Amplitude Modulation Bandwidth (Amb)

1

Amplitude Modulation (AM)

In am transmission, the carrier signal is modulated so that its amplitude varies with the changing amplitudes of the modulating signal. The frequency and phase of carrier remain same; only the amplitude changes to follow variations in the information.

Amplitude Modulation (AM)

AMPLITUDE MODULATION BANDWIDTH (AMB)

The bandwidth of an AM signal is equal to twice the bandwidth of the modulating signal and covers a range centered around the carrier frequency. The bandwidth of an audio signa

is usually 5khz.therefore an am radio station needs a minimum bandwidth of 10 kHz.

The total bandwidth required for am can be determined from the bandwidth of the audio signal;

BWt=2*BWm

BWm =bandwidth of the modulating signal (audio)

BWt=total bandwidth (radio)

fc=frequency of the carrier

(1.1)An Audio Signal

15Sin2π (1500t)

Amplitude modulation a carrier

60Sin2π (100000t)

(a)Sketch the audio signal

(b)Sketch the Carrier

(c)Construct the modulated wave

(d)Determine the modulation factor and percent modulation

(e)What are the frequencies of the audio signal and carrier

(f)What frequencies would show up in the spectrum analysis if the modulation wave

Solution

Given

Audio Signal = 15Sin2π (1500t)

Carrier = 60Sin2π (100000t)

Find

(a)Sketch the audio signal

(b)Sketch the Carrier

(c)Sketch of modulated wave

(d)m, M

(e)ƒa , ƒc

(f)Frequency content of modulation wave

(a)

(b)

(c)

Converting modulation factor and percent modulation

(d) Using the following question for modulation factor

(1.3) How many AM broadcast station can be accommodated in a

100KHZ bandwidth if highest frequency modulation carrier

is 5KHZ

Solution

Give:

BW = 100KHz

ƒa max = 5KHz

Find:

Number of station

Any station being modulated a 5-KHz signal will produce an

upper side frequency 5 KHz above its carrier and a lower-side

frequency 5KHz below its carrier .there by requiring a band–

-width of 10 KHz

(1.5)

The total power contain of an AM signal is 1000 W. Determine the power being transmitted at the carrier frequency and at each of the sideband when the percent modulation is 100%

Solution

Given: Pr = 100 w

M = 100%; there fore m=1

Find: Pc, PUSB + PLSB

From the equation of total power ,

This level 1000 – 666.67 = 333.33 W to be share equally between upper and lower sideband

(1.7)

The power content of the carrier of an AM wave is 5 KW Determine the power content of each of the side band and the total power transmitted when the carrier the modulation 75%

Solution:

Given : Pc=5 KW

M=5%, m=0.75

Find : Pr, PUSB , PLSB

Since in an AM Wave the powering each of the power band side is equal

The total power is the sum of the carrier power and the power in the two side band

Pr = Pc + PUSB + PLSB

= 5000+703.13+703.13

(1.9)

Determine the percent modulation of the amplitude modulation wave which has a power content at the carrier of 8KW and 2KW in each of its sidebands when the carrier is modulation by a simple audio tone

Solution

Given : Pc =8KW

PUSB = PLSB= 2KW

Find: M

Knowing the power content of the sidebands and the carrier the relationshipofsidebands power can be used to determine the modulation factor . Once the modulation factor is know merely multiplying it by 1000 providing percent modulation

FREQUENCY MODULATION (FM)

In FM transmission the frequency of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulation signal. The peak amplitude and phase of the carrier signal remain constant, but as the amplitude of the information signal changed, the frequency of the carrier changes correspondingly.

The figure frequency modulation

Frequency modulation bandwidth (FMB)

The bandwidth of an fm signal is equal to10 time to bandwidth of the modulation signal and like AM bandwidths, covers a range centered around the carrier frequency. The ban- dwidth of an audio signal broadest in stereo is almost15khzdach fm radio station, therefore needs a minimum bandwidth of 150 kHz.

The total bandwidth required for FM can be determined from the bandwidth of the audio signal;

BWt=10*BWm

BWm =bandwidth of the modulating signal (audio)

BWt=total bandwidth (radio)

fc=frequency of the carrier

(2.2)

Determine the frequency deviation and carrier swing for a frequency modulation signal which has a resting frequency of 105.000 MHz and whose upper frequency is 105.007 MHz when the modulation by the particular wave .find the lower frequency reaches by the FM wave

Solution

Give:ƒ0 =105.000 MHz

ƒUpper =105.007 MHz

Find:Δƒ , cs , ƒlower

Frequency deviation is define as the maximum change in frequency of the modulation signal away from the rest carrier frequency

Δƒ =(105.007 – 105.000) x 106

= 0.007 x 106

= 7000

Carrier swing can now be determine

cs = 2Δƒ

= 2(7 x 103)

= 14 x103

The low frequency reached by the modulation wave can be found by subtracting the frequency deviation from the carrier or rest frequency.

ƒlower =ƒ0– Δƒ

=(105.000-0.007) x 106

(2.4)

The frequency modulation signal which is modulation by 3KHz sine wave reach at maximum frequency of 100.02 MHz and minimum frequency of 99.98 MHz.

(a)Determine the carrier swing

(b) Find the carrier frequency

(c)Calculate the frequency deviation of the signal

(d) what is the modulation index of signal

Solution

Given:ƒmax =100.02 MHz

ƒmin = 99.98 MHz

Find:(a)cs (b) ƒc(c) Δƒ (d) mƒ

(a)The carrier swing is define as the total variation in frequency from the highest to lowest reached by the modulation wave.

cs = ƒmax-ƒmin

= 100.02 x 106-99.98x 106

= 0.04 x 106

(b)The carrier frequency or rest frequency is midway between themaximum frequency and minmun frequency reached by the modulation wave

(c)Since the carrier swing is equal to twice the frequency deviation

(d)Modulation index for a frequency modulation wave is define

(2.6)

(a) What is the frequency deviation and carrier swing necessary to provide 75% modulation in the FM broadcast band

(b) repeat for an Fm signal serving as the audio portion of TV band

Solution:

Given: M = 75%

Find: ΔƒFM , c.s FM , ΔƒTV, c.sTV

(a) Frequency deviation define as

The max frequency deviation permitted in the FM bred cast 88-108 MHz by the FCC 75 KHz

Carrier swing related to frequency deviation by

c.s FM = 2ΔƒFM

= 2 x 56.25 x 103

(b)

The maximum frequency deviation permitted by the FCC for the audio portion of TV signal 25KHz

c.s TV = 2ΔƒTT

= 2 x 18.75 x 103

(2.8)

The percent modulation of sound portion of the signal 80% Determine the frequency deviation of carrier swing of the signal.

Solution:

Given:M = 80%

Find:Δƒ , c.s

The percent modulation of FM signal

The maximum frequency deviation for the sound portion of the TV Specified by the FCC is 25 KHz

Carrier Swing is related to frequency deviation by

c.s = 2 Δƒactua

= 2 x 20 x 103

(2.10)

Determine the frequency of modulation signal which is produce an FM signal having a bandwidth of 50 KHz when the frequency deviation of the FM signal 10 KHz

Solution:

Given:BW = 50 KHz

Δƒ = 10 KHz

Find:ƒa

In order to find ƒa refrence must be made to the Schwartz bandwidth carve FIG in order to entered this carve ,determine BW/Δƒ

SATELLITE COMMUNICATION
Satellite transmission is much like line-of-sight microwave transmission in which one of the stations is a satellite orbiting the earth. The principle is the same as terrestrial microwave, with a satellite acting as a super tall antenna and repeater ( see fig: 7.34). although in satellite transmission signals must still travel in straight lines, the limitation imposed on distance by the curvature of the earth are reduced. In this way, satellite relays allow micro waves signals to span contents and oceans with a signal bonus.


satellite micro waves can provide transmission capability to and from any location on earth ,no matter how remote. this advantage makes highly quality communication available to undeveloped parts of the world without quiring a huge investment in ground based infrastructure satellite them selves are extremely expensive ,officered ,but lazing time or frequencies on one can be relatively cheep.

GEOSYNCHRONOUS SATELLITES
Lines of sight propagation requires that the sending and receiving antennas be logged on to each others location at all time (one antenna must have the other in sight ). for this reason , a satellite that’s moves faster or slower than the earth’s rotation is useful only for short period of time (just as stopped clock is accurate twice a day) to ensure constant communication , the satellite must move at the same speed as the earth so that it seems to remain fixed above a certain spot. such satellites are called Geosynchronous.
Such orbital speed is based on distance from the planet .only the orbit can be geosynchronous. This orbit occurs at the equatorial plane is approximately 22000 miles surface of the earth.
But on geosynchronous cannot cover the whole earth, on satellite in orbital has the line-of-sight contact with the vast number so station ,but the curvature of the equidistant from each other in geosynchronous orbit to provide full global transmission Fig:7.35hows three satellite each 1290 degree from another in geosynchronous orbit around the equator orbit around the equator. The view is from north poll

F R EQU ENCY BAN DS FOR SA T ELLI T E CO M MUN ICA T ION
The frequencies reserved for the satellite communication are in the gigahertz (GHZ) range. Each satellite send and receive over two different bands . Transmission from the earth to the satellite is called uplink. Transmission from the satellite to the earth is called downlink. table 7.2 gives the band names and frequencies for each range,

CELLULAR TELEPHONY
Cellular telephony is designed to provide stable communication connections between two moving devices or between one mobile unit and one stationary unit (land) unit. A service provider must be able to locate as the caller moves out of the range of one channel and into the range of another.
To make this tracking possible each cellular service area is divided into small regions called cell. each cell contains an antenna and is control by a small office called the cell office each cell office . in tern, controlled by a switching office called a mobile telephone switching office(MTSO)the MTSO coordinates communication center between all of the cell offices and the telephone central office. it’s a computerized center that is responsible for connection cells as well as recording cell information and the billing (see fig: 7.36).

Cell size is not fixed and can be increased or decreased depending on the population of the area. the typical radius of a cell is 1 to 12 miles . high density areas require more ,geographically smaller cells to meet traffic demands than do lower density areas .once determines ,cell size is optimize to prevent the interface of the adjacent cell signals.
The transmission power of each cell id kept low to prevent its signal from interfering with those of other cell.
CELLULA R B EN DS
Traditional cellular transmission is analog. To minimized noise, frequency modulation (FM) is used for communication between the mobile telephone its self and the cell office. the F CC has the assigned to banned for cellular used (see fig:7.37).
The band between 824 and 849 MHz carries that communication that initiate from the mobile phone the band between 869 and 894 MHz carries those communication that initiate from land phones. Carries frequencies are spaced every 30 KHz, allowing each band to support up to 833 carries . how ever , to carries are required for full duplex communication , which doubles the require width of each channel to 60 KHz and leaves only 416 channels available for each band .

Each band, therefore, is divide into 416 FM channel (for a total of 832 channels). Of these , some are reserved for control and setup data rather than voice communication .in addition , to prevent interface , channel are distribute among the cells in such a way that adjacent cell do not used the same channel . this restriction means that each cell normally has access only 40 channel.
T R ANS MI T T ING
To place a cell from a mobile phone, the caller enters a code of 7 or 10 digits (a phone number ) and presses the send button. The mobile phone than scan the band, seeking a setup channel with a strong signal and sends the data(phone number) to the closest cell office using that channel. The cell office relays the data to the MTSO . the MTSO sends the data on to the telephone central office. if the called party is available , a connection is made and the result is relayed that to the MTSO.at this point ,the MTSO assigns an unused voice channel to the cell and a connection is established. The mobile phone automatically adjust its tuning to the new channel and voice communication can begin.
RECIEVING
When a land phone places cell to a mobile phone, the telephone central office sends the number to the MTSO .the MTSO searches the location of the mobile phone by sending query signals to each cell in a process called paging
Once the mobile phone is found, the MTSO transmits a ringing signal and, when the mobile phone is answered assigns a voice channel to the cell, allowing voice communication is begin
HANDOFF
It may happen that, during a conversation, the mobile phone moves from one cell to another cell. When it does, the signal may become weak. two solve this problem, the MTSO monitors the level of the signal every few seconds .if the strength of the signal the diminishes, the MTSO seeks a new cell that can accommodate the communication better. the MTSO than changes the channel carrying the cell (hands the signal off from the old channel to a new one). Hand off are performed so smoothly that most of the time they are transparent to the user.
DIGITAL
Analog (FM) cellular services are based on a standard called analog circuit switched cellular (ACSC) to transmit digital data using an ACSC.service requires a modem with a max:speed of 9600 to 19,200 bps.
Since 1993, however, several services provider have been moving a cellular data stranded called cellular digital packet data (CDPD).CDPD provides low speed digital service over the existing cellular network. It’s based on the OSI model.
To use the existing digital service, such as 56 kilo switch service, CDPD use what is called a trisected. A trisected is a combination of 3 cell each using 19.2 KBPS, for a total of 57.6 KBPS (which can be accommodate on a 56 k switched line by eliminating some over head ). Under this scheme united states is divided into 12,30 trisected. For every 60 trisected ,there is one router.
ITEGREATION WITH SATELLITES AND PCS
Cellular telephone is moving fast towards integrating the existing system with satellite communication. This integration will make it possible to have mobile communication between any two points on the globe an other goal is to combine cellular telephony and the personal computer communication under a scheme called mobile personal communication to an able people to use small, mobile personal computes to send and received data, voice, image, and the video .