Avionics MCQs – Frequency Division Multiplexing

1 - Question

Multiplexing increases the number of communication channels for transmission.
a) True
b) False
View Answer Answer: a
Explanation: Multiplexing is the process of simultaneously transmitting two or more individual signals over a single communication channel, cable or wireless. In effect, it increases the number of communication channels so that more information can be transmitted.

2 - Question

In which of the following systems multiplexing is not necessary?
a) Telemetry
b) TV broadcasting
c) Satellites
d) Continuous wave transmission
View Answer Answer: d
Explanation: Continuous wave transmission such as morse code, multiplexing is not necessary since only two voltage levels are present and each bit is sent one by one. Also, only one information signal is transmitted whereas in telemetry, TV and satellite communications numerous information is transmitted hence multiplexing is required.

3 - Question

Time division multiplexing: Digital signal:: Frequency division multiplexing:?
a) Pulse code modulated signal
b) Continuous wave signals
c) Analog signal
d) Pulse position modulated signal
View Answer Answer: c
Explanation: The two most common types of multiplexing are frequency-division multiplexing (FDM) and time-division multiplexing (TDM). Two variations of these basic methods are frequency-division multiple access (FDMA) and time-division multiple access (TDMA). In general, FDM systems are used for analog information and TDM systems are used for digital information.

4 - Question

What type of multiplexing is widely used in cellphones?
a) Time division multiplexing
b) Frequency division multiplexing
c) Code division multiplexing
d) Spatial multiplexing
View Answer Answer: c
Explanation: Another form of multiple accesses is known as code-division multiple access (CDMA). It is widely used in cell phone systems to allow many cell phone subscribers to use a common bandwidth at the same time. This system uses special codes assigned to each user that can be identified. CDMA uses a technique called spread spectrum to make this type of multiplexing possible.

5 - Question

The transmission of multiple signals in a common frequency without interference is called _______
a) Time division multiplexing
b) Frequency division multiplexing
c) Code division multiplexing
d) Spatial multiplexing
View Answer Answer: d
Explanation: Spatial multiplexing is the term used to describe the transmission of multiple wireless signals on a common frequency in such a way that they do not interfere with one another. One way of doing this is to use low-power transmissions so that the signals do not interfere with one another. When very low power is used, the signals do not travel very far. The transmission distance is a function of the power level, frequency, and antenna height.

6 - Question

For frequency division multiplexing who defines the channel bandwidth?
a) FCC
c) FAA
d) CCA
View Answer Answer: a
Explanation: The bandwidths of radio channels vary, and are usually determined by FCC regulations and the type of radio service involved. Regardless of the type of channel, a wide bandwidth can be shared for the purpose of transmitting many signals at the same time.

7 - Question

What is the individual carrier frequency of each signal called?
a) Subcarrier
b) Frequency carrier
c) Modulated carrier
d) Coded carrier
View Answer Answer: a
Explanation: Each signal to be transmitted feeds a modulator circuit. The carrier for each modulator (fc) is on a different frequency. The carrier frequencies are usually equally spaced from one another over a specific frequency range. These carriers are referred to as subcarriers.

8 - Question

Which circuit does the actual multiplexing process in frequency division multiplexing?
a) Linear mixer
b) Oscillator
c) RF amplifier
d) Duplexer
View Answer Answer: a
Explanation: The modulator outputs containing the sideband information are added algebraically in a linear mixer; no modulation or generation of sidebands takes place. The resulting output signal is a composite of all the modulated subcarriers. This signal can be used to modulate a radio transmitter or can itself be transmitted over the single communication channel. Alternatively, the composite signal can become one input to another multiplexed system.

9 - Question

Which of the following device is used to demultiplex the received signal?
a) Allpass filters
b) Bandpass filters
c) Bandstop filters
d) Differential filters
View Answer Answer: b
Explanation: A receiver picks up the signal and demodulates it, recovering the composite signal. This is sent to a group of bandpass filters, each centered on one of the carrier frequencies. Each filter passes only its channel and rejects all others. A channel demodulator then recovers each original input signal.

10 - Question

The system which uses FM for the subcarriers is called _____
a) FM II system
b) FM/FM system
c) FM/AM system
d) 2 stage FM system
View Answer Answer: b
Explanation: Generally the individual signals which require multiplexing are frequency modulated. These signals are then added up by the mixer and the resulting output signal is again frequency modulated before transmission.

11 - Question

A cable TV service uses a single coaxial cable with a bandwidth of 860 MHz to transmit multiple TV signals to subscribers. Each TV signal is 6 MHz wide. How many channels can be carried?
a) 143
b) 123
c) 100
d) 150
View Answer Answer: a
Explanation: Total channels = 860/6 = 143.33 or 143.

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