COMMUNICATIONS
Refers to the sending, reception and processing of
information by electrical means
Block Diagram of a Communications System
Information Source
Recipient or Destination Noise
ReceiverTransmitter Channel
Information Source
selects symbols (letters, numbers, words, sounds, etc) from an alphabet (or ensemble) of possible symbols
Types of Information
Transmitter
a collection of electronic components and circuits designed to convert the information into a signal suitable for transmission over a given
modulation, multiplexing, encoding, encryption, and pre-emphasis (FM)
Processes Involved
Channel
the medium by which the electronic signal is sent from one place to another
Receiver
another collection of electronic components and circuits that accept the transmitted message from the channel and convert it back into a form understandable by humans
Processes Involved
Noise
Basic Requirements
Types of Signal
1. ANALOGtelephone, radio broadcast or TV signals
2. DIGITAL comprises of pulses at discrete intervals of time
1. Accurate Communication2. Fast Communication
Transmission Paths
1. Line Communication guided media which include coaxial cable, twisted pair, optical fibers and waveguides
2. Radio Communications unguided media
Frequency number of times a
particular phenomenon occurs at a given time
expressed in hertz (1/1 sec) Hz
Wavelength distance between two points of
similar cycles of a periodic wave
Bandwidth portion of the electromagnetic
spectrum occupied by a signal
Basic Concepts
Nomenclature of Frequency Bands
*microwave frequency band: 1 GHz to 300 GHz **frequencies beyond EHF are optical frequencies
DATES EVENTS
1830 American scientist and professor Joseph Henry transmitted the first practical electrical signal .
1837 Samuel Finley Breeze Morse invented the Telegraph and patented it in 1844.
1843 Alexander Bain invented the facsimile.
1847 James Clerk Maxwell postulated the Electromagnetic Radiation Theory.
1860 Johann Philipp Reis, a German who produces a device called Telephone that could transmit a musical tone over a wire to a distant point but incapable of reproducing it.
1864 James Clerk Maxwell, a Scottish physicist established the Theory of Radio or Electromagnetism which held the rapidly oscillating electromagnetic waves exist and travel at through space with the speed of light.
Significant Historical Events in Electronic Communications
DATES EVENTS
1875 Thomas Alba Edison invented Quadruplex telegraph, doubling existing line qualities. J. M. Emile Baudot invented the first practical Multiplex Telegraph and another type of telegraphy codes which consisted of pre – arranged 5 - unit dot pulse. A. C. Cowper introduced the first Facsimile Machine or writing telegraph using a stylus.
1876 Alexander Graham Bell and Thomas A. Watson invented the Telephone capable of transmitting voice signals (March 10).
1877 Thomas Edison invented the Phonograph.
1878 Francis Blake invented the Microphone Transmitter using platinum point bearing against a hard carbon surface.
1882 Nikola Tesla outlined the basic principles of radio transmission and reception.
1887 Heinrich Hertz detected electromagnetic waves with an oscillating circuit and establishes the existence of radio waves.
DATES EVENTS
1889 Hertz discovered the progressive propagation of electromagnetic action through space using a spark – gap wave generator, to measure the length and velocity of electromagnetic waves and their direct relation to light and heat as their vibration, reflection, refraction and polarization.
18790 Almon Strowger introduced the dial – switching system transmitting the desired telephone number electrically without the assistance of a human telephone operator.
1895 Marchese Guglielmo Marconi discovered ground – wave radio signals.
1898 Guglielmo Marconi established the first radio link between England and France.
1901 Reginald A. Fessenden transmits the world’s first radio broadcast using continuous waves. Marconi transmits telegraphic radio messages from Cornwall, England to Newfoundland, first successful transatlantic transmission of radio signals.
1904 John Ambrose Fleming invented the Vacuum Tube Diode.
DATES EVENTS
1906 Reginald Fessenden invented Amplitude Modulation (AM). Lee De Forest added a grid to the diode and produced triode.Ernst F. W. Alexanderson invented the Tuned Radio Frequency Receiver (TRF) an HF Alternator to producing AC contributing to better voice broadcasting.
1907 Reginald Fessenden developed the Heterodyne Receiver.
1918 Edwin H. Armstrong invented the Superheterodyne Receiver.
1923 J. L. Baird and C. F. Jenkins demonstrated the transmission of Black and White Silhouettes in motion. Vladymir Zworykin and Philo Farnsworth developed television cameras, the Iconoscope and the Image Detector. The first practical television was invented in 1928.
1931 Edwin Armstrong invented the Frequency Modulation, greatly improving the quality of the signals.
1937 Alec Reeves invented the Pulse Code Modulation for digital encoding of PCM signals.
1945 Arthur C. Clarke proposed the use of satellites for long distance radio transmissions.
DATES EVENTS
1946 AT&T introduced the first mobile telephone system for the public called the MTS (Mobile Telephone System).
1947 John Bardeen, Walter Brattain and William Shockley introduced the bipolar junction transistors which started a new trend in radio receiver design; December 4.
1951 First transcontinental microwave system began operation.
1954 J. R. Pierce showed how satellites could orbit around the earth and effect transmission with earth stations.
1957 Troposcatter Radio Link was established between Florida and Cuba by using antennas and high powered transmitters to force microwaves beyond LOS obstructed by earth’s curvature bulge. Russia launched Sputnik I, the first active earth satellite, capable of receiving, amplifying and retransmitting information to earth stations.
1958 Jack Kilby developed the first Monolithic Integrated Circuit Semiconductor chip with active and passive elements.
1959 Robert Noyce invented the Very Large Scale Integrated Circuit (VLSIC).
DATES EVENTS
1962 AT&T launched Telstar I, the first satellite to received and transmit simultaneously. A year later, Telstar II was launched and used for telephone, TV fax and data transmission .
1965 COMSAT and INTELSAT launched the first communications satellite code name Early Bird at approximately 34000 km above sea level.
1967 K. C. Kao and G. A. Bockam of Standard Telecommunications Laboratories in England proposed the use of cladded fiber cables as new transmission medium.
1977 First commercial use of optical fiber cables
1983 Cellular telephone networks introduced.
1991 Tim Berners – Lee developed World Wide Web (WWW).
Self Test
Principles of Communications
Choose the letter which answer each question.
1. He is recognized as the Father of Electromagnetism a. Joseph Henry b. Andre Marie Ampere c. Hans Christian Oersted d. Michael Faraday
2. An American scientist who discovered electromagnetic self –induction. He was a pioneer of modern forecasting techniques.
a. Joseph Henry b. Andre Marie Ampere c. Hans Christian Oersted d. Michael Faraday
3. He discovered Photoelectricity through a photosensitive element,Selenium that could carry current in direct proportion to the amount of light that struck it.
a. Jonas Jacob Berzelius b. Charles Babbage c. James Maxwell d. Michael Faraday
4. The Father of Electrodynamics who exposed the phenomenon of electromagnetism following the discovery of magnet needle by Oersted.
a. Joseph Henry b. Andre Marie Ampere c. James Maxwell d. Michael Faraday
Self Test
Principles of Communications
Choose the letter which answer each question.
5. The range of frequencies represented in a signal. a. Beam width b. Crisp c. Guard band d. Bandwidth
6. It refers to the sending, receiving and processing of information by electronic means
a. Communications b. Telemetry c. Telephony d. Broadcasting
Self Test
Principles of Communications
Choose the letter which answer each question.
7. The big breakthrough in electronics communications came with the invention of the ____________ in 1844.
a. Transistor b. Phonograph c. Telegraph d. Telephone
8. Term for transmission of printed picture by radio (ECE Board April 1998).
a. Facsimile b. ACSSBc. Xerography d. Television
Self Test
Principles of Communications
Choose the letter which answer each question.
9. A Scottish physicist who established the Theory of Radio or Electromagnetism which held that rapidly oscillating electromagnetic waves exist and travel at through space with thespeed of light.
a. James Maxwell b. Andre Marie Ampere c. Hans Christian Oersted d. Michael Faraday
10. The first words transmitted over the telephone a. Mr. Watson, come here, I want you b. What God has wrought c. This is the day d. None of these
Self Test
Principles of Communications
Choose the letter which answer each question.
11. Most of the communications receiver is in form of ____________ type. a. Superheterodyne b. TRFc. Single – ended d. High level
12. He was the one who developed the wireless telegraph. a. Edison b. Armstrong c. Marconi d. Bell
Self Test
Principles of Communications
Choose the letter which answer each question.
13. He invented the first practical Multiplex Telegraph and another type of telegraphy codes which consisted of pre – arranged 5 – unit dot pulse patterns. This multiplexer allowed signals of up to 6 different telegraph machines to be transmitted over a single wire pair.
a. JM Emile Baudot b. AC Cowper c. Thomas Edison d. Thomas Doolittle
14. Introduced the first Facsimile Machine or writing telegraph using stylus.
a. JM Emile Baudot b. AC Cowper c. Thomas Edison d. Alexander Graham Bell
Choose the letter which answer each question.
Self Test
Principles of Communications
15. Invented the Microphone transmitter using platinum point bearing against a hard carbon surface
a. Francis Blake b. Thomas Watson c. Thomas Doolittle d. Henry Hunnings
16. What type of field does Faraday shield stop? a. Common b. Array c. Electrostatic d. Magnetostatic
Self Test
Principles of Communications
Choose the letter which answer each question.
17. Which of the following is not normally tested in a transmitter? a. Power b. Modulation c. Frequency d. Amplitude
18. What other term is essentially synonymous with beating? a. Heterodyning b. Scrambling c. Filtering d. Suppressing
Self Test
Principles of Communications
Choose the letter which answer each question.
19. Term in Communication that is referred “to send in all directions”. a. Announce b. Broadcast c. Transmit d. Media
20. Invented the telephone transmitter using granular carbon and thin platinum diaphragm and a gold – plated electrode
a. Nikola Tesla b. Henry Hunnings c. Thomas Watson d. AC Cowper
Self Test
Principles of Communications
Choose the letter which answer each question.
21. He introduced the Dial Switching System, transmitting the desired telephone number electrically without the assistance of a human telephone operator
a. Thomas Edison b. Almon Strowger c. JJ Thomson d. Lee De Forest
22. An Italian physicist who invented the first wireless telegraph, initially transmitting messages over a distance of 1 mile but later improved it to 8000 miles in 1902. He also made the first successful transatlantic radio transmission.
a. Francis Blake b. Ernst Alexanderson c. KF Braun d. Guglielmo Marconi
Self Test
Principles of Communications
Choose the letter which answer each question.
23. The Father of Quantum Theory. He introduced the revolutionary idea that the energy emitted by an oscillator could take only on discrete values or quanta
a. Guglielmo Marconi b. John Fleming c. Max Karl Ernst Ludwig Planck d. Christian Hulsmeyer
24. Who developed the CRT capable of tracing curves on phosphor screen?
a. John Fleming b. Max Karl Ernst Ludwig Planck c. KF Braun d. Christian Hulsmeyer
Self Test
Principles of Communications
Choose the letter which answer each question.
25. He invented the superheterodyne receiver a. Edwin Armstrong b. Alec Reeves c. Veldemar Poulsen d. Boris Rosing
26. He developed a simple radio echo device to prevent ship collisions. He was considered as the forerunner of RADAR
a. Veldemar Poulsen b. Christian Hulsmeyer c. Boris Rosing d. Reginald Fessenden
Self Test
Principles of Communications
Choose the letter which answer each question.
27. Who invented the Pulse Code Modulator for digital encoding of PCM signals
a. Alec Reeves b. Edwin Armstrong c. Boris Rosing d. Reginald Fessenden
28. He was the one who developed the mathematical solution to a complex repetitive waveform
a. Carson b. P. H. Smith c. J. Fourier d. Y. Uda
Self Test
Principles of Communications
Choose the letter which answer each question.
29. The volume of transaction proceed in a specific unit of timea. Response time b. Throughout c. Availability d. Reliability
30. ____________ is a device that measures the internal open circuit voltage of an equivalent noise generator having an impedance of 600 ohms and delivering noise power to a 600 ohms load.
a. Phosphometer b. Barometer c. Reflectometer d. Voltmeter
Self Test
Principles of Communications
Choose the letter which answer each question.
MODULATION
mixing of low frequency signals modulating signal) with high frequency signals (carrier signal)
process by which some characteristic of a high frequency sine wave is varied in accordance with the instantaneous value of the signal
imposition of information on a given signal
modification of one signal by another signal
Types of Modulation
a. According to carrier used
1. Continuous Wavecarrier is a sinusoid
1. Pulsecarrier is a train of pulses (discrete)
a. According to the method used
Types of Modulation
1. Analogmodulated parameter is made proportional
to the modulating signal
2. Digital change the form of a given signal
Reasons for Modulation
1. To reduce the antenna lengths
2. To reduce noise / interference
3. For frequency assignments
4. For multiplexing
5. To overcome equipment limitations
Analog Modulation
1. Amplitude Modulation
b. Phase Modulation
2. Angle Modulation
a. Frequency Modulation
Digital Modulation
1. Pulse Code Modulation 2. Delta
Modulation
ѵ(t)= V sin (ωt + ѳ)
ѵ = instantaneous amplitudeV = peak amplitudeω = 2Πf; angular frequencyt = instantaneous time ѳ = phase angle in radians
Consider any sinusoid
NOTE Any type of modulation should be reversible (get back to the original signal) by process of demodulation.
amplitude
phase
t = 1/f
Emission of unmodulated carrier N
Emission in which the main carrier is amplitude
-> Double – sideband A
-> Single – sideband, full carrier H
-> Single –sideband, reduced or variable level carrier R
-> Single – sideband, suppressed carrier J
-> Independent sideband B
-> Vestigial sideband C
Emission in which the main carrier is angled modulated
-> Frequency Modulation F
-> Phase Modulation G
Basic Emission Classification First Symbol – Type of Modulation of the Main Carrier
Emission in which the main carrier is angled and amplitude modulated simultaneously
D
Emission of pulses
-> Sequence of unmodulated pulses P
-> Sequence of pulses
->> Modulated in amplitude K
->> Modulated in width/duration L
->> Modulated in position/phase M
->> The carrier is angle – modulated during the period of the pulse
Q
->> Combination of the foregoing or is produced by other means
V
Cases not covered above or combination of two or more of the following modes: amplitude, angle, angle and phase
W
Cases otherwise not covered X
First Symbol – Type of Modulation of the Main Carrier
No modulating signal 0
Digitally keyed carrier 1
Digitally keyed tone 2
Analog 3
Multichannel digital 7
Multichannel analog 8
Combination 9
Cases not otherwise covered X
Second Symbol – Nature of the Signals Modulating the Main Carrier
No information transmitted N
Telegraphy – for aural reception A
Telegraphy – for automatic reception B
Facsimile C
Data transmission, telemetry, telecommand D
Telephony (including sound broadcasting) E
Television (video ) F
Combination of the above W
Cases otherwise not covered X
Third Symbol – Type of Information to be Transmitted
Two – condition code with elements of differing numbersand/or durations
A
Two – condition code with elements of the same number andduration without error correction
B
Two – condition code with elements of the same number and duration with error correction
C
Four – condition code in which each condition represents asignal element
D
Multicondition code in which each condition represents a signal element
E
Multicondition code in which each condition or combinationof conditions represents a character
F
Fourth Symbol – Details of Signals
Sound of broadcasting quality (monophonic) G
Sound of broadcasting quality (stereophonic or quadraphonic) H
Sound of commercial quality J
Sound of commercial quality with the use of frequencyinversion or band splitting
K
Sound of commercial quality with separate frequency –modulated signals to control the levels of demodulated signal
L
Monochrome M
Color N
Combination of the above W
Cases not otherwise covered X
Fourth Symbol – Details of Signals
None N
Code – division multiplex C
Frequency – division multiplex F
Time – division multiplex T
Combination of frequency – division multiplex and time – division multiplex
W
Other types of multiplexing X
Fifth Symbol – Nature of Multiplexing
A system of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating voltage.
Carrier voltage is made proportional to the instantaneous modulating signal
AMPLITUDE MODULATION
Information or
modulating signal (νm)
Information or
modulating signal (νm)
AM modulator
Carrier (νc )
Carrier (νc )
Output (νAM)
Output (νAM)
let the carrier voltage be given by
and the modulating voltage be given by
then the amplitude resulting from modulation is
Since and Vm = Vc ma, then
Therefore A = Vc + Vc ma sin ωm t A = Vc (1 + ma sin ωm t )
The voltage of the resulting AM wave envelope at any instant is
νc(t) = Vc sin ωc t
νm(t) = Vm sin ωm t
A = Vc + νm (t) = Vc + Vm sin ωm t
νAM(t) = A sin ωc t
General Equation of the AM wave
General Form
Standard Form
whereVc = carrier signal peak voltageωc = 2Π fc = carrier signal angular frequency ωm = 2Π fm = modulating signal angular frequency t = instantaneous timema = modulation index
carrier lower sideband LSB
upper sideband USB
νAM(t) = Vc (1 + ma sin ωm t) sin ωc t
νAM(t) = Vc sin ωc t + cos (ωc - ωm) t + cos (ωc + ωm) t
Time Domain of Standard AM
Frequency Domain of Standard AM
graph of relative amplitude of signal against frequency
USBLSB
fc - fm fc + fm
carrierνAM(t)
fc
Envelope
the curve produced by joining the tips of the individual RF cycles of the AM waveform
envelope
AM Modulation Index (ma)
Modulation index (modulation factor, modulation coefficient, degree of modulation, depth of modulation)
; 0 < ma < 1
where
Percent Modulation (Ma)
modulation index expressed as a percentage
Ma = ma x 100%
Degrees of Modulation
1. Ma < 100% undermodulation
2. Ma = 100%modulation
3. Ma > 100% overmodulation
ExampleA modulated wave has a peak value of 2 volts. The carrier wave equation for the voltage is 1.2 sin(20t + 15). Determine whether the signal is overmodulated or not.
GivenVm = 2 V Vc = 1.2 V
Solution
ma = 1.67 (overmodulated)
NOTE PC is constant value before and after modulation. PT is the total power after modulation and is dependent on the modulation index. The higher the modulation index, the higher the output power.
Power Content of an AM Signal
PT = PC
PT = PC + PUSB + PLSB
PSBT =
NOTEPUSB = PLSB
where PUSB = PLSB =
ExampleA transmitter supplies 8 kW to the antenna when unmodulated. Determine the total power radiated whenmodulated to 30%.
Given Pc = 8 kW ma = 0.3
Solution
PT = 8.36 kW
where IC = unmodulated carrier IT = total or modulated current VC = unmodulated carrier voltage VT = total or modulated voltage ma = modulation index
Current and Voltage Relationships
ExampleThe antenna current of an AM transmitter is 8 A when only the carrier is sent, but it increases to 8.93 A when the carrieris sinusoidally modulated. Find the percentage modulation.
GivenIC = 8 A IT = 8.93 A
Solution
ma = 0.701
Ma = 70.1%
Simultaneous Modulation
modulation by several carrier
where VT = total modulated voltageIT = total modulated current maT = effective total modulation index
ExampleA 360 W carrier is simultaneously modulated by 2 audio waves with modulation percentage of 55 and 65, respectively. What is the total sideband power radiated?
GivenPC = 360 W Ma1 = 55% Ma2 = 65%
Solution
PSBT = 130.5 W
Bandwidth Formula for AM
BW = 2 x fm
BW = 2 x fmhighest
Effiency
Percentage Power Saving
NOTEEfficiency and power saving depends on the type of transmission but the total transmitted power is computed on the basis of double sideband full
carrier.
(single carrier modulation)(single carrier modulation)
(simultaneous modulation)
ExampleHow many AM broadcast stations can be accommodated in
a 100 kHz bandwidth if the highest modulating frequency is 5 kHz?
GivenBW = 100 kHz fmhighest = 5 kHz
Solution BW = 2 x fmhighest
BW = 2 x 5 kHzBW = 10 kHz
10 stations
Types of AM Transmission Frequency Domain
1. A3E – Standard AM (DSBFC)- Double Sideband Full Carrier - used for broadcasting
2. A3J – DSBSC- Double Sideband Suppressed Carrier
3. H3E – SSBFC - Single Sideband Full Carrier - could be used as a compatible AM
broadcasting system with A3E receivers
4. J3E – SSBSC - Single Sideband Suppressed Carrier - The carrier is suppressed by at least 45 dB
in the transmitter
Types of AM Transmission Frequency Domain
5. R3E – SSBRC- Single Sideband Reduced Carrier - An attenuated carrier is reinserted into the
SSB signal to facilitate receiving tuning and demodulation.
6. B8E – Independent Sideband Emission - Two independent sidebands, with a carrier
that is most commonly attenuated or suppressed.- Used for HF point – to – point radiotelephony
in which more than one channel is required.
7. C3F – Vestigial Sideband - A system in which a vestige, i.e., a trace, of the unwanted sideband is transmitted usually with a full carrier.
- Used for video transmissions in all the world’s various TV systems to conserve bandwidth.
8. Lincompex (for PCM)- “Linked Compressor and Expander”- Basically a system in which all audio
frequencies above 2.7 kHz are filtered out to allow the presence of a control tone of 2.9 kHz of 120 Hz.
NOTEa) Information is present only in the sidebands. b) During 100% modulation, 2/3 of the total power is
wasted in the carrier. The carrier power percentage is 33.33% of the transmitted AM.
c) Information on the LSB is the same as the information on the USB.
Single Sideband Modulation
single sideband suppressed carrier (J3E) a form of amplitude modulation in which the carrier has been cancelled out with the balanced modulator and one of the sidebands has been removed by using one of the three different methods.
ExampleAn AM transmitter is rated 100 W at 100% modulation. How much is required for the carrier? What is the power required for the audio signal?
GivenPT = 100 W Ma = 100%
Solution
PC = 66.67 W
PSBT = PT – PC
PSBT = 33.33 W
1. Filter Method the simplest system the unwanted sideband is removed, actually heavily
attenuated by a filter the filter maybe LC, crystal, ceramic or mechanical,
depending on the carrier frequency and other requirements
Crystal oscillator
Balanced modulator
Audio amplifier
Filter for other sideband
Sideband suppression
filter
Balanced mixer
Crystal oscillator orsynthesizer
SSB out to linear
amplifier
AF in
Buffer
2. Phase – Shift Method avoids filters and some of their inherent disadvantages makes use of two balanced modulators and two – phase
shifting networks
Audio amplifier
Balancedmodulator
M2
Balancedmodulator
M1
Carrier source
AF90° phase
shifter
Carrier 90° phase
shifterAdder
AF in SSB out to linearamplifier
3. “Third” or Weaver Method developed by Weaver has the advantages of the phase- shift method, such as its
ability to generate SSB at any frequency and use low audio frequencies, without the associated disadvantage of an AF phase shift network required to operate over a large range of audio frequencies.
Balanced modulator M1
Low – passfilter
Balanced modulator M2
90° phase shifter
Balanced modulator M3
AF carrier generator
RF carrier generator
90° phase shifter
Balanced modulator M4
Low – passfilter
2cos ωc
t
2cos ωc
t
2sin ωc t 2sin ωc t
E F
A
B C
sin ωc t
AF in
D
SSB out
Adder
Product Detector
a means of single sideband detection mixing of high frequency component and an
incoming modulated signal on a multiplier circuit followed by a low pass filter
Advantages of SSB
1. power saving 2. less bandwidth 3. less noise 4. less fading
multiplex techniques used for high – density point – to – point communications
simultaneously convey a totally different transmission, to the extent that the upper sideband could be used for telephony while the lower sideband carries telegraphy. It consist of two SSB channels added together to form two sidebands around reduced carrier.
Independent Sideband (ISB) Systems
Standard AM
TransmitterTransmitter Requirements
1. frequency accuracy and stability 2. frequency agility 3. special purity
(absence of spurious signals)4. power output rating 5. efficiency 6. modulation fidelity
RF crystal
oscillator
Class A RF buffer amplifier
Class C RF power amplifier
Class C RF output amplifier
Class B RF linearpower amplifier
High – level Modulation
Low – level modulation
Antenna
AFprocessing
and filtering
AF pre –
amplifier
AF Class B power
amplifier
Modulator (AF Class B output amplier)
AF in
AM Transmitter Functional Block Diagram
Crystal Oscillator
It provides a stable carrier frequency at low power.
Buffer Amplifier
A low gain, high input impedance amplifier that isolates the crystal oscillator to improve its ability.
Class C RF Power Amplifier (Driver)
Provide enough gain to sufficiently drive the modulated amplifier.
Class C RF Output Amplifier
Termed as the modulated amplifier or the RF output amplifier. This is the output stage for high level systems.
Linear Power Amplifier
Amplifier which provides linear power amplification of the amplitude – modulated output signal from the Class C modulated power amplifier (used for low – level modulation ).
AF Processing and Filtering
AF is processed / filtered so as to occupy the correct bandwidth and compressed somewhat to reduce the ratio of maximum to minimum amplitude.
AF Preamplifier
Boost the AF signal in order to provide enough gain sufficiently to drive the modulator.
Modulator
Output is mixed with the carrier to generate the AM signal.
Sample Board Problems
1. Modulation is used to a. increase the carrier power b. reduce the bandwidth usedc. allow the use of practical antennasd. shorten the transmission channel
2. Modulation is done at a. the transmitterb. the receiverc. the channel d. the antenna
Sample Board Problems
3. In an AM wave, useful power is carried by a. the sidebandsb. the carrier c. both d. either
4. Which of these could not be used to remove the unwanted sideband in SSB?
a. Filter Methodb. Phase – Shift Method c. Third Methodd. Balanced Modulator
Sample Board Problems
5. In amplitude modulation, the ____________ of the carrier is varied according to the strength of the signal.
a. amplitudeb. phase c. frequency d. period
6. The output of a balanced modulator is a. USB and LSBb. LSBc. USBd. carrier
Sample Board Problems
7. A 900 kHz carrier is amplitude modulated with 4000 Hz audio tone. The lower and upper sideband frequencies are
a. 450 and 1800 kHzb. 800 and 1000 kHzc.896 and 904 kHzd. 4000 and 8000 kHz
8. The HF band covers the frequency range a. 3 – 30 MHz b. 30 – 300 MHzc. 30 – 300 kHzd. 300 kHz – 3 MHz
Sample Board Problems
9. In amplitude modulation, if the peak signal is 0.5 volt and the carrier peak is 1 volt, the depth of modulation is
a. 25%b. 50%c. 75%d. 78.5%
10. In a communications system, noise is most likely to affect the signal at the
a. transmitterb. channelc. information sourced. destination
Sample Board Problems
11. Another term for Amplitude Modulation a. DSBSCb. SSBSCc. DSBFCd. SSBFC
12. The bandwidth of an AM transmitter if the carrier frequency is 1000 kHz and the modulation frequency varies from 1 kHz to 10 kHz is
a. 2000 kHzb. 2 kHzc. 20 kHzd. 20.1 kHz
Sample Board Problems
13. If the modulation index of an AM wave is increased from 0 to 1, the transmitter power will be
a. unchanged b. increased by 50% c. increased by 100%d. decreased by 50%
14. When the modulation index of an AM amplifier is 1, the amplitude of the carrier is ____________ the amplitude of the modulating signal.
a. greater than b. equal to c. less thand. double
Sample Board Problems
15. Overmodulation results in ____________. a. weakening of the signal b. excessive carrier powerc. distortiond. either a or b
16. If the modulation index of an AM amplifier is 0.80, the % modulation is equal to
a. 20%b. 80%c. 0.80d. 0.20
Sample Board Problems
17. Both frequency and phase modulation utilize ____________ modulation.
a. phase b. amplitudec. digitald. angle
18. The maximum percentage modulation without distortion in an amplitude modulation system is
a. 20%b. 50%c. 110%d. 100%
Sample Board Problems
19. A form of AM in which the carrier is transmitted at full power, but only one of the sidebands is transmitted.
a. SSBFCb. SSBSCc. DSBFCd. DSBSC
20. Emission with only one sideband transmission a. A3E b. J3Ec. 11BEd. H3E
type of angle modulation wherein the frequency of the carrier is varied relative to the amplitude of modulating frequency (signal)
General Equation of the FM Wave
νFM (t) = Vc sin (ωc t + mf sin ωm t )
whereVc = carrier signal peak voltageωc = 2Πfc = carrier signal angular frequency ωm = 2Πfm = modulating signal angular frequency t = instantaneous time mf = modulation index
FREQUENCY MODULATION
Frequency Domain of Standard FM
fc - nfm
νFM(t)
fc
fc + nfm
Time Domain of Standard FM
Frequency Deviation (δ)
the amount of change in carrier frequency produced by modulating signal
the maximum frequency deviation occurs at the maximum amplitude of the modulating signal
δ = fmax - fc
δ = fc - fmin
where fc = carrier frequency fmax = maximum frequency
change of the carrier fmin = minimum frequency
change of the carrier
δ
δ
δ = fmax + fc
δ = fc - fmin
CS
CS (Carrier Swing)
NOTEThe intelligence amplitude determines the a mount of carrier frequency deviation. The intelligence frequency determines the rate of carrier frequency deviation.
δ α Em
δ = k Em
k =
ExampleFind the carrier and modulating frequencies , the modulation index and the maximum deviation of the FM wave represented by the voltage equation νFM(t) = 12 sin (6 x 108 + 5 sin 1250t). What power will this FM wave dissipate in a 10Ω resistor?
GivenνFM(t) = 12 sin (6 x 108 + 5 sin 1250t)
Solution ωc = 6 x 108 ωm = 1250
fc = 95. 49 MHz fc = 198.94 Hz
mf = 5
Solution δ = mf fm
= (5 x 198.94)
δ = 994.72 Hz
2
P = 7.2 W
Carrier Swing
the total variation in frequency of the carrier
Modulation Index
NOTE The modulation index determines the number of significant sidebands in an FM signal. The modulation index for FM is directly proportional to the modulating voltage and at the same time inversely proportional to the modulating signal frequency.
ExampleA modulation frequency range from 30 to 15000 Hz is
permitted in an FM system, together with a maximum deviation of 50 kHz. What will be the minimum and maximum possible values
of modulation index in the system.
Givenfm = 30 – 15000 Hz δmax = 50 kHz
Solution
mf max = 1666.67 mf min = 3.33
Deviation Ratio
The ratio of the maximum permissible frequency deviation to the maximum permissible modulating frequency.
Percentage of Modulation
Wideband FM vs. Narrowband FM
Descriptions Wideband FM Narrowband FM
Applications FM broadcast and
entertainment
Mobile communications
Modulation index (mf)
5 – 2500 ≈ 1
Maximum deviation (δmax)
75 kHz 5 kHz
Modulating frequency (fm)
30 Hz – 15 kHz 3 kHz (max)
ExampleFor an FM signal in the 88 – 108 MHz broadcast band with a frequency deviation of 15 kHz, determine the percent modulation.
Givenfm = 88 – 108 Mhzδmax = 15 kHz
Solution
mf = 20%
Frequency Spectrum of the FM Wave
νFM = J0 (mf) sin ωc t + J1 (mf ) [ sin (ωc + ωm)t – sin (ωc - ωm)t ]
amplitude of the carrier
signal
sidebands with
frequencies
NOTEFor a single fm , you can produce an infinite number of sidebands, in pairs, but most of them would be
of negligible amounts.
Transmitted Power
In FM, PT is constant.
NOTEIn FM, the total transmitted power always remains constant, but with increased depth of modulation the required bandwidth is increased.
Bandwidth Formulas for FM
a. Theoretical BW for FM
b. Approximate BW
c. NBFM
d. WBFM
BW = 2 x fm x no. of highest needed sideband
BW = 2(fm + δ) John Carsons Formula
BW ≈ 2fm
BW ≈ 2δ
ExampleWhat is the bandwidth of a narrowband FM signal generated by a 2 kHz audio signal and a 110 MHz FM carrier?
Givenfm = 2 kHzfc = 110 MHz
Modulation Index (Mf)
Sidebands or J – coefficients
1 3
2 4
3 6
4 7
5 8
SolutionB = 2 x fm x no. of highest needed sidebandB = 2 x 3 kHz x 7
B = 42 kHz
NOTE
The theoretical bandwidth needed in FM is infinite.
negligible negligible
Advantages of FM over AM
1.The amplitude of the FM carrier remains constant.
2. Increase in the S/N ratio (less noise)3. It is possible to reduce noise still further by
increasing deviation and fitting with amplitude limiters.
4. Less adjacent channel interference because there are guard bands between FM stations provided by the FCC and CCIR.
5. It is possible to operate several independent transmitters on the same frequency with considerably less interference.
Disadvantages of FM
1. A much wider channel is required by FM, 7 to 15 times as large as that needed by AM.
2. FM transmitting and receiving equipment tends to be more complex, particularly for modulation and demodulation, therefore more expensive.
3. Since reception is limited to line of sight, the area of reception for FM is much smaller than AM.
Capture Effect
The inherent ability of FM to minimize the effect of undesired signals (noise), also applies to the reception of a strongest signal or stations and minimizing other signals operating on the same frequency.
Block Diagram of an FM Transmitter
RF oscillator
Exciter (the means of providing an
FM RF signal )
Power amplifier
Audioamplifier
Pre-emphasis network
Pre -emphasis
The boosting of the higher modulating frequencies at the
transmitter, in accordance with the
pre –arranged curve to improve noise
immunity at FM and prevent the higher
frequency component of the transmitted intelligence being
degraded.
De -emphasis
Reducing the amplitude of the
higher modulating frequencies at the
receiver to the same amount as it was
before the pre – emphasis
circuit.
NOTEThe pre –emphasis and de – emphasis networks have a
time constant of 75 μs (150 μs for Europe , 25 μs for Dolby) and a cut – off frequency of 2122 Hz.
NOTEIf two modulating signals have the same initial amplitude and one of them is pre – emphasized to twice this amplitude, whereas the other is unaffected, being at a much lower frequency , then the receiver will naturally have to de –emphasize the first signal by a factor of 2 to ensure that both signals have the same amplitude in the output of the receiver.
Forms of Interference in FM
1. Image Frequency effect of two stations being received
simultaneously
2. Co – channel Interference
true to mobile receivers; when travelling from one transmitter toward another. This minimizes by capture effect.
3. Adjacent Channel Interference created between tuning two adjacent
stations due the imperfection of filter circuits like channel 2 and 3.
Stereophonic FM Multiplex System (1961)
modulation system in which two – channel system with left channel and a right channel transmitted simultaneously and independently
NOTEStereophonic uses 38 kHz subcarrier from a pilot carrier of 19 kHz to produce sum and difference of the two channels. Also, a 67 kHz subsidiary communications authorization (SCA) is added for optional transmission in FM broadcasting.
sum channel (L + R)
sub –carrier
difference channel (L - R)
Optional SCA transmission
DSBSC AM FMaudio0 15 19 23 38 53 59.5 67 74.5
Generation of FM Signals
A. Direct Method
Varying the frequency of the carrier oscillator directly
1. Reactance Modulator Reactance of the capacitive or inductive components of the tank circuit vary in direct relationship to the audio signal which causes the oscillator frequency to vary thereby directly producing AM.
The equivalent capacitance depends on the device transconductance. The capacitance can be originally adjusted to any value, within reason, by varying the components R and C
Ceq = gm RC
The expression gmRC has the correct dimensions of capacitance; R, measured in ohms, and gm measured in siemens (S).
Since
Therefore
ExampleDetermine the value of the capacitive reactance obtainable
from a reactance FET whose gm is 12 millisiemens (12 mS). Assume that the gate – to – source resistance is 1/9 of the reactance of the gate – to – drain capacitor and that frequency is 5 MHz.
Givengm = 12 mS R = 1/9 Xc Xc = 9R
Solution
n
Xc eq = 750 Ω
2. Varactor Diode Modulator
Uses a varactor (voltage variable capacitor) diode that when reversed biased exhibits a junction capacitance that varies inversely with the amount of reverse voltage.
B. Indirect Method
Armstrong Method The modulating signal is modulated using balanced modulator then fed to a summing device together with a crystal that is shifted 90°. Wideband FM then obtain by successive frequency multiplication of the output.
Crystal oscillator
Buffer
90° Phase shifter
Combining network
Balanced Modulator
Audio equalizer
1st group of multipliers
Mixer 2nd group of multipliers
Class C power
amplifiers
Crystal oscillator
equalized audio
FM wave(very low fc and mf )
carrier only
carrier at 90°
medium fc
and low mf
low fc
and mf
high fc
and mf
high fc
and mf
sidebands only
modulation wherein the phase of the carrier is made proportional to the instantaneous value of the modulating signal
Phase Modulation
General Equation of the PM Wave
ѵPM(t) = Vc sin(ωc t + mp sin ωm t)
where
mp = modulation indexkp = proportionality constant
mp = kpVm
Under identical conditions: mp = mf but when fm is changed mp k
mf changed
ExampleThe equation of an angle modulated wave is ν(t) = 15 sin (3x108t + 20 sin 2000t). Calculate the maximum deviation. Rewrite this equation if the modulating frequency is halved, but all else remains constant, assuming that the wave
is: a. frequency modulatedb. phase modulated
GivenνPM (t) = 15 sin (3x108t + 20 sin 2000t)
Solution
fm = 318.31 Hz δ = 6.37 kHz
Solutiona. frequency modulated
b. phase modulated
νFM (t) = 15 sin (3x108t + 40 sin 1000t)
νPM (t) = 15 sin (3x108t + 20 sin 1000t)
Frequency Modulation vs. Phase Modulation
Comparisons Phase Modulation Frequency Modulation
deviation (δ) δp is proportional to Vm, independent on
fm
δf is proportional to Vm
modulation index mp is proportional to Vm
mf is proportional to Vm
and inversely proportional to fm
when fm is changed mp will remain constant
mf will increase as fm is reduced, vice
versa
Sample Board Problems
1. Which of the is not an advantage of FM over AM. a. better noise immunity b. lower bandwidth required c. transmitted power is useful d. less modulating power
2. In an FM Stereo Multiplex transmission , the a. sum signal modulates the 19 kHz subcarrier b. difference signal modulates the 67 kHz subcarrier c. difference signal modulates the 38 kHz subcarrierd. difference signal modulates the 19 kHz subcarrier
Sample Board Problems
3. When modulating frequency is doubled, the modulation index ishalved, and the modulating voltage remains constant, the modulation system is
a. AM b. PAM c. FMd. PM
4. Pre – emphasis and de – emphasis are used in the FM stereo multiplex system to
a. SWR b. power ratio c. S/N d. CMMR
Sample Board Problems
5. The commercial FM radio broadcast band is a. 535 – 1605 kHz b. 27 – 29 kHzc. 88 – 108 MHz d. 300 – 3000 MHz
6. For an FM broadcast station, the maximum deviation produced by audio modulation is 45 kHz. The percent modulation is ____________.
a. 10b. 45c. 60d. 100
Sample Board Problems
7. The modulation index of an FM signal is one half the original index if the modulating frequency is
a. doubled b. halvedc. increasedd. decreased
8. An FM signal with a modulation index, mf is passed through a frequency tripler. The wave in the output of the tripler will have a modulation index of
a. mf /3b. mf
c. 3mf
d. 9mf
Sample Board Problems
9. Which of the following is an indirect method of generating FM? a. Reactance FET Modulatorb. Varactor Diode Modulatorc. Armstrong Modulatord. Reactance BJT Modulator
10. What is the deviation ratio for the station in the commercial FM broadcast band?
a. 5.5b. 5c. 4d. 6
Sample Board Problems
11. What is the frequency deviation necessary for 80% modulation in the FM broadcast?
a. 75 kHzb. 75 MHzc. 60 kHzd. 60 MHz
12. In frequency modulation, noise components which affect the amplitude can be eliminated using
a. an oscillatorb. an integratorc. an FM limiter circuitd. differentiator
Sample Board Problems
13. In FM, if a carrier is modulated by a single frequency, the number of SB’s produced is
a. 1b. 2c. 3d. greater than 3
14. The maximum frequency deviation of a standard FM radio broadcast station is
a. 25 kHz b. 40 kHzc. 75 kHzd. 200 kHz
Sample Board Problems
15. The modulation index of an FM signal having a carrier swing of 100 kHz when the modulating signal has a frequency of 2 kHz is
a. 75 kHzb. 25 kHzc. 50 kHzd. 0.50 kHz
16. In FM stereophonic transmission standards, the suppressed subcarrier frequency is
a. 19 kHz b. 67 kHzc. 38 kHzd. 69 kHz
Sample Board Problems
17. Maximum FM broadcast deviation is 75 kHz and maximum modulating frequency is 15 kHz. The maximum bandwidth requirement is
a. 180 kHzb. 160kHzc. 140 kHzd. 90 kHz
18. An FM signal with a deviation δ is passed through a mixer, and has its frequency reduced fivefold. The deviation in the output of the mixer is
a. 5δb. indeterminatec. δ /5d. δ
Sample Board Problems
19. The permissible range in maximum modulating index for commercial FM that has 30 Hz to 15 kHz modulating frequencies is
a. 5 and 2500 b. 50 and 250c. 50 and 2500 d. 5 and 50
20. The carrier swing necessary to provide 80% modulation in the FM broadcast band is
a. 150 kHzb. 120 kHzc. 75 kHzd. 60 kHz
Demodulation
the process by which the modulating signal is recovered from the modulated carrier
found in receiversFunctions of a Receiver
1. selects the desired signal 2. amplifies3. demodulates / detects 4. displays
Standard AM Receivers
1. Tuned Radio Frequency 2. Superheterodyne Receiver
1st RF amplifier
2nd RF amplifier
AMdetector
a simple “logical” receiver simplicity and high sensitivity aligned at broadcast frequencies 535 – 1640 Hz
Tuned Radio Frequency (TRF) Receiver
AFamplifier
Poweramplifier
ganged
It amplifies weak signal from the antenna. It has a variable resistor that controls the RF gain and sensitivity.
It provides rectification and detection for modulated signals.
Volume – controlled amplifier that raises the power level of the audio (AF) signal to a value sufficient to drive the loudspeaker of the receiver.
RF Amplifier
Detector
AF Amplifier
Problems in TRF Receivers
NOTE RF stage provides greater gain, prevention of re –
radiation of the local oscillator, improved rejection of adjacent unwanted signals and better coupling of antenna with the receiver.
1. instability 2. insufficient adjacent frequency
rejection 3. bandwidth radiations
RF amplifier
1st IF amplifier
2nd IF amplifier
AMdetector
AFamplifier
Crystal oscillator
Mixerfo
fs
fIF
fsi
RF AmplifierAmplifies the weak RF signal received from the antenna. Selectivity of the receiver depends on this section.
NOTE The RF section provides discrimination or selectivity against image and intermediate frequency signals, provide an efficient coupling between the antenna and the first stage of the RF amplifier.
AM Superheterodyne Receiver
ganged
Local OscillatorA crystal oscillator whose frequency “beats” with the incoming signal to produce the correct intermediate frequency.
Mixer (First Detector)Combines the incoming RF signal and the signal from the oscillator, and produces two original frequencies, their sum and differences, and harmonics. It produces desired intermediate frequency (IF).
1st IF AmplifierTuned to 455 kHz (IF of AM), amplifies it and rejects the remaining output frequencies
AM Detector (2nd Detector)It demodulates the IF signal and recovers or extracts the original audio signal.
AF AmplifierIt raises the power level of the audio signal; to a value sufficient to drive the loudspeaker of the receiver.
2nd IF AmplifierFurther amplification and selectivity of the IF signal. Most of the gain (sensitivity) of the receiver are achieved on the IF amplifiers.
FM Superheterodyne Receiver
RF amplifier and
preselector
Local oscillator
IF amplifier (10.7 MHz)
De-emphasis network
Audio amplifierMixer Limiter Detector
Demodulation
The process of shifting the spectrum back to the original baseband frequency range and reconstructing the original form. Also known as detection.
Sensitivity
Ability to amplify weak signals
Selectivity
Ability to reject unwanted signals(adjacent)
Image Rejection Ratio (α)
Provided by tuned circuits to block fsi The ratio of the gain at desired frequency (fs)
to the gain of image frequency signal (fsi)
whereQ = quality factor provided by
tuned circuits to block fsi fs = signal frequency fo = oscillator frequency fIF = intermediate frequencyfsi = image frequency signal
NOTE
fsi = fs + 2fIF
fsi = fo + fIF
; the higher the fIF, the better the image rejection
ExampleIn a broadcast superheterodyne receiver having loaded Q of
100 in the antenna coupling circuit. If fIF is 455 kHz. Calculate the image frequency and its rejection ratio of 1000 kHz.
GivenQ = 100 fIF = 455 kHz fs = 1000 kHz
Solution fs = fs + 2fIF
= 1000 kHz + 2(455 kHz)
α = 138.65
fsi = 1910 kHz
1. Modulation means ____________. a. Varying of some parameters of a carrier such as its amplitude to
transmit informationb. Transmit pulses in DC form of a copper wire c. Varying information d. Utilization of a single transmission channel
2. The modulation system used for telegraphy is ____________. a. Single tone modulation b. Two –tone modulation c. Frequency – shift keying d. Pulse code modulation
Self Test
Modulation
Choose the letter which answer each question.
3. A process that occurs in the transmitter.a. Demodulation b. Modulation c. Mixing d. Beating
4. A process that occurs in the receiver a. Beating b. Modulation c. Mixing d. Demodulation
Self Test
Modulation
Choose the letter which answer each question.
5. What is the amount of carrier swing necessary to produce an 80% modulation for the audio portion of the TV band?
a. 40 kHzb. 50 kHz c. 20 kHz d. 25 kHz
6. The letter number designation B8E is a form of modulation is also known as ____________?
a. Pilot – carrier system b. Independent sideband emission c. LINCOMPEX d. Vestigial sideband transmission
Self Test
Modulation
Choose the letter which answer each question.
7. Which symbol indicates that only one sideband is transmitted?a. A3E b. B8E c. C3F d. H3E
8. Refers to an emission designation for facsimile a. J3E and F4E b. A3J and A4E c. A3E and F3C d. R3E and A3E
Self Test
Modulation
Choose the letter which answer each question.
9. Which of the following refers to a double sideband full carrier? a. A3E b. A3J c. F3d. R3A
10. What is emission of F3F? a. Facsimile b. Modulated CW c. RTTY d. Television
Self Test
Modulation
Choose the letter which answer each question.
11. In a filter system of J3E, how many circuits must be balanced? a. 0 b. 1 c. 2 d. 3
12. Independent sideband emission a. H3E b. R3Ec. J3Ed. B8E
Self Test
Modulation
Choose the letter which answer each question.
13. Type of emission produced when an amplitude – modulated transmitter is modulated by a television signal.
a. F3Fb. A3Cc. F3Cd. A3F
14. The output of a balanced modulator is a. AM b. FM c. SSB d. DSB
Self Test
Modulation
Choose the letter which answer each question.
15. What is the emission designation for FM telephony? a. F3Eb. G3E c. J3Ed. H3E
16. What is the maximum power of output of an A3E emission? a. 4 W b. 10 W c. 3 kW d. 10 kW
Self Test
Modulation
Choose the letter which answer each question.
17. If the modulation index of an AM wave is doubled, the antenna current is doubled, the AM system being used is ____________.
a. A5C b. A3J c. A3Hd. A3
18. What is the power saving in J3E system at 100% modulation? a. 33% b. 100% c. 83.3% d. 66.67%
Self Test
Modulation
Choose the letter which answer each question.
19. What is the letter number designation for an FM facsimile? a. F3E b. F3Cc. A3Ed. C3C
20. Which symbol indicates that only one sideband is transmitted? a. A3Eb. B8Ec. C3Fd. H3E
Self Test
Modulation
Choose the letter which answer each question.
21.Vestigial sideband emission a. C3Fb. R3Ec. J3Ed. B8E
22. The third symbol radio emission which represent telephone transmission including sound broadcasting.
a. Wb. Fc. Cd. E
Self Test
Modulation
Choose the letter which answer each question.
23. What is the shape of a trapezoidal pattern at 100% modulation? a. Circle b. Square c. Triangle d. Rectangle
24. ____________ is the maximum sideband suppression using filter system?
a. 50 dB b. 60 dB c. 40 dB d. 30 dB
Self Test
Modulation
Choose the letter which answer each question.
25. Which test instruments displays the carrier and the sidebands amplitude with frequency to frequency?
a. Oscilloscope b. Spectrum Analyzer c. Frequency Analyzer d. Amplitude Analyzer
26. In what type of transmitters is balanced modulators used? a. SSB b. A3Ec. F3E d. FM
Self Test
Modulation
Choose the letter which answer each question.
27. What is emission C3F? a. Facsimile b. Modulated CW c. RTTY d. Television
28. Which one of the following emission transmits the lower sideband and half of the upper sideband?
a. A5C b. J3Ec. A3J d. A3H
Self Test
Modulation
Choose the letter which answer each question.
29. Single sideband reduced carrier emission type. a. H3E b. R3E c. J3E d. B8E
30. Phase modulation type. a. F3E b. F3Cc. F3F d. G3E
Self Test
Modulation
Choose the letter which answer each question.
31. A type of emission is produced when an amplitude – modulated transmitter is modulated by a facsimile signal.
a. A3F b. F3F c. A3C d. F3C
32. Double sideband full carrier emission type. a. A3J b. H3E c. R3A d. A3E
Self Test
Modulation
Choose the letter which answer each question.
33. What type of emission is frequency modulation?a. F3E b. G3E c. A3E d. B3E
34. Standard way of designating AM a. A3E b. B3E c. AHE d. C3F
Self Test
Modulation
Choose the letter which answer each question.
35. If the input to a detector stage is an amplitude – modulated (A3E) IF signal then the output from the stage is
a. A lower frequency carrier b. The audio – voice information c. A Morse code signal d. The upper of lower signal sidebands
36. What is the major advantage of FM over AM? a. High frequency b. No interference c. Simple circuits d. Less noise
Self Test
Modulation
Choose the letter which answer each question.
37. What percent of the radiated power is in the sidebands with 50% sinusoidal modulation (AM)?
a. 11% b. 16.67% c. 33%d. 20%
38. The carrier in an AM transmitter is thea. Transmitter’s output signal when the modulation is zero b. Transmitter’s output signal when the modulation is present c. Output signal from the crystal oscillator d. RMS value of the AM signal
Self Test
Modulation
Choose the letter which answer each question.
39. Two AM transmitting antennas are close together. As a result the two modulated signals are mixed on the final RF stage of both transmitters. What is the resultant effect on the other station?
a. Harmonic interference b. Intermodulation interference c. Spurious interference d. Cross modulation interference
40. The purpose why an RF amplifier is operated under linear class B conditions (as opposed to class C ) is to
a. Generate even only harmonics b. Generate even only odd harmonics c. Increase the efficiency d. Amplify an AM signal
Self Test
Modulation
Choose the letter which answer each question.
41. The type of emission that suffer most from selective fadinga. CW and SSB b. SSB and TV c. FM and double sideband FM d. AATV and CW
42. A receive selectivity of 10 kHz in the IF circuitry is optimum for what type of signals?
a. AB voice b. Facsimile c. FM d. Double – sideband AM
Self Test
Modulation
Choose the letter which answer each question.
43. The negative half of the AM wave is supplied by a/an ____________ in a diode modulator.
a. Tuned circuit b. Transformer c. Capacitor d. Inductor
44. If the percentage modulation of an AM amplifier is 88% and the modulating signal is 1 volt, the carrier has an amplitude of ____________.
a. 1.14 volts b. 0.88 volts c. 1.88 volts d. 0.12 volts
Self Test
Modulation
Choose the letter which answer each question.
45. Mixer is also known as ____________. a. Modulator b. Suppressor c. Converter d. Beater
46. 100% modulation in AM means a corresponding in total power by ___________.
a. 25%b. 75% c. 100% d. 50%
Self Test
Modulation
Choose the letter which answer each question.
47. Three audio waves with 100, 200 and 300 volts amplitude respectively, simultaneously modulate a 450 volts carrier. What is the total percent of the modulation of the AM wave?
a. 69%b. 115.5% c. 50% d. 83%
48. For ____________ percent modulation in AM, the modulation envelope has a peak value double the unmodulated carrier level.
a. 50b. 66.67c. 100 d. 83.3
Self Test
Modulation
Choose the letter which answer each question.
49. The antenna current of an AM transmitter is 6.2 A when unmodulated and rises to 6.7 A when modulated. What is the percent modulation?
a. 57.9% b. 67.5% c. 51% d. 42.8%
50. One of the following can produce AM a. Having the carrier value a resistance b. Having the modulating signal vary a capacitance c. Vary the carrier frequency d. Vary the gain of an amplifier
Self Test
Modulation
Choose the letter which answer each question.
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