# Avionics MCQs – Data Conversion – 2

1 - Question

Sampling an analog signal produces pulse amplitude modulation.
a) True
b) False
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.

2 - Question

What causes a new signal with frequency fs-fm to be created near the original signal?
a) Attenuation
b) Imaging
c) Aliasing
d) Distortion
Explanation: Aliasing occurs when a signal is sampled mistakenly at a frequency lesser than twice the input frequency. It causes a new signal with a frequency of fs-fm to be created near the original signal.

3 - Question

What type of filter is an antialiasing filter?
a) Band pass
b) High pass
c) Low 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.

4 - Question

Which of the following is not used in antialiasing filters?
a) Single stage RC or LC circuits
b) Multistage LC circuits
c) RC active filter
d) High order switched capacitor filter 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.

5 - Question

What is sampling at a rate higher than the Nyquist frequency called?
a) Undersampling
b) Oversampling
c) Distortion
d) Aliasing
Explanation: Sampling at a rate twice the highest frequency is called as Nyquist frequency. When sampling is done at a higher rate than Nyquist frequency it is called as oversampling. Oversampling has both advantages and disadvantages.

6 - Question

Which of the following is not a disadvantage of oversampling?
a) High cost
b) More storage space
c) Less accuracy
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.

7 - Question

What is processing gain?
b) Improvement in SNR
d) Reduction in error by the ADC circuitry
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.

8 - Question

What is the process gain of a signal with bandwidth 20MHz and a sampling frequency of 100MHz?
a) 4dB
b) 5dB
c) 4.6dB
d) 125dB
Explanation: Processing gain(dB) = 10 log[ (fs/2)/BW] = 10 log[ (100/2)/20]= 10 log[ (50/20)] = 4dB.

9 - Question

Which of the following is true with respect to undersampling?
a) High power
b) Greater than Nyquist frequency
c) Use low pass filters
d) Less memory size
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.

10 - Question

Which circuit is used to produce the IF for better selection and filtering?
a) Downconverter
b) Modulator
c) RC circuit
d) Gating circuit
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.

11 - Question

According to the sampling theorem, what is the minimum sampling frequency to retain all the information if the bandwidth of the signal is 20MHz?
a) 20MHz
b) 40MHz
c) 80MHz
d) 2MHz