Introduction. Short History

Transcription

1 Introduction Dr. Šarūnas Paulikas Telekomunikacijų inžinerijos katedra Elektronikos fakultetas, VGTU Short History Samuel Morse developed the first electronic communications system in He called his invention the telegraph. In 1876, Alexander Graham Bell and Thomas A. Watson were the first to successfully transmit human conversation over a crude telephone system. Radio communications began in 1894 when Guglielmo Marconi transmitted the first wireless signals through Earth's atmosphere. Commercial radio began in 1920 when AM radio stations began broadcasting. In 1933, Major Edwin Howard Armstrong invented FM. Commercial broadcasting of FM began in ver Communication Engineering 2 1

2 Electronic Comm. System Electronic communications is the transmission, reception and processing of information between two or more locations using electronic circuits. The information can be: analog (continuous) digital (discrete) ver Communication Engineering 3 Electronic Comm. System All forms of information must be converted to electromagnetic energy before being propagated thorough an electronic communication system. ver Communication Engineering 4 2

3 Electronic Comm. System There are two basic types of electronic communications systems: analog digital An analog communications system is a system in which energy is transmitted and received in analog form (a continuously varying signal such as a sine wave). A digital communications system is a system in which energy is transmitted and received in digital form (discrete levels such as +5 V and ground). ver Communication Engineering 5 Simplified Comm. System ver Communication Engineering 6 3

4 Transmitter Is a collection of one or more electronic devices or circuits that converts the original information to a signal that is more suitable for transmission over a given transmission medium. ver Communication Engineering 7 Transmission Medium Provides a means of transporting signals from a transmitter to a receiver and can be as simple as a pair of copper wires that propagate signals in the form of electric current flow. ver Communication Engineering 8 4

5 Receiver Is a collection of electronic devices and circuits that accepts the transmitted signals from the transmission medium and converts them back to their original form. ver Communication Engineering 9 Modulation and Demodulation It is impractical to propagate information signals over metallic or fiber cables or through Earth's atmosphere: it is extremely difficult to radiate low-frequency signals through Earth's atmosphere in the form of electromagnetic energy. information signals often occupy the same frequency band and, if signals from two or more sources are transmitted at the same time, they would interfere with each other. ver Communication Engineering 10 5

6 Modulation and Demodulation It is necessary to encode the information onto a higher-frequency carrier signal. The information modulates the carrier by changing either its amplitude, frequency, or phase. () t = Asin( π ft +θ ) s 2 ver Communication Engineering 11 Modulation and Demodulation Modulation is simply the process of changing some property of the carrier in accordance with the information. Modulation is performed in a transmitter in a circuit called a modulator. A carrier that has been acted upon by an information signal is called a modulated wave or modulated signal. ver Communication Engineering 12 6

7 Modulation and Demodulation Demodulation is the reverse process of modulation and converts the modulated carrier back to the original information (that is, removing the information from the carrier). Demodulation is performed in a receiver in a circuit called a demodulator. ver Communication Engineering 13 Communication System ver Communication Engineering 14 7

8 Electromagnetic Spectrum The purpose of an electronic communications system is to communicate information between two or more locations commonly called stations. This is accomplished by converting the original information into electromagnetic energy and then transmitting it to one or more receive stations where it is converted back to its original form. ver Communication Engineering 15 Electromagnetic Spectrum Electromagnetic energy can propagate as a voltage or current along a metallic wire, as emitted radio waves through free space, or as light waves down an optical fiber. Electromagnetic energy is distributed throughout an almost infinite range of frequencies. ver Communication Engineering 16 8

9 Band Number Electromagnetic Spectrum Frequency Range 30 Hz- 300 Hz 0.3 khz-3 khz 3 khz-30 khz 30 khz-300 khz 0.3 MHz-3 MHz 3 MHz-30 MHz 30 MHz-300 MHz 0.3 GHz-3 GHz 3 GHz-30 GHz 30 GHz-300 GHz 0.3 THz-300 THz 0.3 PHz-3 PHz 3 PHz-30 PHz 30 PHz-300 PHz 0.3 EHz-3 EHz 3 EHz-30 EHz Designations ELF (extremely low frequencies) VF (voice frequencies) VLF (very low frequencies) LF (low frequencies) MF (medium frequencies) HF (high frequencies) VHF (very high frequencies) UHF (ultrahigh frequencies) SHF (superhigh frequencies) EHF (extremely high frequencies) Infrared light Visible light Ultraviolet light X-rays Gamma rays Cosmic rays ver Communication Engineering 17 Wavelength wavelength = λ = c f velocity frequency ver Communication Engineering 18 9

10 Bandwidth and Inform. Capacity The two most significant limitations on the performance of a communications system are noise and bandwidth. The bandwidth of an information signal is simply the difference between the highest and lowest frequencies contained in the information. The bandwidth of a communications channel is the difference between the highest and lowest frequencies that the channel will allow to pass through it (that is, its passband). ver Communication Engineering 19 Bandwidth and Inform. Capacity The information capacity of a communications system is a measure of how much information can be carried through a system in a given period of time. Hartley s law: I B t ver Communication Engineering 20 10

11 Transmission Modes Simplex (SX) With simplex operation, transmissions can occur only in one direction. Simplex systems are sometimes called one-way-only, receive-only, or transmit-only systems. A location may be a transmitter or a receiver, but not both. An example of simplex transmission is commercial radio or television broadcasting; the radio station always transmits and you always receive. ver Communication Engineering 21 Transmission Modes Half Duplex (HDX) With half-duplex operation, transmissions can occur in both directions, but not at the same time. Half-duplex systems are sometimes called two-wayalternate, either-way, or over-and-out systems. A location may be a transmitter and a receiver, but not both at the same time. Two-way radio systems that use push-to-talk (PTT) buttons to key their transmitters, such as citizensband and police-band radio, are examples of halfduplex transmission. ver Communication Engineering 22 11

12 Transmission Modes Full Duplex (FDX) With full-duplex operation, transmissions can occur in both directions at the same time. Full-duplex systems are sometimes called two-way simultaneous, duplex, or both-way lines. A location can transmit and receive simultaneously; however, the station it is transmitting to must also be the station it is receiving from. A standard telephone system is an example of fullduplex transmission. ver Communication Engineering 23 Transmission Modes Full/Full Duplex (F/FDX) With full/full duplex operation, it is possible to transmit and receive simultaneously, but not necessarily between the same two locations (that is, one station can transmit to a second station and receive from a third station at the same time). Full/full duplex transmissions are used almost exclusively with data communications circuits. The Postal Service is an example of full/full duplex operation. ver Communication Engineering 24 12

13 Noise Electrical noise may be defined as any undesired voltages or currents that ultimately end up appearing in the receiver output. Noise signals at their point of origin are generally very small - the microvolt level. ver Communication Engineering 25 Noise However, the desired signal received is of the same order of magnitude as the undesired noise signal. This situation is made even worse since the receiver itself introduces additional noise. ver Communication Engineering 26 13

14 Noise The noise present in a received radio signal has been introduced in the transmitting medium and is termed external noise. The noise introduced by the receiver is termed internal noise. ver Communication Engineering 27 External Noise MAN-MADE NOISE is produced by spark-producing mechanisms such as engine ignition systems, fluorescent lights, and commutators in electric motors. Man-made noise occurs randomly at frequencies up to around 500 MHz. Another common source of man-made noise is contained in the power lines that supply the energy for most electronic systems. ver Communication Engineering 28 14

15 External Noise ATMOSPHERIC NOISE Is caused by naturally occurring disturbances in the earth's atmosphere (lightning). The frequency content is spread over the entire radio spectrum, but its intensity is inversely related to frequency. Most trouble-some at the lower frequencies, but it is not a significant factor for frequencies exceeding about 20 MHz. ver Communication Engineering 29 External Noise SPACE NOISE External noise arrived from outer space. It is pretty evenly divided in origin between the sun and all the other stars. That originating from our star (the sun) is termed solar noise. Solar noise is cyclical and reaches very annoying peaks about every 11 years. Space noise occurs at frequencies from about 8 MHz up to 1.5 GHz ver Communication Engineering 30 15

16 Internal Noise Internal noise is introduced by the receiver itself. The receiver's major noise contribution occurs in its very first stage of amplification, thus, the first receiver stage must be very carefully designed to have low noise characteristics. It is there that the desired signal is at its lowest level, and noise injected at that point will be at its largest value in proportion to the intelligence signal. ver Communication Engineering 31 Internal Noise Effect ver Communication Engineering 32 16

17 Internal Noise There are two basic types of noise generated by electronic circuits: THERMAL NOISE TRANSISTOR NOISE Additional: EXCESS NOISE TRANSIT-TIME NOISE ver Communication Engineering 33 Thermal Noise Is due to thermal interaction between the free electrons and vibrating ions in a conductor. It causes the rate of arrival of electrons at either end of a resistor to vary randomly, and thereby varies the resistor's potential difference. Resistors and the resistance within all electronic devices are constantly producing a noise voltage. ver Communication Engineering 34 17

18 Thermal Noise Since it is dependent on temperature, it is also referred to as thermal noise. Its frequency content is spread equally throughout the usable spectrum. The terms Johnson, thermal, and white noise may be used interchangeably. ver Communication Engineering 35 Thermal Noise Power of this generated noise is given by P n = ktb k Boltzmann's constant ( T resistor temperature(k) B frequency bandwidth of the system J/K) ver Communication Engineering 36 18

19 Thermal Noise Since this noise power is directly proportional to the bandwidth involved, it is advisable to limit a receiver to the smallest bandwidth possible. The noise is an ac voltage that has random instantaneous amplitude but a predictable rms value: e n = 4kTBR Thus, dissimilar resistors of equal value exhibit different noise levels. ver Communication Engineering 37 Transistor Noise The noise introduced by the transistor, other than its thermal noise. The major contributor of transistor noise is called shot noise. It is due to the discrete-particle nature of the current carriers in all forms of semiconductors. ver Communication Engineering 38 19

20 Transistor Noise These current carriers, even under dc conditions, are not moving in an exactly steady continuous flow since the distance they travel varies due to random paths of motion. The name shot noise is derived from the fact that when amplified into a speaker, it sounds like a shower of lead shot falling on a metallic surface. Shot noise and thermal noise are additive. ver Communication Engineering 39 Transistor Noise The equation for shot noise in a diode is i n = 2qI dc B Unfortunately, there is no valid formula to calculate its value for a complete transistor where the sources of shot noise are the currents within the emitter-base and collector-base diodes. Hence, the device user must refer to the manufacturer's data sheet for an indication of shot noise characteristics. ver Communication Engineering 40 20

21 Excess Noise Two little understood forms of device noise occur at the opposite extremes of frequencies. The low-frequency effect is called excess noise. At frequencies below about 1 KHz. Excess noise is referred as flicker noise, pink noise or 1/f noise. It is present in both bipolar junction transistors (BJT) and field-effect transistors (FET). ver Communication Engineering 41 Transit-time Noise At the vicinity of device high-frequency cut-off, device noise starts to increase rapidly. When the transit time of carriers crossing a junction is comparable to the signal's period, some of the carriers may diffuse back to the source or emitter. This effect is termed transit-time noise. ver Communication Engineering 42 21

22 Overall Noise These high- and low-frequency effects are relatively unimportant in the design of receivers since the critical stages (the front end) will usually be working well above 1 khz and hopefully below the device's high-frequency cutoff area. The low-frequency effects are, however, important to the design of low-level, lowfrequency amplifiers encountered in certain instrument and biomedical applications. ver Communication Engineering 43 Overall Noise The overall noise intensity versus frequency curves for semiconductor devices ver Communication Engineering 44 22

23 Signal-to-Noise Ratio The signal-to-noise ratio (S/N ratio) is a relative measure of the desired signal power to the noise power: s N =10log 10 P P S N ver Communication Engineering 45 Noise Figure S/N very well identifies the noise content at a specific point but is not useful in relating how much additional noise a particular transistor has injected into a signal going from input to output. The term noise figure (NF) is usually used to specify exactly how noisy a device is. It is defined as follows: Sin Nin NF = 10log10 = 10log10 NR Sout Nout NR noise ratio ver Communication Engineering 46 23

24 Cascade Amplifiers Noise Friiss' formula is used to provide the overall noise effect of a multistage system: NR 2 1 NR = NR1 + + L+ P P G power gain. G 1 P G 1 NR N 1 P L P G 2 G N 1 ver Communication Engineering 47 Equivalent Noise Temperature Another way of representing noise is by equivalent noise temperature. T = T0( NR 1), wheret0 eq = 290 K It is used in noise calculations involved with microwave receivers (>1 GHz) and their associated antenna system. It allows easy calculation of noise power at the receiver while the equivalent noise temperature (T eq ) of microwave antennas and their coupling networks are then simply additive. ver Communication Engineering 48 24