MODULE-5

Baseband Transmission of Digital signals: Introduction, Intersymbol Interference, Eye Pattern, Nyquist
criterion for distortionless Transmission, Baseband M-ary PAM Transmission.
[Text2:8.1,8.4,8.5,8.6,8.7]

Noise: Signal to Noise Ratio, External Noise, Internal Noise, Semiconductor Noise, Expressing Noise
Levels, Noise in Cascade Stages.

[Text1:9.5]

Noise

 Noise is an electronic signal that is a mixture of many random frequencies at many amplitudes
that gets added to a radio or information signal as it is transmitted from one place to another or as
it is processed. Noise is not the same as interference from other information signals.

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 The noise level in a system is proportional to temperature and bandwidth, and to the amount of
current fl owing in a component, the gain of the circuit, and the resistance of the circuit.
Increasing any of these factors increases noise.
 Therefore, low noise is best obtained by using low-gain circuits, low direct current, low resistance

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values, and narrow bandwidths. Keeping the temperature low can also help.
In most communication systems, weak signals are normal, and noise must be taken into account
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at the design stage.
 It is in the receiver that noise is the most detrimental because the receiver must amplify the weak
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signal and recover the information reliably.

 Noise can be external to the receiver or originate within the receiver itself. Both types are found
in all receivers, and both affect the SNR.
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Signal-to-Noise Ratio
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 The signal-to-noise (S/N) ratio, also designated SNR, indicates the relative strengths of the signal
and the noise in a communication system.
 The stronger the signal and the weaker the noise, the higher the S/N ratio. If the signal is weak
and the noise is strong, the S/N ratio will be low and reception will be unreliable.
 Signals can be expressed in terms of voltage or power.
 The S/N ratio is computed by using either voltage or power values:

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External noise

 External noise comes from sources over which we have little or no control— industrial,
atmospheric, or space.
 Noise in general contains all frequencies, varying randomly. This is generally known as white
noise.

Industrial Noise

 Industrial noise is produced by manufactured equipment, such as automotive ignition systems,

electric motors, and generators.
 Any electrical equipment that causes high voltages or currents to be switched produces transients
that create noise.
 Noise pulses of large amplitude occur whenever a motor or other inductive device is turned on or

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off.
 The resulting transients are extremely large in amplitude and rich in random harmonics.
 Fluorescent and other forms of gas-fi lled lights are another common source of industrial noise.

Atmospheric Noise.

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The electrical disturbances that occur naturally in the earth’s atmosphere are another source of
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noise.
 Atmospheric noise is often referred to as static. Static usually comes from lightning, the electric
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discharges that occur between clouds or between the earth and clouds.
 Lightning is very much like the static charges that we experience during a dry spell in the winter.
The voltages involved are, however, enormous, and these transient electric signals of megawatt
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power generate harmonic energy that can travel over extremely long distances.
 Like industrial noise, atmospheric noise shows up primarily as amplitude variations that add to a
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signal and interfere with it.

 Atmospheric noise has its greatest impact on signals at frequencies below 30 MHz.

Extraterrestrial Noise

 Extraterrestrial noise, solar and cosmic, comes from sources in space. One of the primary sources
of extraterrestrial noise is the sun, which radiates a wide range of signals in a broad noise
spectrum. The noise intensity produced by the sun varies with time.
 Noise generated by stars outside our solar system is generally known as cosmic noise.
 Although its level is not as great as that of noise produced by the sun, because of the great
distances between those stars and earth, it is nevertheless an important source of noise that must
be considered.
 It shows up primarily in the 10-MHz to 1.5-GHz range, but causes the greatest disruptions in the
15- to 150-MHz range.

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Internal Noise

 Electronic components in a receiver such as resistors, diodes, and transistors are major sources of
internal noise.
 Internal noise, although it is low level, is often great enough to interfere with weak signals.
 The main sources of internal noise in a receiver are thermal noise, semiconductor noise, and
intermodulation distortion.

Thermal Noise

 Most internal noise is caused by a phenomenon known as thermal agitation, the random motion of

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free electrons in a conductor caused by heat.
 Increasing the temperature causes this atomic motion to increase. Since the components are
conductors, the movement of electrons constitutes a current flow that causes a small voltage to be
produced across that component.
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Electrons traversing a conductor as current flows experience fleeting impediments in their path
as they encounter the thermally agitated atoms.
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 The apparent resistance of the conductor thus fluctuates, causing the thermally produced random
voltage we call noise.
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 Thermal agitation is often referred to as white noise or Johnson noise, after J. B. Johnson, who
discovered it in 1928.
 Just as white light contains all other light frequencies, white noise contains all frequencies
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randomly occurring at random amplitudes.

 A white noise signal therefore occupies, theoretically at least, infinite bandwidth.
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Filtered or band-limited noise is referred to as pink noise.

 Noise is a very broadband signal containing a tremendous range of random frequencies, its level
can be reduced by limiting the bandwidth.
 If a noise signal is fed into a selective tuned circuit, many of the noise frequencies are rejected
and the overall noise level goes down. The noise power is proportional to the bandwidth of any
circuit to which it is applied.
 Filtering can reduce the noise level, but does not eliminate it entirely.
 The amount of open-circuit noise voltage appearing across a resistor or the input impedance to a
receiver can be calculated according to Johnson’s formula

 Heat sinks, cooling fans, and good ventilation can help lower noise.

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 Many low-noise receivers for weak microwave signals from spacecraft and in radio telescopes are
super cooled; i.e., their temperature is reduced to very low (cryogenic) levels with liquid nitrogen
or liquid helium.
 Thermal noise can also be computed as a power level. Johnson’s formula is then

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Semiconductor Noise

 Electronic components such as diodes and transistors are major contributors of noise. In addition
to thermal noise, semiconductors produce shot noise, transit-time noise, and flicker noise.
 The most common type of semiconductor noise is shot noise. Current flow in any device is not
direct and linear.
 Shot noise is also produced by the random movement of electrons or holes across a PN junction.
Even though current flow is established by external bias voltages, some random movement of
electrons or holes will occur as a result of discontinuities in the device.
 Shot noise is also white noise in that it contains all frequencies and amplitudes over a very wide
range.
 The amount of shot noise is directly proportional to the amount of dc bias flowing in a device.
 The bandwidth of the device or circuit is also important.
 The rms noise current in a device In is calculated with the formula

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Transit-time noise

 The term transit time refers to how long it takes for a current carrier such as a hole or electron to
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move from the input to the output.

 Transit-time noise shows up as a kind of random variation of current carriers within a device,
occurring near the upper cutoff frequency.
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Transit-time noise is directly proportional to the frequency of operation.

 Since most circuits are designed to operate at a frequency much less than the transistor’s upper
limit, transit-time noise is rarely a problem.
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Flicker noise or Excess noise

 Occurs in resistors and conductors.

 This disturbance is the result of minute random variations of resistance in the semiconductor
material.
 It is directly proportional to current and temperature.
 However, it is inversely proportional to frequency, and for this reason it is sometimes referred to
as 1/f noise.
 Flicker noise is highest at the lower frequencies and thus is not pure white noise.
 Because of the dearth of high-frequency components, 1/f noise is also called pink noise

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Intermodulation Distortion

 Intermodulation distortion results from the generation of new signals and harmonics caused by

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circuit nonlinearities.
 As stated previously, circuits can never be perfectly linear, and if bias voltages are incorrect in
an amplifier or it is driven into clipping, it is likely to be more nonlinear than intended.
 Nonlinearities produce modulation or heterodyne effects.
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Any frequencies in the circuit mix together, forming sum and difference frequencies.
When many frequencies are involved, or with pulses or rectangular waves, the large number of
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harmonics produces an even larger number of sum and difference frequencies.
 IMD products are small in amplitude, but can be large enough to constitute a disturbance that can
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be regarded as a type of noise.

 This noise, which is not white or pink, can be predicted because the frequencies involved in
generating the intermodulation products are known.
 Because of the predictable correlation between the known frequencies and the noise,
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intermodulation distortion is also called correlated noise. Correlated noise is produced only when
signals are present.
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 The types of noise discussed earlier are sometimes referred to as uncorrelated noise.
 Correlated noise is manifested as the low-level signals called birdies. It can be minimized by
good design.

Expressing Noise Levels

 The noise quality of a receiver can be expressed as in terms of noise figure, noise factor, noise
temperature, and SINAD. Noise Factor and Noise Figure.
 The noise factor is the ratio of the S/N power at the input to the S/N power at the output. The
device under consideration can be the entire receiver or a single amplifier stage.
 The noise factor or noise ratio (NR) is computed with the expression

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  The lower the noise figure, the better the amplifier or receiver.
 Noise figures of less than about 2 dB are excellent.

Noise Temperature

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 Most of the noise produced in a device is thermal noise, which is directly proportional to
temperature.
 Therefore, another way to express the noise in an amplifi er or receiver is in terms of noise
temperature TN.
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Noise temperature is expressed in kelvins.
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Remember that the Kelvin temperature scale is related to the Celsius scale by the relationship
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TK =TC +273.
 The relationship between noise temperature and NR is given by
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 Noise temperature is used only in circuits or equipment that operates at VHF, UHF, or microwave
frequencies.
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 The noise factor or noise fi gure is used at lower frequencies.

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SINAD

 Another way of expressing the quality and sensitivity of communication receivers is SINAD—
the composite signal plus the noise and distortion divided by noise and distortion contributed by
the receiver.

 In symbolic form,
 Distortion refers to the harmonics present in a signal caused by nonlinearities.
 The SINAD ratio is also used to express the sensitivity of a receiver.
 The SINAD is a power ratio, and it is almost always expressed in decibels:

 SINAD is the most often used measure of sensitivity for FM receivers used in two-way radios.
 It can also be used for AM and SSB radios.

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Noise in the Microwave Region

 Noise is an important consideration at all communication frequencies, but it is particularly critical

in the microwave region because noise increases with bandwidth and affects high-frequency
signals more than low-frequency signals.
 The limiting factor in most microwave communication systems, such as satellites, radar, and
radio telescope astronomy, is internal noise.
 In some special microwave receivers, the noise level is reduced by cooling the input stages to the
receiver, as mentioned earlier.
 This technique is called operating with cryogenic conditions, the term cryogenic referring to very
cold conditions approaching absolute zero.

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Noise in Cascaded Stages

 Noise has its greatest effect at the input to a receiver simply because that is the point at which the
signal level is lowest.
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The noise performance of a receiver is invariably determined in the very first stage of the
receiver, usually an RF amplifier or mixer.
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 Design of these circuits must ensure the use of very low-noise components, taking into
consideration current, resistance, bandwidth, and gain figures in the circuit.
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 Beyond the first and second stages, noise is basically no longer a problem.
 The formula used to calculate the overall noise performance of a receiver or of multiple stages of
RF amplification, called Friis’ formula, is
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