Archive for September, 2009
September 30th, 2009 | 
Author: Amplitude Modulation:
Amplitude modulation is the process of changing the amplitude of a relatively high frequency carrier signal in proportion with the instantaneous value of the modulating signal. It is a relatively inexpensive, low-quality form of modulation that is used the commercial broadcasting of both audio and video signals. Amplitude modulation is used two-way mobile radio communications, such as citizens band radio. Am modulators are nonlinear devices with two inputs and one output.
One input is a single, high-frequency carrier signal of constant amplitude and the second input is comprised of relatively low-frequency information signals that may be a single frequency or a complex waveform made up of many frequencies. The information signal may be a single frequency or more likely consist of a range of frequencies. For example, typical voice-grade communications systems utilize a range of information frequencies between 300 HZ and 3000 HZ. The modulated output waveform from an AM modulator is often called an AM envelope.
AM Low-level Transmitters:
For voice or music transmission, the source of the modulating signal is generally am acoustical transducer, such as a microphone, a magnetic tape, CD, or a phonograph record. The preamplifier is typically a sensitive, class a linear voltage amplifier with high impedance. The function of the preamplifier is to raise the amplitude of the source signal to a usable level while producing minimum nonlinear distortion and adding as little thermal noise a possible. The driver for the modulating signal is also a linear amplifier that simply amplifies the information signal to an adequate level to sufficiency drive modulator. It is used predominantly for low-power, low-capacity systems, such as wireless intercoms, remote- control units, pagers, and short-range walkie-talkies.
AM High-level Transmitters:
The modulating signal is processed in the same manner as in the low-level transmitter except for the addition of a power amplifier. With high-level transmitters, the power of the modulating signal must be considerably higher with low-level transmitters. This is because the carrier is at full power at the point in the transmitter where modulation occurs and, consequently, requires a high-amplitude modulating signal to produce 100% modulation. The RF carrier oscillator, its associated buffer, and the carrier driver are also essentially the same circuits used in low-level transmitters.
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Atmospheric Noise and Extraterrestrial Noise:
Atmospheric noise is naturally occurring electrical disturbances that originate within earth’s atmosphere. Atmosphere noise is commonly called static electricity and is the familiar sputtering, crackling, and so on often heard from a speaker when there is no signal present. The source of most static electricity is naturally occurring electrical conditions, such as lightning. Static electricity is often in the form of impulses that spread energy throughout a wide range of frequencies. The magnitude of this energy, however, is inversely proportional to its frequency. Consequently, at frequencies above 30MHZ or so, atmosphere noise is relatively insignificant. Extraterrestrial noise consists of signals that originate from outside earth’s atmosphere and is therefore sometimes called deep-space noise. Extraterrestrial noise originates from the Milky Way, other galaxies, and the sun.
Cosmic Noise and Man-made Noise:
Cosmic noise sources are continuously distributed throughout the galaxies. Because the sources of galactic noise are located much farther away than our sun, their noise intensity is relatively small. Cosmic noise is often called black- body noise and is distributed fairy evenly throughout the sky. Man-made noise is simply noise that is produced by mankind. The predominant sources of man-made noise are spark-producing mechanisms, such as commutators in electric motors, automobile ignition systems, c power- generating and switching equipments, and fluorescent lights. Man-made noise is impulsive in nature and contains a wide range of frequencies that are propagated through space in the same manner as radio waves. Man-made noise is most intensive in the more densely populated metropolitan and industrial areas and is therefore sometimes called industrial noise.
Shot Noise and Transit-time Noise:
Shot noise is caused by the random arrival of carriers at the output element of an electronic device, such as a diode, field-effect transistor, or bipolar transistor. Shot noise was first observed in the anode current of a vacuum-tube amplifier and was described mathematically in 1918. The current carriers are not moving in a continuous, steady flow, as the distance they travel varies because of their random paths of motion. Shot noise is randomly varying and is superimposed onto any signal present. When amplified shot noise sounds similar to metal pellets falling on a tin roof Shot noise is sometimes called transistor noise and is additive with thermal noise. Any modification to a stream of carriers as they pass from the input of a device produces an irregular, random variation categorized as transit-time noise. When the time it takes for a carrier to propagate through a device is an appreciable part of the time of one cycle of the signal, the noise becomes noticeable. Transit-time noise in transistors is determined by carrier mobility, bias voltage, and transistor construction.
Thermal Noise:
Thermal noise is associated with the rapid and random movement of electrons within a counter due to thermal agitation. Thermal noise is present in all electronic components and communications systems. Because thermal noise is uniformly distributed across the entire electromagnetic frequency spectrum, it is often referred to as white noise. Thermal noise is a form of additive noise, meaning that it cannot be eliminated, and it increases in intensity with the number of devices in a circuit and with circuit length. Therefore, thermal noise sets the upper bound on the performance of a communications system. The ac component produced from thermal agitation has several names, including thermal noise, because it is temperature dependent; Brownian noise in its frequencies. Hence, thermal noise is the noise is the random motion of free electrons within a conductor caused by thermal agitation.
Because it is often impractical to propagate information signals over standard transmission media, it is often necessary to modulate the source information onto a higher- frequency analog signal called a carrier. In essence, the carrier signal carries the information through the system. The information signal modulates the carrier by changing either its amplitude, frequency, or phase. Modulation is simply the process of changing one or more properties of the analog carrier in proportion with the information signal. The two basic types of electronic communications systems are analog and digital. An analog communications system is a system in which energy is transmitted and received in analog form. With analog communications systems, both the information and the carrier are analog signals.
The term digital communications, however, covers a broad range of communications techniques, including digital transmissions and digital radio. Digital transmission is a true digital system where digital pulses are transferred between two or more points in a communications system. With digital transmission, there is no analog carrier, and the original source information may be in digital or analog form. If it is in analog form, it must be converted to digital pulses prior to transmission and converted back to analog form at the receive end. Digital transmission systems require a physical facility between the transmitter and receiver, such as a metallic wire or an optical fiber cable. Equation 1-8 is the general expression for a time- varying sine wave of voltage such as a high- frequency carrier signal.
Modulation is performed in a transmitter by circuit called a modulator. A carrier that has been acted on by an information signal is called a modulated wave or modulated signal. Demodulations the reverse process of modulation and converts the modulated carrier back the original information. Demodulation is performed in a receiver by a circuit called a demodulator. It is extremely difficult to radiate low- frequency signals from an antenna in the form of electromagnetic energy, and information signals often occupy the same frequency band, if signals from two or more sources are transmitted at the same time, they would interfere with each other.
For example, all commercial FM stations broadcast voce and music signals that occupy the audio-frequency band from approximately 300 Hz to 15 KHZ. To avoid interfering with each other, each station converts its information to a different frequency band of frequencies allocated a particular service. A standard voice- band channel occupies approximately a 3-KHZ band width and is used for transmission of voice-quality signals; commercial AM broadcast channels occupy approximately a 10-KHZ frequency band, and 30 MHZ or more of bandwidth is for microwave and satellite radio channels.
FDM/FM system using a single satellite transponder .with earth coverage antennas and for full full-duplex operation, each link required two RF satellite channels. To avoid interfering with earth station 1 earth station 2 must transmit and receive different RF carrier frequencies. The RF carrier frequencies are fixed and the satellite transponder is simply an RF to RF repeater provides the uplink/downlink frequencies translation. This arrangement is economically impractical and also extremely inefficient. The RF satellite channels are fixed between any two earth stations thus the voice-band channels from each station are committed to a single destination.
In systems where three or more earth stations wish to communicate with each other, fixed frequency or dedicated-channel systems such as those are inadequate a method pf multiple accessing is required .that is each earth station using the satellite has a mean of communicating with each of the other earth stations in the systems through a common satellite transponder .multiple accessing is sometimes called multiple destination. Because the like RS449 standard the RS 530 interface standard does not specify electrical specifications for the RS 530 are outlined by either the RS442A standard.
The principal use of the RS336 is for dial backup of private up data circuits and for automatic dialing of remote terminals. Satellite multiple accessing implies that more than that one user has access to one or more radio channels within satellite communication channel. Transponders are typically leased by a company or o common carrier for the purpose of providing voice or data transmissions to a multiple of users. The method by which a satellite transponder bandwidth is used or accessed depends on the multiple-accessing method utilized.
The three most commonly used multiple accessing arrangements frequency –division multiple accessing (FDMA), time – division multiple accessing (TDMA), and code –division multiple accessing (CDMA). With FDMA, each earth station transmissions are assigned uplink and down link frequency bands within an allotted satellite bandwidth they may be reassigned or demand assigned. Consequently, FDMA transmissions are separated in the frequency domain and therefore must share the total available transponder bandwidth as well as the total transponder power. Signal separation is accomplished with envelope encryption/decryption techniques.
