Microwave-System-Module-2.pdf

Microwave System Engr. Harry Bert Rolle Module 2 Learning outcomes: Discuss microwave communication system. Discuss the different components of microwave system. Microwave Microwaves are generally described as electromagnetic waves with frequencies that range from approximat...

Microwave System Engr. Harry Bert Rolle Module 2 Learning outcomes: Discuss microwave communication system. Discuss the different components of microwave system. Microwave Microwaves are generally described as electromagnetic waves with frequencies that range from approximately 500 MHz to 300 GHz. The wavelengths for microwave frequencies fall between 1 cm and 60 cm, slightly longer than infrared energy. To calculate the wavelength given the frequency, you can use the formula: λ=c/f Where: - λ is the wavelength (in meters, m) - c is the speed of light (approximately 3×1083×108 m/s) - f is the frequency (in Hertz, Hz) Microwave Radio Frequency Assignments Advance Electronic Communication System by Wayne Tomasi (p530) Microwave For full-duplex (two-way) operation as is generally required of microwave communications systems, each frequency band is divided in half with the lower half identified as the low band and the upper half as the high band. The vast majority of electronic communications systems established since the mid-1980s have been digital in nature and, thus, carry voice, video, and data information in digital form. Terrestrial (earth-based) microwave radio relay systems using frequency (FM) or digitally modulated carriers (PSK or QAM) still provide approximately 35% of the total information-carrying circuit mileage in the United States. Microwave Communication System Operating distance that vary from 15 miles to 4000 miles in length. Type of Microwave operating system: Interstate or feeder service (short haul) Interstate and backbone (long haul) System capacities range from less than 12 voice-band channels to more than 22,000 channels. Modulation Technique used in microwave communication system: Frequency modulation - Frequency division multiplexing Pulse code modulation – time division multiplexing Digital modulation techniques – PSK and QAM Microwave Communication System Microwave radios propagate signals through Earth’s atmosphere between transmitters and receivers often located on top of towers spaced about 15 miles to 30 miles apart. Microwave Radio Communication Advance Electronic Communication System by Wayne Tomasi (p531) Microwave Radio Communication Advance Electronic Communication System by Wayne Tomasi (p531) Advantage of Microwave Radio Radio systems do not require a right-of-way acquisition between stations. Each station requires the purchase or lease of only a small area of land. Because of their high operating frequencies, microwave radio systems can carry large quantities of information. High frequencies mean short wavelengths, which require relatively small antennas. Radio signals are more easily propagated around physical obstacles such as water and high mountains. Fewer repeaters are necessary for amplification. Distances between switching centers are less. Underground facilities are minimized. Minimum delay times are introduced. Minimal crosstalk exists between voice channels. Increased reliability and less maintenance are important factors. Disadvantage of Microwave Radio It is more difficult to analyze and design circuits at microwave frequencies. Measuring techniques are more difficult to perfect and implement at microwave frequencies. It is difficult to implement conventional circuit components (resistors, capacitors, inductors, and so on) at microwave frequencies. Transient time is more critical at microwave frequencies. It is often necessary to use specialized components for microwave frequencies. Microwave frequencies propagate in a straight line, which limits their use to line-of-sight applications. Microwave Radio Communication Frequency modulation (FM) is used in microwave radio systems, FM signals are relatively insensitive to this type of nonlinear distortion and can be transmitted through amplifiers that have compression or amplitude nonlinearity with little penalty. FM signals are less sensitive to random noise and can be propagated with lower transmit powers FM microwave systems are also easily expandable. Simplified block diagram of microwave Tx Microwave Radio Communication The baseband is the composite signal that modulates the FM carrier and may comprise one or more of the following: 1. Frequency-division-multiplexed voice-band channels 2. Time-division-multiplexed voice-band channels 3. Broadcast-quality composite video or picturephone 4. Wideband data FM Microwave Radio Transmitter The pre-emphasis network provides an artificial boost in amplitude to the higher baseband frequencies. An FM deviator provides the modulation of the IF carrier that eventually becomes the main microwave carrier. Typically, IF carrier frequencies are between 60 MHz and 80 MHz. IF and its associated sidebands are up-converted to the microwave region by the mixer, microwave oscillator, and bandpass filter. Mixing, rather than multiplying, is used to translate the IF frequencies to RF frequencies because the modulation index is unchanged by the heterodyning process. Microwave generators consist of a crystal oscillator followed by a series of frequency multipliers. Simplified block diagram of microwave Rx FM Microwave Radio Receiver The channel separation network provides the isolation and filtering necessary to separate individual microwave channels and direct them to their respective receivers. The bandpass filter, AM mixer, and microwave oscillator down- convert the RF microwave frequencies to IF frequencies. The FM demodulator is a conventional, noncoherent FM detector (i.e., a discriminator or a PLL demodulator). At the output of the FM detector, a deemphasis network restores the baseband signal to its original amplitude-versus-frequency characteristics. Microwave Repeater A microwave repeater is a receiver and a transmitter placed back to back or in tandem with the system. The repeater station receives a signal, amplifies and reshapes it, and then retransmits the signal to the next repeater or terminal station down line from it Path The location of intermediate repeater sites is greatly influenced by the nature of the terrain between and surrounding the sites. Preliminary route planning generally assumes relatively flat areas, and path (hop) lengths will average between 25 miles and 35 miles between stations. Transmitter output power and antenna gain will similarly enter into the selection process. The exact distance is determined primarily by line-of-site path clearance and received signal strength. For frequencies above 10 GHz, local rainfall patterns could also have a large bearing on path length. Microwave Repeater Types of repeater IF – heterodyne repeater Baseband repeater RF-to-RF repeater. the received RF carrier is down-converted to an IF frequency, amplified, reshaped, upconverted to an RF frequency, and then retransmitted. the received RF carrier is down-converted to an IF frequency, amplified, filtered, and then further demodulated to baseband. The baseband signal, which is typically frequency-division-multiplexed voice-band channels, is further demodulated to a mastergroup, supergroup, group, or even channel level. IF Repeater the received RF carrier is down-converted to an IF frequency, amplified, reshaped, upconverted to an RF frequency, and then retransmitted. Baseband repeater the received RF carrier is down-converted to an IF frequency, amplified, filtered, and then further demodulated to baseband. The baseband signal, which is typically frequency-division-multiplexed voice-band channels, is further demodulated to a mastergroup, supergroup, group, or even channel level. Baseband repeater RF-to-RF repeater. the received microwave signal is not down-converted to IF or baseband; it is simply mixed (heterodyned) with a local oscillator frequency in a nonlinear mixer. The output of the mixer is tuned to either the sum or the difference between the incoming RF and the local oscillator frequency, depending on whether frequency up- or down-conversion is desired. RF to RF Repeater