Analogue And Digital Modulation Computer Science Essay

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Modulation is the process of varying some characteristic of a periodic wave with an external signal and another word can best describe about modulation is a process of mixing the signal with a sinusoid [f (t) = Asin (ωt + φ)] to produce a new signal. Modulation is utilized to send an information bearing signal over long distances. Radio communication superimposes this information bearing signal onto a "carrier signal". These high frequency carrier signals can be transmitted over the air easily and are capable of travelling long distances. The characteristics are (amplitude, frequency, or phase) of the carrier signal are varied in accordance with the information bearing signal. In the field of communication engineering, the information bearing signal is also known as the modulating signal. The modulating signal is a slowly varying signal - as opposed to the rapidly varying carrier frequency.

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Amplitude modulation (AM) is a method of impressing data onto an alternating-current (AC) carrier waveform. The highest frequency of the modulating data is normally less than 10 percent of the carrier frequency. The instantaneous amplitude (overall signal power) varies depending on the instantaneous amplitude of the modulating data.

In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating data appears in the form of signal components at frequencies slightly higher and lower than that of the carrier. These components are called sidebands. The lower sideband (LSB) appears at frequencies below the carrier frequency; the upper sideband (USB) appears at frequencies above the carrier frequency. The LSB and USB are essentially "mirror images" of each other in a graph of signal amplitude versus frequency, as shown in the illustration. The sideband power accounts for the variations in the overall amplitude of the signal.

When a carrier is amplitude-modulated with a pure sine wave, up to 1/3 (33percent) of the overall signal power is contained in the sidebands. The other 2/3 of the signal power is contained in the carrier, which does not contribute to the transfer of data. With a complex modulating signal such as voice, video, or music, the sidebands generally contain 20 to 25 percent of the overall signal power; thus the carrier consumes75 to 80 percent of the power. This makes AM an inefficient mode.

If an attempt is made to increase the modulating data input amplitude beyond these limits, the signal will become distorted, and will occupy a much greater bandwidth than it should. This is called over modulation, and can result in interference to signals on nearby frequencies. Varying the voltage of a carrier or a direct current in order to transmit analog or digital data. Amplitude modulation (AM) is the oldest method of transmitting human voice electronically. In an analog telephone conversation, the voice waves on both sides are modulating the voltage of the direct current loop connected to them by the telephone company.

FORMS OF AMPLITUDE MODULATION (AM)

In radio communication, a continuous wave radio-frequency signal (a sinusoidal carrier wave) has its amplitude modulated by an audio waveform before being transmitted. In the frequency domain, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands.

Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. Amplitude modulation that results in two sidebands and a carrier is often called double-sideband amplitude modulation (DSB-AM). Amplitude modulation is inefficient in terms of power usage. At least two-thirds of the power is concentrated in the carrier signal, which carries no useful information (beyond the fact that a signal is present).

To increase transmitter efficiency, the carrier can be removed (suppressed) from the AM signal. This produces a reduced-carrier transmission or double-sideband suppressed-carrier (DSBSC) signal. A suppressed-carrier amplitude modulation scheme is three times more power-efficient than traditional DSB-AM. If the carrier is only partially suppressed, a double-sideband reduced-carrier (DSBRC) signals results. DSBSC and DSBRC signals need their carrier to be regenerated (by a beat frequency oscillator, for instance) to be demodulated using conventional techniques.

Improved bandwidth efficiency is achieved-at the expense of increased transmitter and receiver complexity-by completely suppressing both the carrier and one of the sidebands. This is single-sideband modulation, widely used in amateur radio due to its efficient use of both power and bandwidth. A simple form of AM often used for digital communications is "on-off keying", a type of "amplitude-shift keying" by which binary data is represented as the presence or absence of a carrier wave. This is commonly used at radio frequencies to transmit Morse code.

What is amplitude modulation, AM

AMPLITUDE DEMODULATION

Amplitude modulation, AM, is one of the most straightforward ways of modulating a radio signal or carrier. The process of demodulation, where the audio signal is removed from the radio carrier in the receiver is also quite simple as well. The easiest method of achieving amplitude demodulation is to use a simple diode detector. This consists of just a handful of components: - a diode, resistor and a capacitor.

AM diode detector

AM Diode Detector

  In this circuit, the diode rectifies the signal, allowing only half of the alternating waveform through. The capacitor is used to store the charge and provide a smoothed output from the detector, and also to remove any unwanted radio frequency components. The resistor is used to enable the capacitor to discharge. If it was not there and no other load was present, then the charge on the capacitor would not leak away, and the circuit would reach a peak and remain there.

ADVANTAGES OF (AM)

DISADVANTAGES OF (AM)

It can be demodulated using a circuit of consisting of few components

It is not efficient in terms of its power usage and not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that of the highest audio frequency

It is a simple to be implement

It is prone to high levels of noise because most noise is amplitude based and obviously AM detectors are sensitive to it.

AM receiver are very cheap and no need any specialised components are needed

ADVANTAGES & DISADVANTAGES OF (AM)

There are several advantages of amplitude modulation and it is still in widespread used and it is a very basic form of modulation, although its simplicity is one of the major advantages, other more sophisticated systems provide a number of advantages. AM has advantages of simplicity but it is not the most efficient mode to use. Both in terms of the amount of space or spectrum it takes up and the way in which it uses the power that is transmitted. This is the reason why it is not widely used these days both for broadcasting and for two way radio communication. Even the long, medium and short wave broadcasts will ultimately change because of the fact that amplitude modulation, AM, is subject to much higher levels of noise than are other modes. For the moment, its simplicity, and its wide usage, means that it will be difficult to change quickly, and it will be in use for many years to go. So there is advantages and disadvantages in both terms.

PHASE MODULATION (PM)

Phase modulation (PM) is a method of impressing data onto an alternating-current (AC) waveform by varying the instantaneous phase of the wave. This scheme can be used with analog or digital data.

In analog PM, the phase of the AC signal wave, also called the carrier, varies in a continuous manner. Thus, there are infinitely many possible carrier phase states. When the instantaneous data input waveform has positive polarity, the carrier phase shifts in one direction; when the instantaneous data input waveform has negative polarity, the carrier phase shifts in the opposite direction. At every instant in time, the extent of carrier-phase shift (the phase angle) is directly proportional to the extent to which the signal amplitude is positive or negative.

In digital PM, the carrier phase shifts abruptly, rather than continuously back and forth. The number of possible carrier phase states is usually a power of 2. If there are only two possible phase states, the mode is called biphasic modulation. In more complex modes, there can be four, eight, or more different phase states. Each phase angle (that is, each shift from one phase state to another) represents a specific digital input data state.

Phase modulation is similar in practice to frequency modulation (FM). When the instantaneous phase of a carrier is varied, the instantaneous frequency changes as well. The converse also holds: When the instantaneous frequency is varied, the instantaneous phase changes. But PM and FM are not exactly equivalent, especially in analog applications. When an FM receiver is used to demodulate a PM signal, or when an FM signal is intercepted by a receiver designed for PM, the audio is distorted. This is because the relationship between phase and frequency variations is not linear; that is, phase and frequency do not vary in direct proportion.

FORMS OF PHASE MODULATION

Although phase modulation is used for some analogue transmissions, it is far more widely used as a digital form of modulation where it switches between different phases. This is known as phase shift keying, PSK, and there are many flavours of this. It is even possible to combine phase shift keying and amplitude keying in a form of modulation known as quadrate amplitude modulation, QAM.

The list below gives some of the forms of phase shift keying that are used:

PM - Phase Modulation

PSK - Phase Shift Keying

BPSK - Binary Phase Shift Keying

QPSK - Quadrate Phase Shift Keying

8 PSK - 8 Point Phase Shift Keying

16 PSK - 16 Point Phase Shift Keying

QAM - Quadrate Amplitude Modulation

16 QAM - 16 Point Quadrate Amplitude Modulation

64 QAM - 64 Point Quadrate Amplitude Modulation

MSK - Minimum Shift Keying

GMSK - Gaussian filtered Minimum Shift Keying

These are just some of the major forms of phase modulation that are widely used in radio communications applications today. With today's highly software adaptable radio communications systems, it is possible to change between the different types of modulation to best meet the prevailing conditions.

PHASE DEMODULATION

In phase modulation (pm) the intelligence is contained in the amount and rate of phase shift in a carrier wave. You should recall from your study of pm that there is an incidental shift in frequency as the phase of the carrier is shifted. Because of this incidental frequency shift, fm demodulators, such as the Foster-Seeley discriminator and the ratio detector, can also be used to demodulate phase-shift signals.

Another circuit that may be used is the gated-beam (quadrate) detector. Remember that the fm phase detector output was determined by the phase of the signals present at the grids. A QUADRATURE DETECTOR FOR PHASE DEMODULATION works in the same manner.

A basic schematic is shown below. The quadrate-grid signal is excited by a reference from the transmitter. This may be a sample of the unmodulated master oscillator providing a phase reference for the detector.

http://www.tpub.com/neets/book12/0279.GIF

The modulated waveform is applied to the limiter grid. Gating action in the tube will occur as the phase shifts between the input waveform and the reference. The combined output current from the gated-beam tube will be a series of current pulses. These pulses will vary in width as shown in figure 3-20. The width of these pulses will vary in accordance with the phase difference between the carrier and the modulated wave.

ADVANTAGES & DISADVANTAGES OF PHASE MODULATION (PM)

ADVANTAGES OF (PM)

DISADVANTAGES OF (PM)

Phase modulation & demodulation is easy compared to Frequency modulation

We need frequency multiplier to increase phase modulation index.

Phase modulator is used in determining velocity of moving target by extracting Doppler information. Doppler

information needs stable carrier which is possible in phase modulation but not in frequency modulation.

Phase ambiguity comes if we exceed its modulation index pi radian (180 degree).

Even though Phase modulation (PM) is widely used in today's radio communications scene, with phase shift keying is being widely used for digital modulation and data transmission. It has its own advantage & disadvantages which must be analysis before using this component. It is used in all forms of radio communications from cellular technology to Wi-Fi, WiMAX, radio broadcasting of digital audio and TV, and many more forms of transmission.

FREQUENCY MODULATION (FM)

Frequency modulation, FM is widely used for a variety of radio communications applications. FM broadcasts on the VHF bands still provide exceptionally high quality audio, and FM is also used for a variety of forms of two way radio communications, and it is especially useful for mobile radio communications, being used in taxis, and many other forms of vehicle.

In view of its widespread use, frequency modulation, FM, is an important form of modulation, despite many forms of digital transmission being used these days.

FM, frequency modulation has been in use for many years. However its advantages were not immediately apparent. In the early days of wireless, it was thought that a narrower bandwidth was required to reduce noise and interference. As FM did not perform well under these conditions, AM predominated and FM was not used. However, Edwin Armstrong, an American engineer looked at the use of wideband FM for broadcasting and introduced the idea against the trend of the thinking of the time.

Since its first introduction the use of frequency modulation, FM has grown enormously. Now wideband FM is still regarded as a very high quality transmission medium for high quality broadcasting. FM, frequency modulation is also widely used for communications where it is resilient to variations in signal strength.

The most obvious method of applying modulation to a signal is to superimpose the audio signal onto the amplitude of the carrier. However this is by no means the only method which can be employed. It is also possible to vary the frequency of the signal to give frequency modulation or FM. It can be seen below that the frequency of the signal varies as the voltage of the modulating signal changes.

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