Avionics - Radio Transmitters and Communication Receivers Quiz(MCQ)

1 .

The transmission of data by dots and dashes is an example of ________

A)

Amplitude modulation

B)

Frequency modulation

C)

Discrete signal transmission

D)

Continuous wave transmission

Correct Answer : Continuous wave transmission

Explanation : The oscillator generates the carrier and can be switched off and on by a telegraph key to produce the dots and dashes of the International Morse code. Information transmitted in this way is referred to as continuous-wave (CW) transmission. Such a transmitter is rarely used today.

2 .

What generates the final carrier frequency?

A)

Oscillator

B)

Antenna

C)

Modulator

D)

Power source

Correct Answer : Oscillator

Explanation : An oscillator, in most applications a crystal oscillator, generates the final carrier frequency. A modulator simply adds the information signal and an antenna transmits the signal.

3 .

What device is used to prevent power from the transmitter from going to the receiver side?

A)

Filter

B)

Duplexer

C)

Multiplexer

D)

Monoplexer

Correct Answer : Duplexer

Explanation : A duplexer is a device that is placed just before the antenna in a transducer system. It is basically a switch which allows both transmission and reception but only one at a time.

4 .

What is the minimum frequency of a 16-MHz crystal with a stability of 200 ppm?

A)

50,00,250Hz

B)

1,60,03,200Hz

C)

1,59,96,800Hz

D)

4,56,42,863Hz

Correct Answer : 1,59,96,800Hz

Explanation : The frequency can vary as much as 200 Hz for every 1 MHz of frequency or 200×16=3200Hz.
The maximum frequency is 16,000,000 – 3200 = 1,59,96,800 Hz.

5 .

Calculate the transmitter output frequency if the transmitter uses a crystal oscillator with a frequency of 14.9 MHz and a frequency multiplier chain with factors of 2, 3, and 3.

A)

1.564MHz

B)

264.3MHz

C)

265.6MHz

D)

268.2MHz

Correct Answer : 268.2MHz

Explaination : Total frequency multiplication factor 2 x 3 x 3 = 18
Transmitter output frequency = 14.9 MHz x 18 = 268.2 MHz.

6 .

______ is used to provide continuous tuning over the desired range?

A)

Frequency multiplier

B)

Frequency synthesizer

C)

Variable frequency oscillator

D)

Variable Frequency producer

Correct Answer : Variable frequency oscillator

Explanation : A variable frequency oscillator (VFO) is used to provide continuous tuning over a desired range. Using VFO different carrier frequencies can be generated.

7 .

What is the only oscillator capable of meeting the precision and stability demanded by the FCC?

A)

Crystal Oscillator

B)

Hartley oscillator

C)

Armstrong Oscillator

D)

RC Phase Shift Oscillator

Correct Answer : Crystal Oscillator

Explanation : The transmitter must remain on the assigned frequency. It must not drift off or wander from its assigned value despite the many operating conditions, such as wide temperature variations and changes in power supply voltage, that affect frequency. The only oscillator capable of meeting the precision and stability demanded by the FCC is a crystal oscillator.

8 .

What is the maximum frequency of a 16-MHz crystal with a stability of 200 ppm?

A)

4,56,42,863Hz

B)

3,16,51,043Hz

C)

2,36,12,500Hz

D)

1,60,03,200Hz

Correct Answer : 1,60,03,200Hz

Explanation : The frequency can vary as much as 200 Hz for every 1 MHz of frequency or 200×16=3200Hz.
The maximum frequency is 1,60,00,000 + 3200 = 1,60,03,200 Hz.

9 .

The device that maintains controlled temperature for crystal oscillators are called __________

A)

Crystal stove

B)

Crystal ovens

C)

Crystal chamber

D)

Temperature chamber

Correct Answer : Crystal ovens

Explanation : greater stability of crystal frequency can be achieved by mounting the crystal in sealed, temperature-controlled chambers known as crystal ovens. These devices maintain an absolute constant temperature, ensuring a stable output frequency.

10 .

Which of the following are the two main characteristics of receivers?

A)

Gain and attenuation

B)

Selectivity and sensitivity

C)

Modulation and demodulation

D)

Multiplexing and demultiplexing

Correct Answer : Selectivity and sensitivity

Explanation : A communication receiver must be able to identify and select the desired signal from thousands of others present in the frequency spectrum (selectivity) and to provide sufficient amplification to recover the modulating signal (sensitivity). A receiver with good selectivity will isolate the desired signal in the RF spectrum and eliminate or at least greatly attenuate all other signals.

11 .

What circuit provides the initial selectivity in a receiver?

A)

Multiplexer

B)

Amplifier

C)

LC tuned circuits

D)

Attenuator

Correct Answer : LC tuned circuits

Explanation : Initial selectivity in a receiver is normally obtained by using LC tuned circuits. By carefully controlling the Q of the resonant circuit, we can set the desired selectivity.

12 .

The ability to pick up week signals by a receiver is called _______

A)

Sensitivity

B)

Gain

C)

Selectivity

D)

Amplification

Correct Answer : Sensitivity

Explanation : A communication receiver’s sensitivity, or ability to pick up weak signals, is mainly a function of overall gain, the factor by which an input signal is multiplied to produce the output signal. In general, the higher the gain of a receiver, the better its sensitivity.

13 .

What type of oscillators are used to input a wide range of frequencies to the IF amplifier?

A)

LC oscillators

B)

Diode oscillators

C)

Crystal oscillators

D)

Variable frequency oscillators

Correct Answer : Variable frequency oscillators

Explanation : The local oscillator is usually a variable frequency oscillator or a frequency synthesizer so that its frequency can be adjusted over a relatively wide range. As the local-oscillator frequency is changed, the mixer translates a wide range of input frequencies to the fixed IF.

14 .

The LNA of a transducer has failed during the transmission of a signal. Which component's failure has caused this?

A)

Duplexer

B)

Mixer

C)

Antenna

D)

RF amplifier

Correct Answer : Duplexer

Explanation : A duplexer is a device which acts as a switch and stops the high power transmission signals from entering into the receiver, which is tuned to handle very low power for better sensitivity. When the duplexer fails, the high power signals enter the receiver where the Low noise amplifier is damaged.

15 .

Which of the following is not an advantage of using RF amplifiers in the initial stage of signal selection?

A)

Low noise

B)

Less power consumption

C)

Better selectivity

D)

Reduced oscillator radiation

Correct Answer : Less power consumption

Explanation : The use of RF amplifiers in the initial stage of signal selection reduces the effect of noise.RF amplifiers improve sensitivity, because of the extra gain; improve selectivity, because of the added tuned circuits; and improve the S/N ratio. Further, spurious signals are more effectively rejected, minimizing unwanted signal generation in the mixer.

16 .

What is connected between the antenna and the mixer to avoid oscillator radiation?

A)

LNA

B)

Duplexer

C)

Attenuator

D)

Power amplifier

Correct Answer : LNA

Explanation : LNA or low noise amplifier is placed in between the antenna and the mixer. The oscillator signal is relatively strong and some of it can leak through and appear at the input of the mixer. The RF amplifier between the mixer and the antenna isolates the two, significantly reducing any local oscillator radiation.

17 .

What is the input signal level that is approximately equal to the average internally generated noise value called?

A)

MBS

B)

MMD

C)

MDS

D)

MSD

Correct Answer : MDS

Explanation : The MDS or Minimum Discernible Signal is the input signal level that is approximately equal to the average internally generated noise value. This noise value is called the noise floor of the receiver. MDS is the amount of signal that would produce the same audio power output as the noise floor signal. The MDS is usually expressed in dBm.

18 .

Analog signals : Signal to noise ratio:: Digital signals _________

A)

Error rate

B)

Bit error rate

C)

Digital noise ratio

D)

Signal Attenuation

Correct Answer : Bit error rate

Explanation : For analog signals, the signal-to-noise ratio is the main consideration in analog signals. For digital signal transmission, the bit error rate (BER) is the main consideration. BER is the number of errors made in the transmission of many serial data bits.

19 .

What type of receivers converts all incoming frequency into a lower frequency for better selectivity and sensitivity?

A)

Digital receiver

B)

Crystal receiver

C)

Analogous receivers

D)

Superheterodyne receivers

Correct Answer : Superheterodyne receivers

Explanation : Superheterodyne receivers convert all incoming signals to a lower frequency, known as the intermediate frequency (IF), at which a single set of amplifiers and filters is used to provide a fixed level of sensitivity and selectivity. Most of the gain and selectivity in a superheterodyne receiver are obtained in the IF amplifiers.

20 .

What is the output of a mixer circuit?

A)

Sum of frequencies

B)

Difference of frequencies

C)

Multiplication of frequencies

D)

Sum and difference of frequencies

Correct Answer : Sum and difference of frequencies

Explanation : The key circuit is the mixer, which acts as a simple amplitude modulator to produce sum and difference frequencies. The incoming signal is mixed with a local oscillator signal to produce this conversion.

21 .

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)

150

C)

162

D)

175

Correct Answer : 143

Explaination : Total channels = 860/6 = 143.33 or 143.

22 .

The system which uses FM for the subcarriers is called _____

A)

FM II system

B)

FM/AM system

C)

FM/FM system

D)

2 stage FM system

Correct Answer : FM/FM system

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.

23 .

Which of the following device is used to demultiplex the received signal?

A)

Bandpass filters

B)

Allpass filters

C)

Bandstop filters

D)

Differential filters

Correct Answer : Bandpass filters

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.

24 .

In which of the following systems multiplexing is not necessary?

A)

Satellites

B)

Continuous wave transmission

C)

Telemetry

D)

TV broadcasting

Correct Answer : Continuous wave transmission

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.

25 .

Time division multiplexing: Digital signal:: Frequency division multiplexing:?

A)

Analog signal

B)

Continuous wave signals

C)

Pulse code modulated signal

D)

Pulse position modulated signal

Correct Answer : Analog signal

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.

26 .

What type of multiplexing is widely used in cellphones?

A)

Spatial multiplexing

B)

Time division multiplexing

C)

Frequency division multiplexing

D)

Code division multiplexing

Correct Answer : Code division multiplexing

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.

27 .

The transmission of multiple signals in a common frequency without interference is called _______

A)

Code division multiplexing

B)

Time division multiplexing

C)

Spatial multiplexing

D)

Frequency division multiplexing

Correct Answer : Spatial multiplexing

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.

28 .

Which circuit does the actual multiplexing process in frequency division multiplexing?

A)

Oscillator

B)

Linear mixer

C)

Duplexer

D)

RF amplifier

Correct Answer : Linear mixer

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.

29 .

For frequency division multiplexing who defines the channel bandwidth?

A)

FAA

B)

FCC

C)

CCA

D)

ARNIC

Correct Answer : FCC

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.

30 .

What is the individual carrier frequency of each signal called?

A)

Subcarrier

B)

Coded carrier

C)

Frequency carrier

D)

Modulated carrier

Correct Answer : Subcarrier

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.

31 .

Frequency division multiplexing:Frequency slots::time division multiplexing:?

A)

Band slots

B)

Coded information

C)

Time slots

D)

Pulsed information

Correct Answer : Time slots

Explanation : In FDM, multiple signals are transmitted over a single channel, each signal being allocated a portion of the spectrum within that bandwidth. In time-division multiplexing (TDM), each signal occupies the entire bandwidth of the channel. However, each signal is transmitted for only a brief time. In other words, multiple signals take turns transmitting over the single channel.

32 .

What device is used to demodulate a time division multiplexed analog wave?

A)

Attenuator

B)

Band stop filter

C)

High pass filter

D)

Low pass filter

Correct Answer : Low pass filter

Explanation : the analog signal is converted to a series of constant-width pulses whose amplitude follows the shape of the analog signal. The original analog signal is recovered by passing it through a low-pass filter. In TDM using PAM, a circuit called a multiplexer (MUX or MPX) samples multiple analog signal sources; the resulting pulses are interleaved and then transmitted over a single channel.

33 .

Which of the following device was used in early TDM/PAM telemetry systems?

A)

DSP

B)

Commuter

C)

Logic gates

D)

Linear switch

Correct Answer : Commuter

Explanation : Multiplexers in early TDM/PAM telemetry systems used a form of rotary switch known as a commutator. Multiple switch segments were attached to the various incoming signals while a high-speed brush rotated by a dc motor rapidly sampled the signals as it passed over the contacts.

34 .

What is the purpose of one shot multivibrator?

A)

Trigger all OR gates at clock frequency

B)

Trigger all AND gates at clock frequency

C)

Trigger all OR gates at signal frequency

D)

Trigger all AND gates at signal frequency

Correct Answer : Trigger all AND gates at clock frequency

Explanation : The one-shot multivibrator is used to trigger all the decoder AND gates at the clock frequency. It produces an output pulse whose duration has been set to the desired sampling interval.

35 .

The circuit used to regenerate clock pulses from the transmitted PAM signals is called ____

A)

Clock recovery circuits

B)

Timer circuits

C)

Clock receiving circuits

D)

Clock demodulator circuits

Correct Answer : Clock recovery circuits

Explanation : Instead of using a free-running clock oscillator set to the identical frequency of the transmitter system clock, the clock for the demultiplexer is derived from the received PAM signal itself. A circuit called the clock recovery circuits are typical of those used to generate the demultiplexer clock pulses.

36 .

What are used to reduce or stop synchronization problems while receiving?

A)

Synchronizer

B)

Demodulators

C)

Clock recovery circuits

D)

Band pass filter

Correct Answer : Clock recovery circuits

Explanation : Clock recovery circuits are used to remedy the synchronization problem encountered in demultiplexing. The clock pulse is derived from the transmitted signal so that synchronization errors are reduced.

37 .

Which of the following type of multiplexing uses pulse code modulation?

A)

Code division multiplexing

B)

Frequency division multiplexing

C)

Amplitude limited multiplexing

D)

Time division multiplexing

Correct Answer : Time division multiplexing

Explanation : The most popular form of TDM uses pulse-code modulation (PCM), in which multiple channels of digital data are transmitted in serial form. Each channel is assigned a time slot in which to transmit one binary word of data. The data streams from the various channels are interleaved and transmitted sequentially.

38 .

Which of the following statements is true with respect to PCM?

A)

Analog data is transmitted directly

B)

Analog signal is amplified before transmission

C)

The parallel binary data is converted into serial before transmission

D)

The analog signal is converted into parallel binary data before transmission

Correct Answer : The parallel binary data is converted into serial before transmission

Explanation : The analog signal is converted into a digital signal. Since the converted digital signal is parallel bits of data, it has to be converted to serial before it can be multiplexed and transmitted.

39 .

Which of the following is false with respect to PCM?

A)

Reliable

B)

Inexpensive

C)

Resistant to noise

D)

Not easily recoverable

Correct Answer : Not easily recoverable

Explanation : When signals have been degraded because of noise, attenuation, or distortion, all the receiver has to do is to determine whether a pulse was transmitted. Amplitude, width, frequency, phase shape, and so on do not affect reception. Thus PCM signals are easily recovered and rejuvenated, no matter what the circumstances.

40 .

A special PCM system uses 16 channels of data, one whose purpose is an identification (ID) and synchronization. The sampling rate is 3.5 kHz. The word length is 6 bits. Find the serial data rate.

A)

451kHz

B)

336kHz

C)

326kHz

D)

152kHz

Correct Answer : 336kHz

Explaination : Serial data rate = sampling rate x no. bits/frame = 3.5 kHz x 96 = 336 kHz.