250+ TOP MCQs on Frequency Division Multiplexing and Answers

Avionics Multiple Choice Questions on “Frequency Division Multiplexing”.

1. Multiplexing increases the number of communication channels for transmission.
a) True
b) False
Answer: a
Clarification: 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. In which of the following systems multiplexing is not necessary?
a) Telemetry
b) TV broadcasting
c) Satellites
d) Continuous wave transmission
Answer: d
Clarification: 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. 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
Answer: c
Clarification: 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. What type of multiplexing is widely used in cellphones?
a) Time division multiplexing
b) Frequency division multiplexing
c) Code division multiplexing
d) Spatial multiplexing
Answer: c
Clarification: 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. 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
Answer: d
Clarification: 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. For frequency division multiplexing who defines the channel bandwidth?
a) FCC
b) ARNIC
c) FAA
d) CCA
Answer: a
Clarification: 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. What is the individual carrier frequency of each signal called?
a) Subcarrier
b) Frequency carrier
c) Modulated carrier
d) Coded carrier
Answer: a
Clarification: Each signal to be transmitted feeds a modulator circuit. The carrier for each modulator (f_c) 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. Which circuit does the actual multiplexing process in frequency division multiplexing?
a) Linear mixer
b) Oscillator
c) RF amplifier
d) Duplexer
Answer: a
Clarification: 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. Which of the following device is used to demultiplex the received signal?
a) Allpass filters
b) Bandpass filters
c) Bandstop filters
d) Differential filters
Answer: b
Clarification: 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. 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
Answer: b
Clarification: 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. 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
Answer: a
Clarification: Total channels = 860/6 = 143.33 or 143.