Analog And Digital Communication Notes 1r68f

A skip zone, also called a silent zone or zone of silence, is a region where a radio transmission can not be received. The zone is located between regions both closer and farther from the transmitter where reception is possible. In the modulation process, some parameter of the carrier wave (such as amplitude, frequency or phase ) is varied in accordance with the modulating signal . This modulated signal is then transmitted by the transmitter . In the modulation process, two signals are used namely the modulating signal and the carrier . Advantages of Modulation 1. Reduction in the height of antenna 2.

Avoids mixing of signals

If the baseband sound signals are transmitted without using the modulation by more than one transmitter, then all the signals will be in the same frequency range i.e. 0 to 20 kHz . Therefore, all the signals get mixed together and a receiver can not separate them from each other . Hence, if each baseband sound signal is used to modulate a different carrier then they will occupy different slots in the frequency domain (different channels). Thus, modulation avoids mixing of signals . 3.

Increase the Range of Communication

The frequency of baseband signal is low, and the low frequency signals can not travel long distance when they are transmitted . They get heavily attenuated . The attenuation reduces with increase in frequency of the transmitted signal, and they travel longer distance . The modulation process increases the frequency of the signal to be transmitted . Therefore, it increases the range of communication. 4.

Multiplexing is possible

Multiplexing is a process in which two or more signals can be transmitted over the same communication channel simultaneously . This is possible only with modulation. The multiplexing allows the same channel to be used by many signals . Hence, many TV channels can use the same frequency range, without getting mixed with each other or different frequency signals can be transmitted at the same time . 5. Improves Quality of Reception With frequency modulation (FM) and the digital communication techniques such as PCM, the effect of noise is reduced to a great extent . This improves quality of reception . - 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. A balanced modulator mixes the audio signal and the radio frequency carrier, but suppresses the carrier, leaving only the sidebands. The output from the balanced modulator is a double sideband suppressed carrier signal and it contains all the information that the AM signal has, but without the carrier. It is possible to generate an AM signal by taking the output from the balanced modulator and reinserting the carrier.

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The balanced modulators are used to suppress the unwanted carrier in an AM wave.

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The carrier and modulating signals are applied to the inputs of the balanced modulator and we get the DSB signal with suppressed carrier at the output of the balanced modulator.

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Thus the output consists of the upper and lower side bands only.

Principle of operation: 

The principle of operation of a balanced modulator states that if two signals at different frequencies are ed through a “nonlinear resistance” then at the output we get an AM signal with suppressed carrier.

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The device having a nonlinear resistance can be diode or a JFET or even a bipolar transistor.

Types of balanced modulator: The suppression of carrier can be done using the following balanced modulators: 1.

Using the diode ring modulator or lattice modulator.

2.

Using the FET balanced modulator.

In telecommunications emphasis is the intentional alteration of the amplitude-vs.-frequency characteristics of the signal to reduce adverse effects of noise in a communication system or recording system. Typically, prior to some process, such as transmission over cable, or recording to phonograph record or tape, the input frequency range most susceptible to noise is boosted. This is referred to as "pre-emphasis" -- "pre-" the process the signal will undergo. Later, when the signal is received, or retrieved from recording, the reverse transformation is applied ("de-emphasis") so that the output accurately reproduces the original input. Any noise added by transmission or record/playback, to the frequency range previously boosted, is now attenuated in the de-emphasis stage. The whole system of pre-emphasis and de-emphasis is called emphasis Frequency domain allows for techniques which could be used to determine the stability of the system. Also, these techniques can be use in conjunction with the S-domain (Laplace transform) which gives more insight to the stability of the system, transient response, and steady state response.

In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a refinement of amplitude modulation which uses transmitter power and bandwidth more efficiently. Amplitude modulation produces an output signal that has twice the bandwidth of the original baseband signal. Single-sideband modulation avoids this bandwidth doubling, and the power wasted on a carrier, at the cost of increased device complexity and more difficult tuning at the receiver. Advantages:

Transfer functions which are complicated to determine the behavior of the experimentally can be determined using the frequency response analysis Design of the system and adjusting the parameters of the system can be easily carried out. Corrective measurement for noise disturbance generated in the system and parameters variation can be easily determined using frequency analysis Absolute and Relative stability of the closed loop system can be estimated from the knowledge of the open loop frequency system Frequency domain analysis can also be carried out for the non linear control systems.

Digital communication: 

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Advantage: o

Digital instruments are free from observational errors like parallax and approximation errors.

o

Can be noise-immune without deterioration during transmission and write/read cycle.

Disadvantage: o

Samples analog wave forms into a limited set of numbers and records them.Hence some information is lost. Less accuracy.

o

Cost is high and not easily portable

Analog Communication: 

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Advantage: o

Analog technology records wave forms as they are (continuous range of values to represent information). Hence more accurate.

o

Analog signal processing can be done in real time and consumes less bandwidth.

o

Low cost and portable.

Disadvantage: o

Analog systems less immune to noise – i.e., random unwanted variation.The noise gets added in the signal path, hence increasing SNR.

o

Anastruments usually have a scale which is cramped at lower end and give considerable observational errors.

o

More likely to get affected reducing accuracy.

Advantages: Analog – No quantization errors, requires less bandwidth, low cost and can be easily constructed because of less pre-processing requirements. Digital – Fast processing, easier for storage, strong immunity to noise, parallel processing possibility, error correction possibilities, easy portability

Disadvantages: Analog – Quality often degraded due to noise, Requires high quality processing which in turn demands costly hardware, costly storage requirements due to more data, high power requirements Digital – Quantization errors, high bandwidth requirements due to pulse signals, less accurate due to finite set of data Aliasing is an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled. It also refers to the distortion or artifact that results when the signal reconstructed from samples is different from the original continuous signal There are 2 basic ways to achieve Anti-Aliasing: 1.

Increase the sample rate (used e.g. in MSAA, SSAA and custom modes like EQAA and CSAA)

2.

Blur the edges/contrasts (used e.g. in MLAA, FXAA and SMAA), also called Post-AA or Post-Processing.

The Shannon–Hartley theorem states the channel capacity C, meaning the theoretical tightest upper bound on the information rate of data that can be communicated at an arbitrarily low error rate using an average received signal power S through an analog communication channel subject to additive white Gaussian noise of power N: C=B Log(1+S/N) where C is the channel capacity in bits per second, a theoretical upper bound on the net bit rate (information rate, sometimes denoted I) excluding error-correction codes; B is the bandwidth of the channel in hertz (band bandwidth in case of a band signal); S is the average received signal power over the bandwidth (in case of a carrier-modulated band transmission, often denoted C), measured in watts (or volts squared); N is the average power of the noise and interference over the bandwidth, measured in watts (or volts squared); and S/N is the signal-to-noise ratio (SNR) or the carrier-to-noise ratio (CNR) of the communication signal to the noise and interference at the receiver (expressed as a linear power ratio, not as logarithmic decibels).

Frequency hopping a method of transmitting radio signals more securely by repeatedly and rapidly changing the transmitter frequency, used to counter inteference, monitoring, etc. Jitter is defined as a variation in the delay of received packets. The sending side transmits packets in a continuous stream and spaces them evenly apart. Because of network congestion, improper queuing, or configuration errors, the delay between packets can vary instead of remaining constant Quantization noise is a model of quantization errorintroduced by quantization in the analog-to-digital conversion (ADC) in telecommunication systems and signal processing. It is a rounding error between the anaput voltage to the ADC and the output digitized value.

Stands for

AM stands for Amplitude Modulation

FM stands for Frequency Modulation

Origin AM method of audio transmission was first successfully carried out in the mid 1870s. FM radio was developed in the United states in the 1930s, mainly by Edwin Armstrong.

Modulating differences In AM, a radio wave known as the "carrier" or "carrier wave" is modulated in amplitude by the signal that is to be transmitted. The frequency and phase remain the same. In FM, a radio wave known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted. The amplitude and phase remain the same. Pros and cons AM has poorer sound quality compared with FM, but is cheaper and can be transmitted over long distances. It has a lower bandwidth so it can have more stations available in any frequency range. FM is less prone to interference than AM. However, FM signals are impacted by physical barriers. FM has better sound quality due to higher bandwidth. Frequency Range AM radio ranges from 535 to 1705 KHz (OR) Up to 1200 bits per second. FM radio ranges in a higher spectrum from 88 to 108 MHz. (OR) 1200 to 2400 bits per second. Bandwidth Requirements Twice the highest modulating frequency. In AM radio broadcasting, the modulating signal has bandwidth of 15kHz, and hence the bandwidth of an amplitude-modulated signal is 30kHz. Twice the sum of the modulating signal frequency and the frequency deviation. If the frequency deviation is 75kHz and the modulating signal frequency is 15kHz, the bandwidth required is 180kHz. Zero crossing in modulated signal

Equidistant Not equidistant

Complexity Transmitter and receiver are simple but syncronization is needed in case of SSBSC AM carrier. Tranmitter and reciver are more complex as variation of modulating signal has to beconverted and detected from corresponding variation in frequencies.(i.e. voltage to frequency and frequency to voltage conversion has to be done). Noise AM is more susceptible to noise because noise affects amplitude, which is where information is "stored" in an AM signal. FM is less susceptible to noise because information in an FM signal is transmitted through varying the frequency, and not the amplitude.