November 30, 2017

Srikaanth Kumar

Analog Communication Freshers Advanced Interview Questions And Answers


Define Pam And Write Down Its Drawbacks?

Pulse Amplitude Modulation is the process by which the amplitude of the regularly spaced pulses varies according to the the amplitude of the modulating signal.

The drawbacks are:

Since the amplitude of the pulses varies therefore the peak power of the modulating s/g is much greater.

The bandwidth required for transmitting is greater since the amplitude varies.

What Is Pulse Position Modulation?

Pulse position modulation (PPM) is the process in which the position of a standard pulse is varied as a function of the amplitude of the sampled signal.


What Is The Advantage Of Ppm Over Pwm And Pam?

The phase deviation are usually small. The noise produces a smaller disturbing effect on the time position of the modulating pulse train and as a result, PPM waves have a better performance with respect to signal to noise ratio in comparison to PAM and PWM systems.

What Are The Applications Of Pulse Position Modulation?

It is primarily useful for optical communication systems, where there tends to be little or no multipath interference. Narrowband RF (Radio freuency) channels with low power and long wavelength (i.e., low freuency) are affected primarily by flat fading, and PPM is better suited.

What Is The Purpose Of Using Differential Pulse Position Modulation?

It is possible to limit the propagation of errors to adjacent symbols, so that an error in measuring the differential delay of one pulse will affect only two symbols, instead of effecting all successive measurements.

What Are The Advantage Of Ppm?

One of the principle advantages of pulse position modulation is that it is an Mary modulation techniue that can be implemented noncoherently, such that the receiver does not need to use a phaselocked loop (PLL) to track the phase of the carrier.

This makes it a suitable candidate for optical communications systems, where coherent phase modulation and detecting are difficult and extremely expensive. The only other common Mary noncoherent modulation techniue is Mary freuency shift keying, which is the freuency domain dual to PPM.

The other advantages of pulse position modulation are:

The amplitude is held constant thus less noise interference.
Signal and noise separation is very easy.
Due to constant pulse widths and amplitudes, transmission power for each pulse is same.

How Can Be Aliasing Be Avoided?

Aliasing can be avoided if:

Sampling frequency must be greater than the frequency of the modulating signal.
The frequency should be band limited to maximum frequency of the signal(fm) Htz.
If prealias filter is used.

State The Advantages Of Super Heterodyning?

The advantages are:

High selectivity and sensitivity.
No change in Bandwidth that is bandwidth remains same all over the operating range.
High adjacent channel rejection.

What Do You Mean By Fm And Classify Fm?

Frequency Modulation can be defined as the frequency of the carrier (wc) is varied acc. to the modulating signal about an unmodulated frequency.

FM are of 2 types:

Narrowband FM
Wideband FM

What Do You Mean By Nyquist Rate?

In case of Nyquist rate, the sampling frequency is equal to the maximum frequency of the signal and therefore the successive cycles of the spectrum does not overlap.

What Is Amplitude Modulation?

Amplitude Modulation is defined as the process in which the instantaneous value of the amplitude of the carrier is varied according to the amplitude of the modulating or base band signal.

What Is Modulation? What Happens In Over Modulation?

Modulation is defined as the process in which some characteristics of the signal called carrier is varied according to the modulating or baseband signal. For example – Amplitude Modulation, Phase Modulation, Frequency Modulation.

In case of over modulation, the modulation index is greater than one and envelope distortion occurs.

What Is Multiplexing? Name The Types Of Multiplexing?

Multiplexing is defined as the process in which a number of message signals are combined together to form composite signals so that they can be transmitted through the common channel.

The two types of multiplexing are:

Frequency Division Multiplexing: In this technique, fixed frequency bands are allotted to every user in the complete channel bandwidth. Such frequency is allotted to user on a continuous basis.
Time Division Multiplexing: When the pulse is present for the short time duration and most of the time their is no signal present inbetween them than this free space between the two pulses can occupied by the pulses from other channels. This is known as Time Division Multiplexing.

What Is Sampling? What Is Sampling Theorem?

Sampling is defined as the process in which an analog signals are converted into digital signals. It means that a continuous time signal is converted into a discrete time signal.

Sampling Theorem is defined as : ’The continuous time signal that can be represented in its samples and recovered back if the sampling frequency (fs) is greater than the maximum frequency of the signal (fm) that is fs >2fm’.

What Is Under Sampling?

Under sampling is also known as aliasing effect in which the the sampling frequency is less than the maximum frequency of the signal and therefore the successive cycles of the spectrum overlap.

What Is Modulation?

Modulation may be defined as the process by which some parameters of a high freuency signal termed as carrier, is varie in accordance with the signal to be transmitted.

What Are The Different Types Of Analog Modulation?

Amplitude modulation

angle modulation.

What Si The Need For Modulation?

Consider, for example, picture signal of a T.V camera. It has freuency spectra of DC to 5.5MHz.such a wide band of freuency can’t be propagated through ionosphere. However, if this signal is modulated with a carrier in VHF and UHF range, the percentage bandwidth becomes very small and the signal become suitable for transmission through atmosphere.

What Are The Objectives Met By Modulation?

Length of antenna is shortened, signal loss is reduced, ease of radiation, adjustment of bandwidth, shifting signal freuency of the assigned value.

What Are The Advantage Of Pam And Pwm?

PWM system gives a greater signal to noise ratio as compared to PAM but reuires a larger bandwidth to achieve this.

What Are The Application Of Ppm?

PPM is employed in narrowband RF channel systems, with the position of each pulse representing the angular position of an analogue control on the transmitter, or possible states of binary switch. The number of pulse per frame gives the number of controllable channels available. The advantage of using PPM for this type of application is that the electronics reuired to decode the signal are extremely simple, which leads to small, lightweight receiver/decoder units. (Model aircraft reuire parts that are as lightweight as possible).

Explain The Principle Of Ppm?

The amplitude and the width of the pulse is kept constant in this system, while the position of each pulse, in relation to the position of a recurrent reference pulse is varied by each instantaneous sampled value of the modulating wave. This means that the transmitter must send synchronizing pulses to operate timing circuits in the receiver. The PPM has the advantage of reuiring constant transmitter power output, but the disadvantage of depending on transmitterreceiver synchronization.

What Is The Puprpose Of Ppm?

PPM may be used to transmit analog information, such as continuous speech or data.

What Are The Analog Analogies Of Pam, Ppm & Pwm?

PAM is similar to AM; PPM and PWM is similar to angle modulation.

What Is Freuency Modulation (fm)?

Freuency modulation is the process of varying the freuency of a carrier wave in proportion to the instantaneous amplitude of the modulating signal without any variation in the amplitude of the carrier wave.

What Is Pwm Or Pulse Length Modulation Or Pilse Duration Modulation?

In PWM, the pulse amplitude is kept constant but the leading edge, trailing edge or both may be varied as a function of the amplitude of the sampled signal and care must be taken to ensure that the pulse don’t overlap in a TDM system.

What Are The Disadvantages Of Pwm?

PWM, in general, reuires a greater average power than PAM systems. Also, the PWM system reuires a greater bandwidth than PAM.

Explain The Principle Of Pwm?

Pulsewidth modulation (PWM) of a signal or power source involves the modulation of its duty cycle, to either convey information over a communication channel or control the amount of power sent to a load. PWM uses a suare wave whose pulse width is modulated resulting in the variation of the average value of the waveform is directly dependent on the duty cycle D.

Mention The Applications Of Pwm?

PWM can be used to reduce the total amount of power delivered to a load without losses normally incurred when a power source is limited by resistive means. This is because the average power delivered is proportional to the modulation duty cycle. With a sufficiently high modulation rate, passive electronic filters can be used to smooth the pulse train and recover an average analog waveform.



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