Avionics - Digital Communication Techniques Quiz(MCQ)
A)
Can transmit binary data
B)
Can transmit analog data
C)
LAN is a digital data transmission
D)
Only restricted to communication between computers

Correct Answer :   Only restricted to communication between computers

Explanation : Digital data communication was only restricted to communication between computers. Since analog signals can now be easily converted into digital signals using analog to digital converter, digital transmission can now transmit voice, video and other analog signals in digital form.

A)
Low frequency transmission
B)
High frequency transmission
C)
Cannot easily change binary 1 to 0
D)
High power transmission

Correct Answer :   Cannot easily change binary 1 to 0

Explanation : Noise is inevitable in a signal transmission. In a digital transmission the amplitude of the noise must be higher than the amplitude of the signal at binary ’1’, which is generally not the case, and vice versa. The receiver can differentiate between binary ‘1’ and ‘0’ with a significant amount of noise.

A)
Signal regeneration
B)
Signal cut off
C)
Signal threshold
D)
Data regeneration

Explanation : In the process of signal regeneration a cut off value or threshold value is set in the receiver so that the noise can be clipped off. When the values are set properly only the binary data would pass through the circuit. This produces a clean output pulse.

A)
Bit error
B)
Bit error rate
C)
Error rate
D)
Error bit rate

Correct Answer :   Bit error rate

Explanation : Bit rate error(BER) is the number of error bits that are present for a given number of bits transmitted. It depends on several factors like the environment, equipment and other considerations. BER is calculated for a particular set of conditions.

A)
0
B)
0.542
C)
0.819
D)
1

Explanation : Total number of bits = 512 x 8 = 4096 bits
Average number of errors = (2/10,000) x 4096 = 0.819.

A)
Less power
B)
High power
C)
Continuous data
D)
Discrete data

Explanation : Digital data is discrete and hence can be transmitted via time division multiplexing, whereas analog data is continuous and can only be transmitted as a whole. However, analog data can be converted into digital by analog to digital converters and then be transmitted via time division multiplexing. Time division multiplexing is one of the biggest advantages of digital communication.

A)
True
B)
False
C)
Can Not Say
D)
None of the above

Explanation : In a parallel transmission each bit is transmitted in a separate wire. Since each bit is transmitted individually, each bit requires a separate transmitter and a receiver.

A)
Data line
B)
Data bus
C)
D)
Parallel line

Explanation : In a parallel transmission multiwire cables are used. These cables carry a bit in each wire and are referred as data bus.

A)
Cost factor
B)
Low speed
C)
Signal Attenuation
D)
Number of transmission channels

Explanation : One of the major disadvantages of using parallel transmission for long distance is that laying the data bus is costly. To cover a distance of 10km for an 8 bit system, we need a cable of 80km length. Parallel data are usually faster than serial since all the bits are transmitted simultaneously.

A)
Multiplexer
B)
Preemphasizer
C)
Serializer/Deserializer
D)
Transmission converters

Explanation : Parallel transmission is high speed and serial data can cover longer distances. Hence they are used interchangeably and the devices which convert serial to parallel and parallel to serial transmission is called serializer/deserializer device (SERDES).

A)
Cost
B)
Noise
C)
Resistance
D)
Inductive and capacitance effects

Correct Answer :   Inductive and capacitance effects

Explanation : As the speed of transmission or the frequency of transmission increases the wavelength of transmission becomes comparable with the length of the data bus. This causes inductive and capacitance effects which degrade the signal.

A)
Least significant bit
B)
Most significant
C)
Not in any specific order
D)
All the bits simultaneously

Correct Answer :   Least significant bit

Explanation : In serial transmission, the bits are transmitted one after the other in a specific order. The least significant bit is transmitted first and the most significant bit is transmitted in the end.

13 .
In serial communication, if the time interval to transmit one bit is 10μs. What is the time to transmit an 8-bit word?
A)
8μs
B)
80μs
C)
0.8μs
D)
0.008μs

Explaination : Total transmission time = time for each bit x total number of bits = 8 x 10μs = 80μs.

A)
Amplify
B)
Reduce noise
C)
Increase range
D)
Convert analog to digital

Correct Answer :   Convert analog to digital

Explanation : ADC corresponds to Analog to Digital Converter which is used to convert an analog signal into a digital. Digital signal can be processed and manipulated.

A)
Circular waves
B)
Stairstep wave
C)
Triangular waves
D)
Smooth continuous wave

Explanation : The Digital to analog converter takes the binary number as an input and produces the analog voltage proportional to the binary number. These analog voltages represent specific analog voltage levels and have stairstep characters.

A)
25kHz
B)
45kHz
C)
70kHz
D)
None of the above

Explanation : Fifth harmonic = 5 x signal frequency = 5 x 14kHz = 70kHz.

A)
0.7mV
B)
1mV
C)
1.7mV
D)
5mV

Explanation :

A)
Quantization error
B)
Data error
C)
Sampling error
D)
Acquisition error

Explanation : When the number of bits is not sufficient enough to represent the analog voltage levels, quantization error occurs. The greater the number of bits, the greater the number of increments over the analog range and the smaller the quantizing error.

A)
60Hz
B)
120Hz
C)
6000Hz
D)
12000Hz

Explanation : Minimum sampling frequency= Nyquist frequency = 2x(frequency of analog waveform)
= 2x 6000Hz = 12000Hz.

A)
126
B)
215
C)
255
D)
256

Explanation : The number of voltage increments that can be in an N bit system is given by 2N-1 = 28-1 =255.

A)
True
B)
False
C)
Can Not Say
D)
None of the above

Explanation : The modulator acts like a gating circuit that allows the analog wave for a particular time period producing a pulse. The amplitude of the pulse is the input to the ADC which converts the voltage into binary numbers.

A)
Band pass
B)
Low pass
C)
High pass
D)
Band stop

Explanation : The antialiasing filter is basically just a low pass filter which allows signals with frequencies less than the cut off value to pass through. The cut off frequency is usually half of the sampling frequency.

A)
RC active filter
B)
Single stage RC or LC circuits
C)
Multistage LC circuits
D)
High order switched capacitor filter circuits

Correct Answer :   Single stage RC or LC circuits

Explanation : An antialiasing filter must have good selective characteristics. The roll off rate of a single stage RC or LC circuit is too low and hence Multistage LC circuits, RC active filter and High order switched capacitor filter circuits are used.

A)
Less accuracy
B)
High cost
C)
More storage space
D)
High power consumption

Explanation : Accuracy of the conversion increases with an increase in sampling rate since discretization is reduced and we get a better digital replica of the original signal. As the frequency is high and the time period between samples are very low huge amounts of data are recorded and hence require high power and storage space eventually leading to high costs.

A)
B)
C)
Reduction in error by the ADC circuitry
D)
Improvement in SNR

Correct Answer :   Improvement in SNR

Explanation : One of the major advantages of oversampling is that it increases the signal to noise ratio(SNR). It decreases the quantization noise by spreading it over a wider frequency. The increase in SNR is called the processing gain.

A)
High power
B)
Less memory size
C)
Use low pass filters
D)
Greater than Nyquist frequency

Correct Answer :   Use low pass filters

Explanation : Undersampling causes aliasing which at the output of the ADC results in a wave with much lower frequency than the original signal. To reduce aliasing effects, antialiasing filters are used which acts as a low pass filter.

A)
RC circuit
B)
Modulator
C)
Gating circuit
D)
Downconverter

Explanation : In radio receivers, a high-frequency signal is converted to a lower mixed frequency called an intermediate frequency (IF) where it can be more adequately filtered for improved frequency selection. A downconverter is a circuit that performs this equivalent of aliasing.

A)
Less power
B)
Fast microprocessors
C)
D)
Low memory capacity

Explanation : Since in undersampling the time period between samples is sufficiently large, this allows slower processors to be used for ADC. This also allows low memory capacity for storage and less power consumption.

A)
81.5dB
B)
72.24dB
C)
62.35dB
D)
10.254dB

Explanation : 20 log( 212-1) = 20 log (4096-1) = 20 log (4095) = 72.24dB.

A)
Flash conversion
B)
Pipelined conversion
C)
Delta encoded conversion
D)
Successive approximation conversion

Explanation : The flash converter produces an output as fast as the comparators can switch and the signals can be translated to binary levels by the logic circuits. Comparator switching and logic propagation delays are extremely short. Thus, flash converters are the fastest type of A/D converter.

A)
10MHz
B)
11MHz
C)
12MHz
D)
13MHz

Explanation : Operational frequency = 1 / (100 x 10-9)= 10MHz. Operational frequencies higher than this would produce errors.

A)
True
B)
False
C)
Can Not Say
D)
None of the above

Explanation : Settling time is the amount of time it takes for the output voltage of a DAC, after a change in binary input, to stabilize to within a specific voltage range. The output is thus not an accurate representation of the binary input; it is not usable until it settles down.

A)
0.25V
B)
1.23V
C)
1.25V
D)
1.75V

Explanation : Resolution = reference voltage / 2N = 10/23 = 1.25V.

A)
Phase modulation
B)
Amplitude modulation
C)
Frequency modulation
D)
Pulse modulation

Explanation : In pulse modulation, the information to be transmitted is represented by a series of binary pulses. Since the pulse information is binary in nature analog signal shave to be converted to digital before transmitting.

35 .
What is the output voltage if the input voltage of a compander with a maximum voltage range of 1 V and a μ of 255 is 0.8V0?
A)
1.54V
B)
1.02V
C)
0.458V
D)
0.08V

Explaination :

A)
Pulse code modulation
B)
Pulse width modulation
C)
Pulse position modulation
D)
Pulse amplitude modulation

Correct Answer :   Pulse code modulation

Explanation : The most widely used technique for digitizing information signals for electronic data transmission is pulse code modulation. It has uniform transmission quality and also can be used when the signal traffic is high.

A)
Modulating
B)
Amplification
C)
Compressing
D)
Companding

Explanation : To reduce the effects of noise and distortion in pulse modulation, a process called companding is done. Companding is a process of signal compression and expansion.

A)
Telemetry systems
B)
Communication of airplane with ATC
C)
Switch power modes
D)
Remote control models

Correct Answer :   Communication of airplane with ATC

Explanation : Pulse modulation is used in telemetry systems to monitor spacecraft or missile, RC models, for switching power supplies like regulators and also as audio switching power amplifiers. Communication of airplane with ATC is amplitude modulated waves.

A)
Pulse width modulation
B)
Pulse position modulation
C)
Pulse frequency modulation
D)
Pulse amplitude modulation

Correct Answer :   Pulse frequency modulation

Explanation : There are four basic forms of pulse modulation. They are: pulse amplitude modulation, pulse width modulation, pulse position modulation pulse code modulation. In any form of pulse modulation, the frequency of the signal is not changed.

A)
0
B)
1
C)
2
D)
3

Explanation : The amplitude of PWM is binary in nature meaning that it has only two levels. The amplitude of the modulating signals varies the width of the pulses generated.

A)
Pulse amplitude modulation
B)
Pulse code modulation
C)
Pulse width modulation
D)
Pulse position modulation

Correct Answer :   Pulse amplitude modulation

Explanation : Out of all the pulse modulation techniques, Pulse amplitude modulation is the least expensive and simplest to implement. In pulse amplitude modulation, the amplitude of the pulse varies with the amplitude of the modulating signal.

A)
Pulse is narrow
B)
C)
Modulates a high frequency carrier
D)
Pulse width changes in accordance with the amplitude of modulating signal

Correct Answer :   Pulse width changes in accordance with the amplitude of modulating signal

Explanation : In PPM, the pulses change position according to the amplitude of the analog signal. The pulses are very narrow. These pulse signals may be transmitted in a baseband form, but in most applications, they modulate a high-frequency radio carrier.

A)
True
B)
False
C)
Can Not Say
D)
None of the above

Explanation : DSP is the use of a fast digital computer or digital circuitry to perform processing on digital signals. Any digital computer with sufficient speed and memory can be used for DSP.

A)
Amplified
B)
Stored in a RAM
C)
Attenuated
D)
Changed back to analog

Correct Answer :   Stored in a RAM

Explanation : When the signal is converted from analog to digital it is a sequence of binary numbers which is stored in the RAM. A user defined code that is usually stored in the ROM performs mathematical and other manipulations after which it is converted back into analog signals.

A)
Larry Page
B)
Alan Turing
C)
Ken Thompson
D)
John Von Neumann

Correct Answer :   John Von Neumann

Explanation : Physicist John Von Neumann is generally credited with creating the stored program concept that is the basis of operation of all digital computers. Binary words representing computer instructions are stored sequentially in a memory to form a program. The instructions are fetched and executed one at a time at high speed.

A)
Modulation
B)
Equalization
C)
Data compression
D)
Phase shifting

Explanation : Data compression is done by checking redundancy in data. Data redundancy checking is only possible when the data is in digital form and hence data compression cannot be done in analog signals.

A)
DDFT
B)
FGPA
C)
FDFT
D)
Fast Fourier transforms

Correct Answer :   Fast Fourier transforms

Explanation : The DFT is a complex program that is long and time-consuming to run. In general, computers are not fast enough to perform DFT in real time as the signal occurs. Therefore, a special version of the algorithm has been developed to speed up the calculation. Known as the fast Fourier transform (FFT), it permits real-time signal spectrum analysis.

A)
Spectrum analysis
B)
Data analysis
C)
Signal analysis
D)
Signal decoding

Explanation : Spectrum analysis is the process of examining a signal to determine its frequency content. Algorithms such as discrete Fourier transform (DFT) and FPGA is used to analyze the frequency content of an input signal.

A)
Easily programmable
B)
High sampling frequency
C)
Better processing capabilities
D)
Less power consumption at higher speeds

Correct Answer :   High sampling frequency

Explanation : The time taken for input/output and the processing time together must be smaller than the sampling period to ensure the continuous flow of data. Since high sampling frequencies are needed for accurately converting the analog signal to digital, high speed DSP is a must.

A)
Von Neumann limit
B)
Von Neumann bottleneck
C)
Von Neumann speed
D)
Von Neumann limitation

Correct Answer :   Von Neumann bottleneck

Explanation : The accessibility limitation of only one data or instruction set at a time from the memory is called as Von Neumann bottleneck. This has the effect of greatly limiting the execution speed.

A)
Split cell architecture
B)
Fragmented architecture
C)
Harvard architecture
D)
Von Neumann architecture