ANALOG COMMUNICATION VIVA QUESTIONS WITH ANSWER SET-2
Ans: Pulse amplitude modulation, acronym PAM, is a form of signal modulation where the amplitude of a carrier consisting of a periodic train of rectangular pulses is varied in proportion to the sample values of a message signal. In this type of modulation, the pulse duration is held constant.
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Ans: Before sampling a signal, it must be passed through a low-pass filter, so that the higher frequencies are eliminated from the signal and the signal conforms to the requirement of the sampling circuit. Also, the PAM technique has the same signal to noise ratio as the AM. Thus, it is not employed in the practical circuits but may be employed to produce other forms of pulse modulation.
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Ans: Demodulation is performed by detecting the amplitude level of the carrier at every period.
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Ans: PAM is widely used in baseband transmission of digital data, with non baseband application having been largely superseded by pulse-code modulation and more recently by PPM.
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Ans: PAM is “pulse shaping”. Essentially, communications engineer realize that the shape of the pulse in the time domain can positively or negatively affect the characteristics of that pulse in frequency domain.
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Ans: In PAM, the amplitude of individual pulses in the pulse train is varied from its default value in accordance with the instantaneous amplitude of the modulating signal at sampling intervals. The width of the pulse is kept constant.
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Ans: Main advantages of PAM are simple transmitter and receiver design. PAM is used to carry information as well as to generate other pulse modulations.
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Ans: A filter is a device that passes electric signal at certain frequencies of frequency range while preventing the passage of others.
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Ans: Low-pass, high-pass, Band-pass, Band-elimination ( also referred to as band reject or notch).
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Ans: Active filters are the circuits that use an operational-amplifier as the active device in combination with some resistors and capacitors to provide an LRC-like filters performance at low frequencies.
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Ans: a low pass filter is a filter that passes low frequency signal but attenuates signal with frequencies higher than the cut-off frequency.
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Ans: Inductors are bulky and costly and may have large internal resistive components.
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Ans: An ideal low pass filter is a network which passes all the frequency up to cut-off frequency fc, and blocks all frequencies above fc.
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Ans: The transition region present in practical filters doesn’t exist in an ideal filter. An ideal low-pass filter can be realized mathematically by multiplying a signal by the rectangular function in the frequency domain or, equivalently, Convolution with a sinc function in the time domain.
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Ans: a high pass filter is a circuit that attenuates all signals below a specified cut-off frequency.
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Ans: Pass: to cause or permit to go past or through a barrier.
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Ans: It means that if you set the cut-off frequency for say 450Hz, any signal coming through to the input of the filter is 96dB down at 900 Hz. Set it for 1KHz, signals at 2KHz are down 96dB.
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Ans: Passive filters are implemented using only passive components such as resistors, capacitors and inductors. These filters do not produce any amplification of the input signal.
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Ans: The range of frequencies that is transmitted.
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Ans: The range of frequencies that is attenuated.
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Ans: in many low pass filter applications, it is necessary for the closed loop gain to be as close to 1 as possible within the baseband. Butterworth filter is best suited for this type of application.
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Ans: It passes a band of frequencies and attenuates frequencies on both sides of the pass band. You make take an example of series resonant circuit for the explanation.
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Ans: The ideal band pass filter can be used to isolate the component of a time series that lies within a particular band of frequencies.
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Ans: Band reject filters are tuned circuits that prevent the passage of signal within a specified band of frequencies. These devices are also known as band stop or notch filters.
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Ans: Twin-T & active Wein-Robinson circuit.
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Ans: Signal filters block or decrease unwanted frequencies or signal wave characteristics.
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Ans: Digital filters use digital signal processing (DSP) to perform numerical calculations on sampled values of a signal.
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Ans: It is an op-amp voltage follower, with a diode inserted between the op-amp output terminals and the circuit output point.
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Ans: In diode rectifiers circuit, the voltage drop that occurs with an ordinary semiconductor rectifier can be eliminated to give precision rectification.
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Ans: a full wave precision rectifier consists of a summing circuit and a precision half wave rectifier which has a voltage gain of 2.
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Ans: Gain of upto 100 can be achieved in this configuration, which is useful for signal conditioning of low output of transducers in mV range.
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Ans: The process of conversion of digital signal to analog signal is called so.
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Ans: 1) R-2R DAC 2) Binary weighted DAC.
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Ans: A 4-bit R-2R ladder network consists of digitally controlled switches, current to voltage converter and a resistance network.
Note: For more detail about R-2R ladder, go through your book.
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Ans: An oscillator is a semiconductor device consisting of a semiconductor specimen placed in magnetic field, and a resistor after a power supply.
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