21. In the practical A/D converters, what are the distortions and time-related degradations that occur during the conversion process?
A. Jitter errors
B. Droops
C. Nonlinear variations in the duration of the sampling aperture
D. All of the mentioned
Answer: D
An ideal S/H introduces no distortion in the conversion process and is accurately modeled as an ideal sampler. However, time-related degradations such as errors in the periodicity of the sampling process (“jitter”), nonlinear variations in the duration of the sampling aperture, and changes in the voltage held during conversion (“droop”) do occur in practical devices.
22. In the absence of an S/H, the input signal must change by more than one-half of the quantization step during the conversion, which may be an impractical constraint.
A. True
B. False
Answer: B
The use of an S/H allows the A /D converter to operate more slowly compared to the time actually used to acquire the sample. In the absence of an S/H, the input signal must not change by more than one-half of the quantization step during the conversion, which may be an impractical constraint.
23. The noise power σn2 can be reduced by increasing the sampling rate to spread the quantization noise power over a larger frequency band (-Fs/2, Fs/2).
A. True
B. False
Answer: A
The noise power σn2 can be reduced by increasing the sampling rate to spread the quantization noise power over a larger frequency band (-Fs/2, Fs/2), and then shaping the noise power spectral density by means of an appropriate filter.
24. What is the process of down-sampling called?
A. Decimation
B. Fornication
C. Both Decimation & Fornication
D. None of the mentioned
Answer: A
To avoid aliasing, we first filter out the out-of-band (fl, F J 2) noise by processing the wideband signal.
The signal is then passed through the low pass filter and re-sampled (down sampled) at the lower rate. The down-sampling process is called decimation.
25. If the interpolation factor is I = 256, the A/D converter output can be obtained by averaging successive non-overlapping blocks of 128 bits.
A. True
B. False
Answer: A
If the interpolation factor is I = 256, the A/D converter output can be obtained by averaging successive non-overlapping blocks of 128 bits.
This averaging would result in a digital signal with a range of values from zero to 256 (b as 8 bits) at the Nyquist rate. The averaging process also provides the required anti-aliasing filtering.
26. The crosshatched areas give two types of Quantization error in DM, they are?
A. Slope-overload distortion
B. Granular noise
C. Slope-overload distortion & Granular noise
D. None of the mentioned
Answer: C
The crosshatched areas illustrate two types of quantization error in DM, slope-overload distortion and granular noise.
27. The slope-overload distortion is avoided, if which of the following conditions satisfy?
A. Min|dx(t)/d(t)| ≤ Δ/T
B. Max|dx(t)/d(t)| ≤ Δ/T
C. |dx(t)/d(t)| ≤Δ/T
D. None of the mentioned
Answer: B
The crosshatched areas illustrate two types of quantization error in DM, slope-overload distortion and granular noise types of quantization error in DM, slope-overload distortion, and granular noise.
Since the maximum slope A (T in x (n) is limited by the step size, slope-overload distortion can be avoided if max|dx(t)/d(t)|≤Δ/T).
28. In DM, By increasing Δ, reduces the overload distortion but increases the granular noise, and vice versa.
A. True
B. False
Answer: A
The granular noise occurs when the DM tracks a relatively flat (slowly changing) input signal. We note that increasing Δ reduces overload distortion but increases the granular noise, and vice versa.
29. Which of the following is the right way to reduce distortion in the DM?
A. By setting up an integrator in front of DM
B. By setting up an integrator behind the DM
C. By setting up an integrator in the middle of DM
D. None of the mentioned
Answer: A
We note that increasing Δ reduces overload distortion but increases the granular noise, and vice versa. One way to reduce these two types of distortion is to use an integrator in front of the DM.
30. What are the effects produced by Dm by setting up an integrator at the front of DM?
A. Simplifies the DM decoder
B. Increases correlation of the signal into the DM input
C. Emphasizes the low frequencies of x(t)
D. All of the mentioned
Answer: D
One way to reduce these two types of distortion is to use an integrator in front of the DM. This has two effects. First, it emphasizes the low frequencies of x (t) and increases the correlation of the signal to the DM input. Second, it simplifies the DM decoder because the differentiator (inverse system) required at the decoder is canceled by the DM integrator.
