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Questions and Answers
What is the frequency range typically associated with microwaves?
What is the frequency range typically associated with microwaves?
Which formula is used to calculate the wavelength of microwaves based on frequency?
Which formula is used to calculate the wavelength of microwaves based on frequency?
Which of the following modulation techniques is NOT commonly used in microwave communication systems?
Which of the following modulation techniques is NOT commonly used in microwave communication systems?
What operating distance typically characterizes microwave radios in communication systems?
What operating distance typically characterizes microwave radios in communication systems?
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Which of the following statement type stands true about the channels in microwave systems?
Which of the following statement type stands true about the channels in microwave systems?
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What kind of systems provide a significant portion of information-carrying circuit mileage in the United States?
What kind of systems provide a significant portion of information-carrying circuit mileage in the United States?
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Which division identifies the lower and upper halves of frequency bands in full-duplex microwave systems?
Which division identifies the lower and upper halves of frequency bands in full-duplex microwave systems?
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What is the typical spacing between transmitters and receivers in microwave radio systems?
What is the typical spacing between transmitters and receivers in microwave radio systems?
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What is the purpose of an FM deviator in microwave communication?
What is the purpose of an FM deviator in microwave communication?
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Which component in a microwave generator follows the crystal oscillator?
Which component in a microwave generator follows the crystal oscillator?
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What type of demodulator is used in an FM microwave radio receiver?
What type of demodulator is used in an FM microwave radio receiver?
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What is the average path length between microwave repeater sites in open terrain?
What is the average path length between microwave repeater sites in open terrain?
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How do IF frequencies translate to RF frequencies in microwave communication?
How do IF frequencies translate to RF frequencies in microwave communication?
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What influences the location of intermediate microwave repeater sites?
What influences the location of intermediate microwave repeater sites?
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What type of repeater first down-converts the received RF carrier?
What type of repeater first down-converts the received RF carrier?
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What process restores the baseband signal to its original characteristics after demodulation?
What process restores the baseband signal to its original characteristics after demodulation?
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What is a primary advantage of microwave radio systems compared to other communication systems?
What is a primary advantage of microwave radio systems compared to other communication systems?
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Which of the following is a disadvantage of microwave radio communication?
Which of the following is a disadvantage of microwave radio communication?
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What effect does frequency modulation (FM) have on microwave radio systems?
What effect does frequency modulation (FM) have on microwave radio systems?
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Which component is used in the FM microwave transmitter to enhance higher baseband frequencies?
Which component is used in the FM microwave transmitter to enhance higher baseband frequencies?
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What is a crucial limitation of microwave frequencies in communication?
What is a crucial limitation of microwave frequencies in communication?
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Which of the following components is challenging to implement at microwave frequencies?
Which of the following components is challenging to implement at microwave frequencies?
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What type of multiplexed channels can be included in the baseband signal for FM microwave systems?
What type of multiplexed channels can be included in the baseband signal for FM microwave systems?
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What is typically minimized in microwave radio communication systems in comparison to other systems?
What is typically minimized in microwave radio communication systems in comparison to other systems?
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Study Notes
Microwave Communication Systems
- Microwave communication systems generally use frequencies ranging from 500 MHz to 300 GHz.
- The wavelengths for these frequencies fall between 1 cm and 60 cm.
- Wavelength can be calculated using the equation: λ=c/f (where λ: wavelength in meters, c: speed of light (approximately 3×10^8 m/s), f: frequency in Hertz (Hz))
- The frequency bands used for microwave communications are divided in half: low band and high band, for full-duplex transmission
- Since the mid-1980s, many electronic communication systems have been using digital methods to transmit voice, video and data.
- Microwave relay systems are essential for transmitting info in the U.S., covering approximately 35% of circuit mileage.
- Terrestrial microwave radio relay systems using frequency (FM) or digitally modulated carriers (PSK or QAM) are common.
- Microwave systems can operate over distances ranging from 15 miles to 4000 miles.
- They are used for interstate or feeder service (short-haul) and interstate and backbone (long-haul) communication.
- System capacities range from less than 12 voice-band channels to over 22,000 channels.
- Common modulation techniques for microwave communication include Frequency modulation (FM) with Frequency division multiplexing (FDM), Pulse code modulation (PCM) with Time Division Multiplexing (TDM), and Digital modulation techniques like Phase-Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM).
Microwave Radio Communication
- Microwave radios use signals that propagate through Earth’s atmosphere between transmitters and receivers.
- This often involves towers placed about 15 to 30 miles apart.
- Microwave radio systems do not require extensive right-of-way acquisition, only small areas of land for each station.
- Microwave radio systems can transfer large quantities of information due to their high operating frequencies.
- These high frequencies also allow for smaller antenna sizes.
- Microwave radio signals can readily pass around obstacles like water and mountains.
- They require fewer repeaters for amplification, leading to shorter distances between switching centers and minimal underground facilities.
- This results in reduced transmission delays, minimal crosstalk between voice channels, and improved reliability with less maintenance.
Disadvantages of Microwave Radio
- Microwave frequencies are more challenging to analyze and design for circuitry.
- It is difficult to perfect and implement measuring techniques at these frequencies.
- Conventional circuit components like resistors, capacitors, and inductors are challenging to use at microwave frequencies.
- Transient time is more critical with microwave frequencies, often requiring specialized components.
- Microwave frequencies propagate in a straight line, limiting their use to line-of-sight applications.
Frequency Modulation (FM) in Microwave Systems
- Frequency modulation (FM) is favored in microwave radio systems because FM signals are relatively insensitive to nonlinear distortion.
- FM signals can be transmitted through amplifiers with compression or amplitude nonlinearity with minimal impact.
- FM signals are less sensitive to random noise and can be propagated with lower transmit powers.
- FM microwave systems are easily expandable.
Microwave Transmitter (Tx)
- The baseband signal is the composite signal that modulates the FM carrier. It can include:
- Frequency-division-multiplexed voice-band channels
- Time-division-multiplexed voice-band channels
- Broadcast-quality composite video or picturephone signals
- Wideband data
- A pre-emphasis network boosts the amplitude of higher-frequency baseband signals.
- An FM deviator modulates the Intermediate Frequency (IF) carrier, which eventually becomes the main microwave carrier.
- The IF carrier frequency typically falls between 60 MHz and 80 MHz.
- The IF carrier signal and its associated sidebands are up-converted to the microwave frequency region using a mixer, microwave oscillator and bandpass filter.
- Mixing is preferred over multiplication for frequency translation because it preserves the modulation index.
- Microwave generators consist of a crystal oscillator followed by a series of frequency multipliers.
Microwave Receiver (Rx)
- A channel separation network isolates and filters individual microwave channels, directing them to their respective receivers.
- A bandpass filter, AM mixer, and microwave oscillator down-convert the RF microwave frequencies to IF frequencies.
- The FM demodulator is typically a noncoherent FM detector (like 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 Repeaters
- A microwave repeater acts as a receiver and transmitter, placed back-to-back or in tandem.
- It receives a signal, amplifies and reshapes it, then retransmits the signal to the next repeater or terminal station.
- The location of repeater sites is influenced by terrain between and surrounding the sites.
- Initial route planning assumes relatively flat areas with path (hop) lengths averaging between 25 miles and 35 miles between stations.
- Transmitter output power and antenna gain are also factors in site selection.
- Line-of-sight path clearance and received signal strength determine the exact distance.
- For frequencies above 10 GHz, rainfall patterns can impact path length.
Types of Repeaters
- IF - Heterodyne 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.
- RF-to-RF Repeater: The received RF carrier is amplified and retransmitted directly without frequency conversion. This type of repeater uses an RF amplifier operating at the carrier frequency.
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Description
This quiz covers the fundamentals of microwave communication systems, including frequency ranges, wavelength calculations, and the importance of relay systems. Learn about the applications and technology behind microwave communications, as well as the distinctions between low and high band usage. Test your knowledge on how these systems contribute to modern communication networks.
