Pulse Analog Modulation MCQ Quiz - Objective Question with Answer for Pulse Analog Modulation - Download Free PDF

Concept:

The roll-off factor (β) in a raised cosine filter represents the ratio of:

Correct Option Explanation:

The correct answer is option 4: excess bandwidth to the Nyquist bandwidth.

The raised cosine filter is a type of filter used in digital communication systems to shape the transmitted signal's spectrum, ensuring that intersymbol interference (ISI) is minimized. The roll-off factor, β, is a critical parameter in the design of the raised cosine filter. It determines the excess bandwidth used by the filter beyond the minimum required Nyquist bandwidth.

The Nyquist bandwidth is the minimum bandwidth required to transmit data without ISI, given by the symbol rate (Rs). However, practical filters cannot achieve a perfect rectangular frequency response, so additional bandwidth is introduced. This additional bandwidth is characterized by the roll-off factor (β).

Mathematically, the total bandwidth (B) of a raised cosine filter is given by:

B = Rs × (1 + β)

Where:

• Rs is the symbol rate (also known as the Nyquist rate).
• β is the roll-off factor, which ranges from 0 to 1.

The roll-off factor (β) represents the ratio of the excess bandwidth to the Nyquist bandwidth. In other words, it indicates how much additional bandwidth is introduced by the filter relative to the minimum required Nyquist bandwidth.

Additional Information 

Option 1: Excess power of system to the minimum Nyquist bandwidth

This option is incorrect because the roll-off factor (β) does not represent a ratio involving power. The roll-off factor is strictly related to bandwidth and not power. It describes the additional bandwidth used by the filter beyond the Nyquist bandwidth, not the excess power of the system.

Option 2: Additional bandwidth to total system bandwidth

This option is incorrect because the roll-off factor (β) specifically represents the ratio of excess bandwidth to the Nyquist bandwidth, not the total system bandwidth. The total system bandwidth includes the Nyquist bandwidth and the excess bandwidth. Therefore, this option does not accurately describe the roll-off factor.

Option 3: Total available bandwidth to the Nyquist bandwidth

This option is incorrect because the roll-off factor (β) represents the ratio of excess bandwidth to the Nyquist bandwidth, not the total available bandwidth. The total available bandwidth is the sum of the Nyquist bandwidth and the excess bandwidth, so this option does not accurately describe the roll-off factor.

Concept:

Pulse Position Modulation (PPM)

• Pulse Position Modulation (PPM) is a form of signal modulation where the message information is encoded in the timing of a series of signal pulses.
• In PPM, the position of each pulse, relative to the position of a clock pulse, is varied according to the sampled value of the message signal.
• PPM is used in applications where the presence of the signal is more important than the amplitude or width, such as in optical communication systems.

Explanation of the Correct Option:

The correct option is Option 1: position of the pulse.

In Pulse Position Modulation (PPM), the information is encoded in the position of the pulse. This means that the time at which the pulse occurs is varied to represent the information being sent. Here’s a detailed explanation:

1. PPM Basics: PPM involves the transmission of pulses at different positions within a given time frame. The exact position of each pulse within the frame represents the data being communicated. This method is particularly useful in environments where signal timing can be precisely controlled and detected.

2. How It Works: In PPM, a reference clock or synchronization pulse is used as a baseline. The position of each pulse relative to this reference is adjusted to convey the information. For example, if we are encoding digital data, a pulse might be shifted to one position for a binary '0' and another position for a binary '1'. In analog applications, the pulse position might vary continuously to represent the amplitude of the analog signal.

3. Advantages of PPM: One of the main advantages of PPM is its robustness to amplitude variations and noise. Since the information is conveyed through the timing of the pulses rather than their amplitude or width, PPM signals are less affected by amplitude noise and can be more easily detected in noisy environments. This makes PPM particularly suitable for optical communication systems, where signal strength can vary due to atmospheric conditions.

4. Applications: PPM is widely used in optical communication systems, such as fiber-optic communication and free-space optical communication, where the precise timing of light pulses can be accurately controlled and detected. It is also used in some radio communication systems and in telemetry for encoding sensor data.

5. Implementation: Implementing PPM requires precise timing control and detection mechanisms. The transmitter must be able to generate pulses at exact positions within the time frame, and the receiver must be able to accurately measure the arrival time of each pulse to decode the information. This often involves the use of high-speed clocks and synchronization techniques to ensure accurate timing.

Overall, the position of the pulse is the key parameter in PPM that carries the information, making Option 1 the correct choice.

Important Information

To analyze why the other options are incorrect, let’s look at each one in detail:

Option 2: power of the pulse

• In PPM, the power of the pulse does not convey the information. The power of the pulse remains constant, and it is the timing or position of the pulse that changes to encode the data.
• Power modulation is not a characteristic of PPM; rather, it is used in other modulation schemes such as Pulse Amplitude Modulation (PAM), where the amplitude (and therefore power) of the pulse is varied.

Option 3: amplitude of the pulse

• In PPM, the amplitude of the pulse is not used to encode information. The amplitude of the pulses remains constant, and it is their position within the time frame that varies to represent the data.
• Amplitude modulation is characteristic of schemes like PAM, where the amplitude of the pulses is varied according to the signal being transmitted.

Option 4: width of the pulse

• In PPM, the width of the pulse does not carry the information. The width of the pulses remains constant, and it is their position in time that is modulated.
• Pulse Width Modulation (PWM) is a different modulation scheme where the width of the pulses is varied to represent the signal. In PPM, however, the pulse width is not relevant to the encoding of information.

In conclusion, Pulse Position Modulation (PPM) encodes information in the position of the pulses, making Option 1 the correct choice. The other options—power, amplitude, and width of the pulse—are characteristics of different modulation schemes and do not apply to PPM.

Concept:

A Raised Cosine Filter is a type of filter used in digital communication systems to shape the transmitted signal spectrum and reduce intersymbol interference (ISI).

The filter is characterized by a roll-off factor \( \beta \) and a symbol rate Rs.

Definition:

The roll-off factor \( \beta \) defines how much excess bandwidth is used beyond the Nyquist bandwidth (Rs/2)

The roll-off factor lies between \( 0 \leq \beta \leq 1 \), where:

• β = 0: Ideal brick-wall filter (practically unrealizable)
• β = 1: Maximum bandwidth usage (100% excess)

Formula for Bandwidth:

The total bandwidth (BW) of a raised cosine filter is given by:

\( BW = \frac{R_s}{2}(1 + \beta) \)

Where:

• \( R_s \) = symbol rate
• \( \beta \) = roll-off factor

The correct answer is option 3):  (A monostable multivibrator)

Concept:

The Pulse Width Modulation sends a narrow or a wide pulse within the interval, and the width of the pulse is the value.
Pulse Position Modulation sends a fixed amplitude pulse, but the timing of the pulse within the interval is the value.

Pulse width modulation may be generated by applying trigger pulses (at the sampling rate) to control the starting time of pulses from a monostable miltivibrator, and feeding in the signal to be sampled to control the duration of the pulses. The circuit diagram for such an arrangement is shown in Fig

The emitter-coupled monostable multivibrator shown in Fig makes an excellent voltage to time converter, since its gate width is dependent on the voltage to which the capacitor C is charged. If this voltage is varied in accordance with a signal voltage, a series of rectangular pulses are obtained,with widths varying as required. The circuit performs two functions; it samples, and converts this sample in Pulse Width Modulation

Solution

Aperture Effect

• During Flat top sampling, Sinc pulses are used to convert varying amplitudes of pulses to flat top pulses
• As a result, there is a decrease in the amplitude of the modulated signal
• This distortion in pulse modulated signal is known as the aperture effect
• This distortion is corrected by connecting an equalizer in cascade with a low pass reconstruction filter  
• The equalizer has the ability to decrease the in-band loss of reconstruction filters as the frequency increases in order to compensate for the aperture effect

The distortion in the pulse modulation scheme results in aperture effect , is corrected by equalizer

Therefore the correct answer is option 1

PPM (Pulse Position Modulation): 

Pulse Position Modulation (PPM) is an analog modulating scheme in which the amplitude and width of the pulses are kept constant, while the position of each pulse, with reference to the position of a reference pulse, varies according to the instantaneous sampled value of the message signal. This makes it similar to a Phase modulation.

Important Points

PAM (Pulse Amplitude Modulation):  In analog modulation, if the amplitude of a pulse or duration of a pulse is varied according to the instantaneous values of the baseband modulating signal, then such a technique is called as Pulse Amplitude Modulation (PAM). This is similar to AM modulation in which the carrier amplitude varies according to the modulating signal.

PWM (Pulse Width Modulation): It is a process in which output signals of low frequency are generated from the input pulse of high frequency. This is similar to the FM modulation technique.

PCM (Pulse Code Modulation): It is a technique by which an analog signal gets converted into digital form to have signal transmission through a digital network. ∴ This belongs to a digital modulation scheme.

Concept:

Duty cycle is defined mathematically as:

\(Duty\;cycle = \frac{{{T_{on}}}}{{{T_{on}} + {T_{off}}}}=\frac{T_{on}}T{}\) ------(1)

Where,

T: Total Time (On-Time + Off-time) and defined as:

\(T=\frac{1}{f}\) ------(2)

f: Pulse frequency

Calculation:

Given:

f = 1Mhz

Duty cycle = 90%

From equation 1 and 2,

Duty cycle = T_On x f

0.90 = T_on x 1Mhz

T_on = 900 nsec

Hence, option 1 is correct.

Concept:

The efficiency of a modulation scheme is defined as:

\(Efficiency = \frac{{Sideband\;power}}{{Total\;power}}\)

Mathematically, the power efficiency of an AM signal with single tone modulation is given by:

\({η _{AM}} = \frac{{{μ ^2}}}{{2 + {μ ^2}}}\)

μ = Modulation index

Calculation:

With μ = 0.8, the efficiency will be:

\({η _{AM}} = \frac{{{0.8^2}}}{{2 + {0.8 ^2}}}=\frac{0.64}{2.64}\)

η = 24.24 %

Analysis:-

Pulse modulation:

• In pulse modulation methods the carrier signal consists of a pulse train. Some parameter of which is varied according to the instantaneous value of the modulating signal.
• It can be of two types digital or analog.

Pulse Analog modulation:

• Some parameter of the pulse carrier signal is varied according to the instantaneous value of the analog modulating signal.
• It is of 3 types PAM, PWM, and PPM

PAM: It stands for Pulse Amplitude Modulation. The amplitude of the pulses of the carrier pulse train is varied according to the modulating signal.

PWM: It stands for Pulse Width Modulation. The width of the pulses of the carrier pulse train is varied according to the modulating signal.

PPM: It stands for Pulse Position Modulation. The position of the pulse relative to its un-modulated time occurrence is varied according to the message signal.

In PPM, data transmitted in short pulses have the same width and amplitude. The PPM changes the delay between pulses hence PPM needs synchronisation.

Comparison of different Pulse Modulation Techniques:

PWM :

It stands for pulse width modulation.

In this narrow or wide pulse is sent which means pulse width is varied.

For the generation of PWM 

• Comparator
• MonoStable Vibrator

Hence correct option is "4"

Different types of Pulse modulation are shown as follows:

PPM:

• In PPM, data transmitted in short pulses have the same width and amplitude.
• The PPM changes the delay between pulses hence PPM needs synchronization.

​

Comparison of different Pulse Modulation Techniques:

Pulse Width Modulation (PWM) or Pulse Duration Modulation (PDM) or Pulse Time Modulation (PTM) is an analog modulating scheme in which the duration or width or time of the pulse carrier varies proportional to the instantaneous amplitude of the message signal.

The width of the pulse varies in this method, but the amplitude of the signal remains constant. Amplitude limiters are used to make the amplitude of the signal constant. These circuits clip off the amplitude, to a desired level and hence the noise is limited.

The main difference between analog and digital modulation is that analog modulation is a continuous signal which implies the signal changes with time. On the other hand, digital modulation is a discrete signal which means it will carry only binary information.

There are three types of analog pulse modulation techniques:

1. PAM( Pulse Amplitude Modulation): An analog modulation scheme in which the amplitude of the pulse carrier varies which is proportional to the instantaneous amplitude of the message signal.

2. PWM(Pulse width modulation):  An analog modulation scheme in which the width or the duration of the time of the pulse carrier varies which is proportional to the instantaneous amplitude of the message signal.

3. PPM ( Pulse position modulation):  An analog modulation scheme in which the position of the reference pulse varies according to the instantaneous sampled value of the message signal.

Additional Information

Digital Modulation techniques:

1. DPCM( Differential pulse code modulation): DPCM is a technique of analog to digital signal conversion which samples an analog signal and then quantizes the difference between the sampled value and its predicted value, then encodes the signal to form a digital value.

2. DM( Delta Modulation): DM is an easy way of DPCM in which the data which is to be transmitted is minimized to a 1-bit data stream.

3. ADPCM( Adaptive differential pulse code modulation): ADPCM is a technique used to convert analog signals to binary signals by taking frequent samples of the sound and expressing the value of the sampled modulation in binary form.

Thus from the above definitions, we can conclude that PAM doesn't come under a digital scheme.

Conclusion: option 4 is correct.

Concept:

A Raised Cosine Filter is a type of filter used in digital communication systems to shape the transmitted signal spectrum and reduce intersymbol interference (ISI).

The filter is characterized by a roll-off factor \( \beta \) and a symbol rate Rs.

Definition:

The roll-off factor \( \beta \) defines how much excess bandwidth is used beyond the Nyquist bandwidth (Rs/2)

The roll-off factor lies between \( 0 \leq \beta \leq 1 \), where:

• β = 0: Ideal brick-wall filter (practically unrealizable)
• β = 1: Maximum bandwidth usage (100% excess)

Formula for Bandwidth:

The total bandwidth (BW) of a raised cosine filter is given by:

\( BW = \frac{R_s}{2}(1 + \beta) \)

Where:

• \( R_s \) = symbol rate
• \( \beta \) = roll-off factor