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Modulation

Basic to the field of communications is the concept of modulation. Modulation

is the process of putting information onto a high-frequency carrier for

transmission. In essence, then, the transmission takes place at the high

frequency (the carrier) which has -been modified to "carry" the lower-frequency

information. The low-frequency information is often 'called the intelligence

signal or, simply, the intelligence. It follows that once this information is received,

the intelligence must be removed from the high-frequency carrier-a process

known as demodulation. At this point you may be thinking, why bother to go

through this modulation process? Why not just transmit the information directly?

The problem is that the frequency of the human voice ranges from about 20 to

3000Hz. If everyone transmitted those frequencies directly as radio waves,

interference would cause them all to be ineffective.

Modulation is process of putting information onto a high frequency carrier for

transmission

Intelligence Signal is the low frequency information that modulates the

carrier

Intelligence is low-frequency information modulated onto a high frequency

carrier in a transmitter

Demodulation is process of removing intelligence from the high frequency

carrier in a receiver

The solution is modulation, which allows propagation of the low-frequency

intelligence with a high-frequency carrier. The high-frequency carriers are

chosen such that only one transmitter in an area operates at the same

frequency to minimize interference,. and that frequency is high enough so that

efficient antenna sizes are manageable.

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Communication Systems

Communications systems are often categorized by the frequency of the carrier.

Table l-1 provides the names for various frequency ranges in the radio spectrum.

The extra-high-frequency range begins at the starting point of infrared

frequencies, but the infrareds extend considerably beyond 300 GHz (300 X 109

Hz). After the infrareds in the electromagnetic spectrum (of which the radio

waves are a very small portion) come light waves, ultraviolet rays, X rays,

gamma rays, and cosmic rays.

Figure 1: A communication system block diagram

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Table 1-1.Radio-Frequency Spectrum

FREQUENCY DESCRIPTION

30 GHZ - 300 GHZ extremely high frequency radio astronomy, high-

frequency microwave

radio relay, microwave

remote sensing, amateur

radio, directed-energy

weapon, millimeter wave

scanner

3 GHZ - 30 GHZ Super high frequency Radio astronomy,

microwave

devices/communications,

wireless LAN, most modern

radars, communications

satellites, satellite television

broadcasting, DBS,

amateur radio

300 MHZ - 3 GHZ ultrahigh frequency Television broadcasts,

microwave ovens,

microwave

devices/communications,

radio astronomy, mobile

phones, wireless LAN,

Bluetooth, ZigBee, GPS

and two-way radios such

as Land Mobile, FRS and

GMRS radios, amateur

radio

30 MHZ - 300 MHZ very high frequency FM, television broadcasts

and line-of-sight ground-

to-aircraft and aircraft-to-

aircraft communications.

Land Mobile and Maritime

Mobile communications,

amateur radio, weather

radio

3 MHZ - 30 MHZ high frequency Shortwave broadcasts,

citizens' band radio,

amateur radio and over-

the-horizon aviation

communications, RFID,

Over-the-horizon radar,

Automatic link

establishment (ALE) / Near

Vertical Incidence

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Skywave (NVIS) radio

communications, Marine

and mobile radio

telephony

300 KHZ - 3 MHZ medium frequency AM (medium-wave)

broadcasts, amateur

radio, avalanche beacons

30 KHZ - 300 KHZ low frequency Navigation, time signals,

AM longwave

broadcasting (Europe and

parts of Asia), RFID,

amateur radio

3 KHZ - 30 KHZ very low frequency Navigation, time signals,

submarine

communication, wireless

heart rate monitors,

geophysics

300 HZ - 3 KHZ voice frequency Submarine

communication,

Communication within

mines

Up to 300 HZ extremely low frequency Communication with

submarines

Table 1-1 represents a simple communication system in block diagram form.

Notice that the modulated stage accepts two inputs, the carrier and the

information (intelligence) signal. It produces the modulated signal, which is

subsequently amplified before transmission. Transmission of the modulated signal

can take place by any one of four means: antennas, waveguides, optical fibers,

or transmission lines.

Many of the performance measurements in communication systems are

specified in dB (decibels). The two basic limitations on the performance of a

communications system. (1) Electrical noise and (2) the bandwidth of

frequencies allocated for the transmitted signal.

Decibels (dBs) are used to specify measured and calculated values in noise

analysis, audio systems, and microwave system gain calculations, satellite

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system link-budget analysis, antenna power gain, light-budget calculations, and

many other communications system measurements.

The dB value is calculated by taking the log of the ratio of the measured or

calculated power (P2) with respect to a reference power (P1) level. This result is

then multiplied by 10 to obtain the value in dB.

Decibel formula for power comparisons

The most basic form for decibel calculations is a comparison of power levels.

The decibel formula or equation for power is given below:

Where:

Ndb is the ratio of the two power expressed in decibels

P2 is the output power level

P1 is the input power level

If the value of P2 is greater than P1, then the result is given as a gain, and

expressed as a positive value, e.g. +10dB. Where there is a loss, the decibel

equation will return a negative value, e.g. -15dB.

Decibel equations for voltage and current

Although the decibel is used primarily as comparison of power levels, decibel

current equations or decibel voltage equations may also be used provided that

the impedance levels are the same. In this way the voltage or current ratio can

be related to the power level ratio.

In the first instance for voltage because power = voltage squared upon the

resistance:

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Where:

Ndb is the ratio of the two power expressed in decibels

V2 is the output voltage level

V1 is the input voltage level

Similarly because power = current squared upon the resistance, the decibel

current equation becomes:

Where:

Ndb is the ratio of the two power expressed in decibels

I2 is the output current level

I1 is the input current level

NOISE

Electrical noise may be defined as any undesired voltages or currents that

ultimately end up appearing in the receiver output. To the listener this electrical

noise often manifests itself as static. .Noise signals at their point of origin are

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generally very small, for example, at the microvolt level. You may be wondering,

therefore, why they create so much trouble. Well, a communications receiver is

a very sensitive instrument that is given a very small signal at its input that must

be greatly amplified before it can possibly drive a speaker.

The noise present in a received radio signal that has been introduced in the

transmitting medium is termed external noise. The noise introduced by the

receiver is termed internal noise. The important implications of noise

considerations in the study of communications systems cannot be

overemphasized.

External Noise

Human--Made Noise. The most troublesome form of external noise is usually the

human-made variety. It is often produced by spark-producing mechanisms such

as engine ignition systems, fluorescent lights, and commutators in electric

motors. This noise is actually "radiated" or transmitted from its generating sources

through the atmosphere in the same fashion that a transmitting antenna

radiates desirable electrical signals to a receiving antenna.

Atmospheric Noise Atmospheric noise is caused by naturally occurring

disturbances in the earth's atmosphere, with lightning discharges being the most

prominent contributors. The frequency content is spread over the entire radio

spectrum, but its intensity is inversely related to frequency. It is therefore most

troublesome at the lower frequencies. It manifests itself in the static noise that

you hear on standard AM radio receivers. Its amplitude is greatest from a storm

near the receiver, but the additive effect of distant disturbances is also a factor.

Space Noise The other form of external noise arrives from outer space and is

called space noise. It is pretty evenly divided in origin between the sun and all

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the other stars. That originating from our star (the sun) is termed solar noise. Solar

noise is cyclical and reaches very annoying peaks about every eleven years.

Internal Noise

Thermal Noise: Noise caused by thermal interaction between free

electrons and vibrating ions in a conductor.

Shot Noise: Noise introduced by carriers in the pn junctions of

semiconductors

Excess Noise: Noise occurring at frequencies below 1khz, varying in

amplitude inversely proportional to the frequence

Transit-Time Noise: Noise produced in semiconductors when the transit

time of the carriers crossing a junction is close to the signals period.

Signal-To-Noise Ratio

Signal-To-Noise Ratio: Relative measure of desired signal power to noise

power

Noise Figure (NF): A figure describing how noisy a device is in decibels

Noise ratio (NR): A figure describing how noisy a device is as a ratio

having no units

The most fundamental relationship used is known as the signal-to-noise ratio

(S/N ratio), which is a relative measure of the desired signal power to the

noise power. The SIN ratio is often designated simply as S/N and can be

expressed mathematically as

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at any particular point in an amplifier. It is often expressed in decibel form as

Example:

A transistor amplifier has a measured S/N power of 10 at its input and 5 at its

output.

(a) Calculate the NR.

(b) Calculate the NF

(c) Using the results of part (a), verify that S/N Equation can be rewritten

mathematically as

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Their difference (10 dB - 7 dB) is equal to the result of 3 dB determined in part (b)

Information and Bandwidth

By now you should have a good grasp on the noise limitation. Quite simply, if the

noise level becomes too high, the information is lost. The other limitation is the

bandwidth utilized by the communications system. Stated simply once again,

the greater the bandwidth, the greater the information that can be transferred

from source to destination. The study of information in communications systems is

a science in itself (given the title information theory) that uses a highly

theoretical method of analysis. Information theory is the study of information to

provide for the most efficient use of a band of frequencies (a channel) for

electrical communications.

Information Theory is concerned with optimization of transmitted

information.

Channel is a band of frequencies

You might ask: Why is efficient channel utilization so important? The band of

usable frequencies is limited, and we are living in a world increasingly

dependent on electrical communications.

Regulatory agencies (TCRA] in Tanzania) allocate the channel that may be

used for a given application in a given area. This is done to minimize

interference possibilities that will exist with two different signals working at the

same frequency. The information explosion of recent years has taxed the total

available frequency spectrum to the point where getting the most information

from the smallest range of frequencies is in fact quite important.

A formal relationship between bandwidth and information was developed by R.

Hartley of Bell Laboratories in 1928 and is called Hartley's law. It states that the

information that can be transmitted is proportional to the product of the

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bandwidth utilized times the time of transmission. In simpler terms it means the

greater the bandwidth, the more information that can be transmitted.

Exercises

1. Define modulation.

2. What is carrier frequency?

3. Describe the two reasons that modulation is used for communications

transmissions.

4. List the three parameters of a high-frequency carrier that may be varied

by a low-frequency intelligence signal.

5. What are the frequency ranges included in the following frequency

subdivisions: MF (medium frequency), HF (high frequency), VHF (very high

frequency), UHF (ultra high frequency), and SHF (super high frequency)?

6. Define electrical noise, and explain why it is so troublesome to a

communications receiver.

7. Explain the difference between external and internal noise.

8. List and briefly explain the various types of external noise.

9. Calculate the S/N ratio for a receiver output of 4 V signal and 0.48 V noise

both as a ratio and in decibel form.

10. Define information theory.

11. What is Hartley's law? Explain its significance.