What is digital modulation? Describe briefly its methods.

In digital modulation, an analog carrier signal is modulated by a digital bit stream. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet).

Fundamental digital modulation methods

These are the most fundamental digital modulation techniques:

• In the case of PSK, a finite number of phases are used.
• In the case of FSK, a finite number of frequencies are used.
• In the case of ASK, a finite number of amplitudes are used.
• In the case of QAM, a finite number of at least two phases, and at least two amplitudes are used.

In QAM, an inphase signal (the I signal, for example a cosine waveform) and a quadrature phase signal (the Q signal, for example a sine wave) are amplitude modulated with a finite number of amplitudes, and summed. It can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK an

In digital modulation, an analog carrier signal is modulated by a digital bit stream. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet).

Fundamental digital modulation methods

These are the most fundamental digital modulation techniques:

• In the case of PSK, a finite number of phases are used.
• In the case of FSK, a finite number of frequencies are used.
• In the case of ASK, a finite number of amplitudes are used.
• In the case of QAM, a finite number of at least two phases, and at least two amplitudes are used.

In QAM, an inphase signal (the I signal, for example a cosine waveform) and a quadrature phase signal (the Q signal, for example a sine wave) are amplitude modulated with a finite number of amplitudes, and summed. It can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK and ASK.

In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits. Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises the symbol that is represented by the particular phase.

If the alphabet consists of M = 2^N alternative symbols, each symbol represents a message consisting of N bits. If the symbol rate (also known as the baud rate) is f_S symbols/second (orbaud), the data rate is Nf_S bit/second.

For example, with an alphabet consisting of 16 alternative symbols, each symbol represents 4 bits. Thus, the data rate is four times the baud rate.

In the case of PSK, ASK or QAM, where the carrier frequency of the modulated signal is constant, the modulation alphabet is often conveniently represented on a constellation diagram, showing the amplitude of the I signal at the x-axis, and the amplitude of the Q signal at the y-axis, for each symbol.

d ASK.

In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits. Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises the symbol that is represented by the particular phase.

If the alphabet consists of M = 2^N alternative symbols, each symbol represents a message consisting of N bits. If the symbol rate (also known as the baud rate) is f_S symbols/second (orbaud), the data rate is Nf_S bit/second.

For example, with an alphabet consisting of 16 alternative symbols, each symbol represents 4 bits. Thus, the data rate is four times the baud rate.

In the case of PSK, ASK or QAM, where the carrier frequency of the modulated signal is constant, the modulation alphabet is often conveniently represented on a constellation diagram, showing the amplitude of the I signal at the x-axis, and the amplitude of the Q signal at the y-axis, for each symbol.

What are the applications of DSB-SC modulation?

Applications:

- Analogue TV systems: to transmit color information.

- For transmitting stereo information in FM sound broadcast at VHF

- One important application of DSB is the transmission of color information in a TV signal.

- CB radio

- TV broadcasting

- Air traffic control radios

- Garage door opens keyless remotes

- DSB-SC is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave.

- DSB-SC used in stereo transmission of FM radio.

- Two way radio communications.

What is sidebands ?

sideband: One of two frequency bands on either side of the carrier wave, containing the modulated signal.

In electronic signal transmission, a sideband is the portion of a modulated carrier wave that is either above or below the basic ( baseband ) signal. The portion above the baseband signal is the upper sideband ; the portion below is the lower sideband . In regular amplitude modulation ( AM ) transmission, both sidebands are used to carry a message. In some forms of tranmission, one sideband is removed ( single-sideband transmission ) or a portion of one sideband is removed.

Write down the difference between AM,FM and Digital signals?

Write Down the comparison between STS & TST networks?

Comparison between STS & TST networks:

What is STS switching? Write down important features of our STS.

Static Transfer Switch (STS): Static Transfer Switch (STS) is an electrical device that switches between two power sources without delay.

Based on Current & Voltage capacity of STS, it is classified as Low Voltage STS (LV-STS) & Medium Voltage STS (MV-STS). LV-STS have current rating from 50A to 4000A & voltage up to 600V.

Some of the important features of our STS are as follow:

·         Increased power quality

·         Easy monitoring all parameters on LCD display

·         Fast microcontroller. Power blackout protection

·         Automatic static switching. Remote monitoring of input power sources

·         Easy static and mechanical transfer between separate input sources

·         Remote management of power events. Power event logging

·         Advanced RS232 communication features. DRY contact alarm interface

·         Password protected login system from remote site (timed Access)

·         2 redundant power supplies form electronic boards (hot swappable)

·         Easy front access to all components inside of the STS

·         Second protection cover on live circuits which prevents electrical shock

·         Input sources protected by fuses

·         3 positioned Maintenance bypass switch which prevents cross currents between input sources

·         User adjustable parameters by entering a password. Built in real time clock

·         history (with their date and time)

·         Automatic transfer test from a remote site or using front panel. Front panel Lamp test

·         External emergency shutdown (EPO) input

·         Hot plug construction during maintenance bypass

·         High current output tolerant up to 1000%.

·         Temperature sensor inside the Cabinet

What is QAM?

Quadrature Amplitude Modulation: A modulation technique that generates four bits out of one baud. For example, a 600 baud line (600 shifts in the signal per second) can effectively transmit 2,400 bps using this method. Both phase and amplitude are shaped with each baud, resulting in four possible patterns. Using two carriers out of phase by 90° and modulated by separate signals.

What are the advantages of single sideband transmission?

The advantages of SSBSC are

1.Power conservation: Normally ,with single side band transmission ,only one sideband is transmitted and the carrier is suppressed. So less power is required to produce essentially the same quality signal.

2.Bandwidth conservation: Single sideband transmission requires half as much bandwidth as conventional AM double side band transmission.

3.Noise reduction: Because a single side band system utilizes half as much bandwidth as conventional AM, the thermal noise power is reduced to half that of a double side band system.

What are the disadvantages of conventional (or) double side band full carrier system?

In conventional AM ,carrier power constitutes two thirds or more of the total transmitted power. This is a major drawback because the carrier contains no information ;the sidebands contain the information.

 Second ,conventional AM systems utilize twice as much bandwidth as needed with single sideband systems.

Define Heterodyning.

Heterodyne means to mix two frequencies together in a nonlinear device or to translate one frequency to another using nonlinear mixing.

Define tracking error.How it is reduced?

The difference between the actual local oscillator frequency and the desired frequency is called tracking error. It is reduced by a technique called three point tracking.

Define High side injection tracking & Low side injection tracking.

High side injection tracking: In high side injection tracking , the local oscillator should track above the incoming RF carrier by a fixed frequency equal to f_RF +f_IF .

Low side injection tracking: In low side injection tracking ,the local oscillator should track below the RF carrier by a fixed frequency equal to f_RF -f_IF .

Define Local Oscillator tracking.

Tracking is the ability of the local oscillator in a receiver to oscillate either above or below the selected radio frequency carrier by an amount equal to the intermediate frequency throughout the entire radio frequency band.

Define Local Oscillator tracking.

Tracking is the ability of the local oscillator in a receiver to oscillate either above or below the selected radio frequency carrier by an amount equal to the intermediate frequency throughout the entire radio frequency band.

Define Local Oscillator tracking.

Tracking is the ability of the local oscillator in a receiver to oscillate either above or below the selected radio frequency carrier by an amount equal to the intermediate frequency throughout the entire radio frequency band.

Define image frequency. Define image frequency rejection ratio.

 An image frequency is any frequency other than the selected radio frequency carrier that ,if allowed to enter a receiver and mix with the local oscillator ,will produce a cross product frequency that is equal to the intermediate frequency.

The image frequency rejection ratio is the measure of the ability of pre selector to reject the image frequency.

Mathematically ,IFRR is

IFRR =(1+Q²r²)^1/2

Where r= (f_im/f_RF)-(f_RF/f_im)

Distinguish between low level and high level modulation.

In low level modulation, modulation takes place prior to the output element of the final stage of the transmitter.It requires less power to achieve a high percentage of modulation.

In high level modulators, the modulation takes place in the final element of the final stage where the carrier signal is at its maximum amplitude and thus, requires a much higher amplitude modulating signal to achieve a reasonable percent modulation.

Define High level Modulation.

In high level modulators, the modulation takes place in the final element of the final stage where the carrier signal is at its maximum amplitude. For high level modulator class C amplifier is used.

Define Low level Modulation. What is the advantage of low level modulation?

In low level modulation, modulation takes place prior to the output element of the final stage of the transmitter. For low level AM modulator class A amplifier is used.

An advantage of low level modulation is that less modulating signal power is required to achieve a high percentage of modulation.

Define Modulation index and percent modulation for an AM wave.

Modulation index is a term used to describe the amount of amplitude change present in an AM waveform .It is also called as coefficient of modulation.

Mathematically modulation index is

m = Em

Ec

Where m = Modulation coefficient

Em = Peak change in the amplitude of the output waveform voltage.

Ec = Peak amplitude of the unmodulated carrier voltage.

Percent modulation gives the percentage change in the amplitude of the output wave when the carrier is acted on by a modulating signal.

What is Companding? What is the main reason of using Companding?

Companding: An operation in which the dynamic range of signals is compressed before transmission and is expanded to the original value at the receiver

                     The use of companding allows signals with a large dynamic range to be transmitted over facilities that have a smaller dynamic range capability. Companding reduces the noise and crosstalk levels at the receiver.

Define and state the causes of fold over distortion.

The minimum sampling rate(fs) is equal to twice the highest audio input

frequency(fa).If fs is less than two times fa ,distortion will result. The distortion is

Called aliasing or fold over distortion.

The side frequencies from one harmonic fold over into the sideband of another

harmonic. The frequency that folds over is an alias of the input signal hence ,the

names “aliasing” or “fold over distortion” .

What is Foldover Distortion? Show the appropriate spectrum of fold over distortion.

 Foldover Distortion: Signal distortion arising from those parts of the aliasing components that fall within the spectrum of the original signal. 

                 
               Fig: Foldover spectrum (distoryion) caused by undersampling of the i/p message.

What is Side tone coupling? Explain with circuit diagram.

Side tone coupling: In telephones, it is the hearing of one's voice in the receiver when one speaks. Too much side tone make people speak lower and can cause feedback. Too little side tone and people may think the phone is not working or may shout.

                                 Fig: Smooth Tone Clickless CW Sidetone Generator

                                     Fig: Schematic diagram of the sidetone circuit

Figure Description: This is a simplified schematic diagram of a receiver sidetone circuit. Because current will not flow through a balanced circuit, the turns ratio of inductors L1 and L2 is unbalanced by a predetermined amount and the value of resistor R is changed so as not to match the line impedance. This way, a controlled amount of signal can be induced into L3 to be used as receiver sidetone.

What is Amplitude Modulation? Write down its Advantages and Disadvantages.

•      Amplitude Modulation: Amplitude Modulation is a process where the amplitude of a carrier signal is altered according to information in a message signal. The frequency of the carrier signal is usually much greater than the highest frequency of the input message signal.

- Advantage of AM

1- Corverage area of AM Reciever is wider than FM because atmospheric propagation

2- AM is long distance propagation because λ

3- AM Circuit is cheapter and non complex than FM.

4- AM have bandwidth limited and FM unlimited

- Disadvantage of AM

1- The only one way to withance to noise happen is increasing power transmite

2- Signal of AM is not stronger than FM when it propagate to obstacle. (Urband)

3- Only one sideband of AM transmites Information Signal, So it loss power on other sideband and Carrier.

4- Noise mixes AM Signal in amplitude when it propagates in free space that it make difficulty to recover Original Signal at reciever.