Avionics Questions and Answers for Experienced people on “Data Conversion – 2”.
1. Sampling an analog signal produces pulse amplitude modulation.
a) True
b) False
Answer: a
Clarification: 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. What causes a new signal with frequency fs-fm to be created near the original signal?
a) Attenuation
b) Imaging
c) Aliasing
d) Distortion
Answer: c
Clarification: 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. What type of filter is an antialiasing filter?
a) Band pass
b) High pass
c) Low pass
d) Band stop
Answer: c
Clarification: 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. 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
Answer: a
Clarification: 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. What is sampling at a rate higher than the Nyquist frequency called?
a) Undersampling
b) Oversampling
c) Distortion
d) Aliasing
Answer: b
Clarification: 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. Which of the following is not a disadvantage of oversampling?
a) High cost
b) More storage space
c) Less accuracy
d) High power consumption
Answer: c
Clarification: 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. What is processing gain?
a) Gain added by the ADC
b) Improvement in SNR
c) Amplitude increment by ADC
d) Reduction in error by the ADC circuitry
Answer: b
Clarification: 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. 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
Answer: a
Clarification: Processing gain(dB) = 10 log[ (fs/2)/BW] = 10 log[ (100/2)/20]= 10 log[ (50/20)] = 4dB.
9. 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
Answer: c
Clarification: 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. Which circuit is used to produce the IF for better selection and filtering?
a) Downconverter
b) Modulator
c) RC circuit
d) Gating circuit
Answer: a
Clarification: 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. 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
Answer: b
Clarification: According to sampling theorem, minimum sampling frequency = 2 x 20MHz = 40MHz.
Avionics for Experienced people,