Short Questions and Answers Sampling Theory and Pulse Analog Modulation

Q.1. What is pulse position modulation (PPM)?

Ans. In PPM, the position of a pulse relative to its unmodulated time of occurrence is varied in accordance with the message signal.

Q.2. What is Pulse Amplitude Modulation (PAM)?

Ans. The process in which amplitudes of regularly spaced rectangular pulses vary with the instantaneous sample values of a continuous message signal in a one to one fashion is known as pulse amplitude modulation.

Q.3. What do you mean by Nyquist rate?

Ans. fs ≥ 2fm i.e., sampling rate greater than or equal to twice of the message bandwidth or highest frequency component of the message signal (fm) is called Nyquist rate.

Q.4. Define Nyquist rate.

Ans. It is the minimum sampling rate required to represent the continuous time signal faithfully in its sampled form.

Nyquist rate = 2fm.

Where fm is the highest frequency component in the signal to be sampled.

Q.5. What do you mean by aperture effect?

Ans. During flat top sampling, to convert varying amplitudes of pulse to flat top pulses we use a sin c function. Because of this, there would be decrease in the amplitude. This distortion is named as Aperture effect. This may be eliminated by using an equalizer in cascade with the output low pass filter.

Q.6. What is sampling?

Ans. The process of converting an analog signal into a discrete signal or making an analog or continuous signal to occur at a particular interval of time is known as sampling.

Q.7. What do you mean by Pulse-Time Modulation?

Ans. The modulation technique in which the time (or) duration of the pulses is varied in accordance with the amplitude of the message signal keeping the amplitude of the pulse constant, is referred to as pulse-time modulation.

Q.8. Define Nyquist rate.

Ans. It is the minimum sampling rate required to avoid the aliasing.

Nyquist rate = fs min = 2W Hz.

Q.9. What are the different types of PTM systems?

Ans. There are two kinds of Pulse-time modulation schemes. They are:

(i) Pulse duration (or) pulse width (or) pulse length modulation (PDM (or) PWM (or) PLM)

(ii) Pulse position modulation

Q.10. What is Pulse duration modulation (PDM)?

Ans. The method in which the samples of the message signal are used to vary the duration (or) width of the individual pulses. This is referred to as pulse duration modulation.

Q.11. What is aliasing?

Ans. Aliasing is the error or distortion introduced due to overlapping of adjacent spectrums when the sampling is performed at a rate which is lower than the Nyquist rate.

Q.12. Name the analog pulse modulation system.

Ans. PAM, PWM and PPM are pulse analog modulation systems.

Q.13. Name some of the digital pulse modulation systems.

Ans. PCM, delta modulation (DM) and adaptive delta modulation (ADM) are digital pulse modulation systems.

Q.14. Why is PAM not used in practice?

Ans. As PAM does not have a high noise immunity.

Q.15. What is the difference between pulse analog modulation and other analog modulation systems?

Ans. In the analog pulse modulation system, the carrier is a train of pulse and either amplitude or width or position of this carrier is varied in proportion to the amplitude of the message signal.

Q.16. Define the sampling process.

Ans. It is a process of converting a continuous time signal to an equivalent discrete time signal.

Q.17. What is Guard band?

Ans. When the sampling rate is chosen much higher than the Nyquist rate then a small space occurs between the samples. This space is said to be Guard band.

Q.18. How to recover the original modulating signal back from the sampled signal?

Ans. By passing the sampled signal through a low pass filter or adequate cut off frequency.

Q.19. Why is noise immunity of PWM is better than that of PAM?

Ans. Because noise modifies the amplitude of PWM signal and the information which is contained in the width variation remains unaffected.

Q.20. How do you transmit information using PWM?

Ans. The information is contained in the width variation of the carrier in a PWM system. The width of carrier pulses is varied in proportion to the instantaneous amplitude of the message signal.

Q.21. Define a band pass signal.

Ans. If the frequency spectrum of a signal is centered around a non-zero frequency fc, then the signal is called as a band pass signal.

Q.22. Does the transmitted power vary in PAM and PPM?

Ans. In PAM, it does vary but in PPM, it is constant.

Q.23. Define a low pass signal.

Ans. If the frequency spectrum of a signal extends from 0 to W Hz, then, it is called as a low pass signal.

Q.24. In PPM, the information is contained in……….

Ans. Relative position of pulses.

Q.25. Why should the sampling rate be greater than 2W?

Ans. Because then guard bands are produced, in the frequency spectrum which allows us to use practical filters.

Q.26. What are the merits and demerits of PPM transmission?

Ans. Merits are high noise immunity, possible to use repeaters, power remains constant while the demerits are synchronization is necessary and large bandwidth.

Q.27. What is aliasing?

Ans. It is the signal distortion caused due to the overlapping of adjacent spectrums of a sampled signal if the sampling frequency is less than the Nyquist rate.

Q.28. Why is the PAM signal contaminated by noise?

Ans. Due to the additive noise, the shape of the PAM signal will get modified. As the PAM signal contains the information in its amplitude variations, the noise will contaminate the information.

Q.29. What is undersampling?

Ans. Undersampling is sampling at a rate which is lower than Nyquist rate.

Q.30. PPM pulses are derived form………

Ans. PWM pulses.

Q.31. What are the demerits of PWM transmission?

Ans. Larger bandwidth requirement and fluctuating transmitted power.

Q.32. What is Nyquist interval?

Ans. It is defined as the reciprocal of Nyquist rate.

Q.33. State an example of a bandpass signal.

Ans. The audio signals occupy the frequency band from 20 Hz to 20 kHz. So, it is a bandpass signal.

Q.34. How to avoid aliasing?

Ans. By using antialiasing filter and by increasing the sampling rate above the Nyquist rate.

Q.35. How to recover the original signal from the flat top smapled signal?

Ans. By sing a low pass filter (LPF).

Q.36. The bandwidth of a PWM signal is……..than a PAM signal.

Ans. Larger.

Q.37. What is meant by a time limited signal?

Ans. It is a signal which is present only over a finite interval of time is called as a time limited signal.

Q.38. State an example of time limited signal.

Ans. A rectangular pulse.

Q.39. Out of natural PAM and flat topped PAM, the practically preferred one is….

Ans. Flat topped PAM.

Q.40. What is a band limited signal?

Ans. It is a signal, the spectrum of which extends over a finite frequency range.

Q.41. The shape of the carrier used for analog pulse modulation is….

Ans. Train of rectangular pulses.

Q.42. The channel noise modifies ……… of the modulated signal.

Ans. Amplitude.

Q.43. State the advantages of PWM.

Ans. The advantages of PWM are better noise immunity, no need of synchronization, it is possible to reconstruct the PWM signal from the noise contaminated signal.

Q.44. PWM signal carries all the information in its….

Ans. Width variations.

Q.45. How to recover the original modulating signal for PAM signal?

Ans. The original modulating signal can be recovered from a PAM signal by passing the PAM signal through a low pass filter (LPF).

Q.46. In all the analog pulse modulation systems the minimum carrier frequency should be….

Ans. Higher than the Nyquist rate.

Q.47. PAM is similar to…………

Ans. Amplitude modulation.

Q.48. The bandwidth of PPM signal is……….

Ans. Always constant

Q.49. The analog pulse modulation having the least practical utility is………..

Ans. PAM.

Q.50. What are the types of PAM?

Ans. The PAM signal can be naturally sampled PAM or it can be flat top sampled PAM signal. Moreover the PAM signal can be single polarity PAM or double polarity PAM.

Q.51. What is the minimum bandwidth required to transmit a PAM signal?

Ans. It is BT ≥ 1/2τ, where, τ is the width of each PAM pulse.