This set of Software Defined Radio Multiple Choice Questions & Answers (MCQs) focuses on “ADC Tradeoffs – 1”.
1. Which of the following is the first step in establishing acceptable aliasing power?
a) Establish the relation between CIR and BER
b) Fix BER threshold
c) Convert BER threshold to CIR threshold
d) Apply the instantaneous dynamic range requirement of ADC
View Answer
Explanation: The first step in establishing acceptable aliasing power is to fix the BER threshold based on QOS requirements of the waveform. The second step is to establish a relationship between CIR and BER. Then the BER threshold is converted to CIR threshold. The fourth and the final step is to apply the instantaneous dynamic range requirement of ADC.
2. Noise must be less than _____ of the ADC.
a) half of the LSB
b) LSB
c) half of the MSB
d) MSB
View Answer
Explanation: The effect of aliasing, spurious response in IF/RF processing and other noise must be less than half of the least significant bit of the ADC. If this condition is satisfied, then all the bits of ADC give only the useful signal power.
3. To avoid aliasing, the wideband ADC is ____ by anti-aliasing filters to suitably alter the _____ waveform.
a) succeeded, digital
b) preceded, digital
c) succeeded, analog
d) preceded, analog
View Answer
Explanation: Aliasing prevents faithful restoration of the signal. To avoid aliasing, the wideband ADC is preceded by anti-aliasing filters to suitably alter the analog waveform. The filters must have satisfactory stop-band attenuation.
4. The shape factor of an anti-aliasing filter is given by the ratio of the frequency at ___ dB attenuation point to the frequency at ___ dB point.
a) 80, 3
b) 3, 80
c) -80, -3
d) -3, -80
View Answer
Explanation: Anti -aliasing filters require large shape factor for suppression of signals closer to the upper band-edge of ADC passband. The shape factor of an anti-aliasing filter is given by the ratio of the frequency at -80 dB attenuation point to the frequency at -3 dB point.
5. Which among the following filters has the slowest roll-off?
a) Cauer
b) Chebychev
c) Butterworth
d) Bessel
View Answer
Explanation: Bessel function has the slowest roll-off factor among the four filters. The approximate value of shape factor is 6. It is a monotonic filter.
6. Which of the following expressions give the relation between power and quantization step size (q)?
a) p=q2/2R
b) p=q/2R
c) p=q2/R
d) p=q2*2R
View Answer
Explanation: p=q2/2R is the relation between power and quantization step size. R is the input resistance.
7. Which of the following expressions give the SNR at the output of ADC?
a) SNR=6.02*B+1.76+10log(fs/2fmax)
b) SNR=6.02*B+1.76+10log(2fmax/fs)
c) SNR=12.04*B+1.76+10log(2fmax/fs)
d) SNR=6.02*B+1.76
View Answer
Explanation: SNR=6.02*B+1.76+10log(fs/2fmax) is the expression for SNR at the output of ADC. B is the number of bits. fs is the sampling frequency. fmax is the maximum frequency component of the signal.
8. In non-monotonic filters, attenuation increases with frequency.
a) True
b) False
View Answer
Explanation: In monotonic filters, attenuation increases with frequency. Non-monotonic filter has less attenuation. It allows energy beyond the passband and exhibits phase distortion.
9. Which of the following is untrue about Sigma-Delta ADC?
a) fs>2fmax
b) Zero crossing detector consists of a sample and hold circuit
c) Aperture jitter is minimized due to integration
d) The integrating filter is preceded by a zero crossing detector
View Answer
Explanation: Sigma-Delta ADC does not require a sample and hold circuit. The zero-crossing detector can be implemented by threshold logic.
10. Quadrature sampling uses ____ to ____ the bandwidth available for a particular sampling rate.
a) integration, quadruple
b) complex numbers, quadruple
c) complex numbers, double
d) integration, double
View Answer
Explanation: Quadrature sampling uses complex numbers to double the bandwidth available for a particular sampling rate. It is a polyphase filter. Real signals are combined with cosine and sine reference signal generated by a local oscillator.
11. ____, ____ and ____ must be properly matched for effective representation of input signal in quadrature sampling.
a) Modulators, signal paths, correlator
b) Signal paths, modulators, zero crossing detector
c) Low pass filters, zero crossing detector, correlator
d) Low pass filters, modulators, signal paths
View Answer
Explanation: Low pass filters, modulators,and signal paths must be properly matched for effective representation of input signal in quadrature sampling. Amplitude mismatch, inconsistent time duration in filters may lead to degradation of complex signal.
12. Which of the following is not a characteristic of IC implementation of quadrature sampling ADC?
a) Thermal difference is less than lumped components
b) Matching of passive component on IC substrate is easy
c) Matching of passive component on IC substrate is difficult
d) Thermal difference among filters is not large
View Answer
Explanation: The integrated circuit implementation of quadrature sampling ADC is effective because of short signal paths, easy matching of passive components on IC substrate and low thermal difference among filters.
13. In Nyquist sampling, the ____ frequency is ______to the duration of the observation interval.
a) maximum recoverable, proportional
b) minimum resolvable, inversely proportional
c) maximum recoverable, inversely proportional
d) minimum resolvable, proportional
View Answer
Explanation: In Nyquist sampling, the maximum recoverable frequency is given by the expression fs/2. The minimum resolvable frequency is inversely proportional to the duration of the observation interval. The observation interval is given by the number of time domain points.
14. Which of the following is not a characteristic of DAC?
a) Linearity
b) Level conversion
c) Grounding
d) Sample and hold circuit
View Answer
Explanation: DAC does not use sample and hold circuit. However it does make use of setup and hold circuit which is similar to sample and hold circuit. Linearity, level conversion, grounding, and isolation of digital from analog waveform are characteristics of a DAC.
15. The principle behind Sigma-Delta ADC is oversampling.
a) True
b) False
View Answer
Explanation: It can be observed from the expression SNR=6.02*B+1.76+10log\((\frac{f_s}{2f_{max}})\), as the sampling frequency increases, SNR increases. This is the principle behind Sigma-Delta ADC. The amount of oversampling is given by the ratio of sampling frequency to Nyquist frequency.
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