitter. Vestigial sideband AM is used to conserve spectrum space. The picture and sound transmitter frequencies are spaced 4.5 MHz apart, with the sound frequency being the higher.
63. TV cameras use either a vacuum tube imaging device such as a vidicon or a solid-state, imaging device such as the charge coupled device (CCD) to convert scene into a video signal.
64. A scene is scanned by the imaging device to break it up into segments that can be transmitted serially. The National Television, Standards Committee (NTSC) standards call for scanning the scene in two 262 ½ line fields which are interlaced to form a single 525-line picture called a frame. Interlaced scanning reduces flicker. The field rate is 59.94 Hz, and the frame or picture rate is 29.97 Hz. The horizontal line scan rate is 15,734 Hz or 63.6 mS per line.
65. The color in a scene is captured by three imaging devices which break a picture down into its three basic colors of red, green, blue using color light filters. Three color signals are developed (R, G, B). These are combined in a resistive matrix to form the Y signal and are combined in other ways to fonti the I and Q signals. The I and Q signals amplitude modulate 3.58 MHz subcarriers shifted 908 from one another in balanced modulators producing quadrature DSB suppressed signals that are added to form a carrier composite color signal. This color signal is then used to modulate the AM picture transmitter along with the Y signal.
66. A TV receiver is a standard superheterodyne receiver with separate sections for processing and recovering the sound and picture. The tuner section consists of RF amplifiers, mixers, and a frequency synthesized local oscillator for channel selection. Digital infrared remote control is used to change channels in the synthesizer via a control microprocessor.
67. The tuner converts the TV signals to intermediate frequencies of 41.25 MHz for the sound and 45.75 MHz for the picture. These signals are amplified in IC IF amplifiers. Selectivity is usually provided by a surface acoustic wave (SAW) filter. The sound and picture IF signals are placed in a sound detector to form a 4.5 MHz sound IF signal. This is demodulated by a quadrature detector or other FM demodulator to recover the sound. .Frequency multiplexing techniques similar to those used in FM radio are used for stereo TV sound. The picture IF is demodulated by a diode detector or other AM demodulator to recover the Y signal.
68. The color signals are demodulated by two balanced modulators fed with 3.58-MHz subcarriers in quadrature. The subcarrier is frequency- and phase-locked to the subcarrier in the transmitter by phase-locking to the color subcarrier burst transmitted on the horizontal blanking pulse.
69. To keep the receiver in step with the scanning process at the transmitter, sync pulses are transmitted along with the scanned lines of video. These sync pulses are stripped off the video detector and used to synchronize horizontal and vertical oscillators in the receiver. These oscillators generate deflection currents that sweep the electron beam in the picture tube to reproduce the picture.
70. The color picture tube contains three electron guns that generate narrow electron beams aimed at the phosphor coating on the inside of the face of the picture tube. The phosphor is arranged in millions of tiny red, green, and blue color dot triads or stripes. The electron beams excite the color dots or stripes in proportion to their intensity and generate light of any color depending upon the amplitude of the red, green, and blue signals. The electron beam is scanned or deflected horizontally and vertically in step with the transmitted video signals. Deflection signals from the internal sweep circuits drive coils in a deflection yoke around the neck of the picture creating magnetic fields that sweep the three electron beams.
71. The horizontal output stage, which provides horizontal sweep, is also used to operate a flyback transformer that steps up the horizontal sync pulses to a very high voltage. These are rectified and filtered into a 30- to 35-kV voltage to operate the picture tube. The flyback also steps down the horizontal pulses and rectifies and filters them into low-voltage dc supplies that are used to operate most of the circuits in the TV set.
72. Cable television is the transmission of multiple TV signals which are frequency multiplexed on a single cable to be used in lieu of over-the-air signal transmission. The headend of a cable TV station collects TV signals from local stations and from other sources by satellite and then modulates and multiplexes these signals on a cable that is sent to subscribers.
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