Radio transmitters

Different Types of radio transmitters

1. LECTURE ON AM/FM TRANSMITTER
PRESENTED BY
A.ABHISHEK REDDY
ASSISTANT PROFESSOR, MECS.
Mobile No.:8121444996
1 SESSION-1

2. UNIT-4 SYLLABUS
SESSION-12
TransmittersandReceivers:
(i)Classificationoftransmitters.Highlevelandlowlevel
AMtransmitters.
(ii)FMtransmitters.
(iii)PrincipleofoperationofTunedradiofrequency
(TRF)andsuperheterodynereceivers.
(iv)SelectionofRFamplifier.ChoiceofIntermediate
frequency.Imagefrequencyanditsrejectionratio
(v)Receivercharacteristics:Sensitivity,Selectivity,
Fidelity,Doublespotting,AutomaticGainControl.

3. Block diagram of communication
system
3 SESSION-1

4. Information or
Message
Transducer Transmitter Communication
Channel or
Medium
Information
in Electrical form
•Ittakestheinformationtobecommunicatedinelectrical
formandconvertitintoanelectronicsignalcompatible
withthecommunicationmedium.
TRANSMITTER
4 SESSION-1

5. TRANSMITTER
In this block diagram of communication
system, the upper section is called the
transmitting section.
5 SESSION-1

6. TRANSMITTER
The main parts of transmitter are explained
as follows :
Microphone : It converts sounds into
electrical signals in wires. It is the opposite
of a loudspeaker.
Modulator : The audio signal is modulated
into the radio frequency carrier in this
modulator stage.
6 SESSION-1

7. TRANSMITTER
Frequency generator : The frequency
generation stage will decide the frequency
on which the transmitter will operate.
RF power amplifier : The power
amplification of the radio signal is carried
out in the final stage. It makes the signal
stronger so that it can be transmitted
through the channel over long distances.
7 SESSION-1

8. TRANSMITTER
An antenna is a transducer which
converts electrical signals into
electromagnetic waves.
8 SESSION-1

9. BASIC BLOCKS OF TRANSMITTER
Modulator
RF oscillator
Power amplifier
9 SESSION-1

10. BASIC FUNCTIONS OF TRANSMITTER
Modulation
Carrier generation
Amplification (Power)
Itisanelectronicunitwhichacceptstheinformation
signaltobetransmittedandconvertsitintoanRFsignal
capableofbeingtransmittedoverlongdistances.
10 SESSION-1

11. BASIC FUNCTIONS OF TRANSMITTER
Everytransmitterhasthreebasicfunctionsasfollows:
Thetransmittermustgenerateasignalofcorrect
frequencyatadesiredpointinthespectrum.
Secondlyitmustprovidesomeformofmodulationto
modulatethecarrier.
Thirditmustprovidesufficientpoweramplificationin
ordertocarrythemodulatedsignaltoalongdistance.
11 SESSION-1

12. CLASSIFICATION OF RADIO
TRANSMITTERS
1.According to the type of modulation used.
2.According to service involved.
3.According to the frequency range involved.
4.According to the power used.
12 SESSION-1

13. CLASSIFICATION BASED ON
TRANSMITTED FREQUENCY
Low frequency (LF) transmitters (30 KHZ-
300KHZ)
Mediumfrequency(MF)transmitters(300KHZ-
3MHZ)
Highfrequency(HF)transmitters(3MHZ-
30MHZ)
13 SESSION-1

14. CLASSIFICATION BASED ON
TRANSMITTED FREQUENCY
Veryhighfrequency(VHF)transmitters
(30MHZ-300MHZ)
Ultrahighfrequency(UHF)transmitters
(300MHZ-3GHZ)
Microwavetransmitters(>3GHZ)
14 SESSION-1

15. CLASSIFICATION BASED ON TYPE
OF SERVICE INVOLVED
Radio telegraph transmitters.
i.AM/FM for point –to-point communication
ii.Transmission of messages are sent by means of dots and dashes
Radio telephony transmitters.
i.used for transmission of telephone signals over long distances.
ii.AM/FM and short waves involve highly directional antennas
iii.Txdpower is order of few KW
Television transmitters.
i.Txmittingof picture/video signal—VSB
ii.Txmittingof sound—FM
iii.Both operate in VHF and UHF band with 7MHZ bandwidth
15 SESSION-1

16. CLASSIFICATION BASED ON TYPE
OF SERVICE INVOLVED
Radar transmitters.
i.Pulse modulation : uses pulses as carrier and requires typically of 100KW
peak power in microwave band typically 3000MHz(10cm wavelength) or
10,000MHz(3cm wavelength)
ii.Continuous wave transmission : requires FM as carrier (dopplereffect)
Navigational transmitters.
i.For seas and air for blind landing of aircraft
ILS-instrumental landing system
GCA-ground control approach
16 SESSION-1

17. CLASSIFICATION BASED ON TYPES
OF MODULATION
CW Transmitters
AM Transmitters
FM Transmitters
SSB Transmitters
17 SESSION-1

18. CONTINIOUS WAVE (CW)
TRANSMITTERS
Itisasimplecrystaloscillatorcircuitwhichgeneratesacarriersignalofthedesired
frequency.
Informationtobetransmittedisexpressedinaspecialformofcodeusingdotsand
dashestorepresentlettersofthealphabetandnumbers.
18 SESSION-1

19. CONTINIOUS WAVE (CW)
TRANSMITTERS
Thekeyisasimplehandoperatedswitchconnectedinemitterofthetransistor.
Byclosingthekeywecanturnonthecrystaloscillatoronandbyopeningthe
keytheoscillatoristurnedoff.
 Whenthekeyisclosed,theoscillatorproducesasinusoidalsignalata
frequencyequaltothecrystalfrequency,whereaswiththekeyopen,the
outputofoscillatoriszero.
 Thekeyisopenedandclosedinordertoproducezerooutputanddotsor
dashes.
19 SESSION-1

20. CONTINIOUS WAVE (CW)
TRANSMITTERS
Dotscorrespondtotheshortdurationoutputwhereasadashcorrespondstoa
longdurationoutput.
Therequiredmessagescanbetransmittedusingdifferentcombinationsof
dotsanddashesfordifferentalphabetsandletters.
Lengthofdashis3timesthelengthofdots
Spacingbetweenlettersinawordisequalto3dotsor1dash
20 SESSION-1

21. ADVANTAGES OF CW TRANSMITTER
Simple to construct
Compact and portable
Can be operated on batteries
21 SESSION-1

22. DISADVANTAGES
Askilledoperatorisrequiredtoconvertthe
messagetobesentintoacodedformofdots
anddashes.
Longdistancecommunicationisnotpossible.
Voiceorpicturecannotbesent.
22 SESSION-1

23. DIFFERENT TYPES OF CWT
Straight Key
Keyer
“Bug” Keyer
23 SESSION-1

24. AM TRANSMITTER
AmplitudemodulationtechniqueisusedinAM
transmitters,heretheamplitudeofcarrierisvaried
inproportionwiththeamplitudeofthemodulating
signal,keepingitsfrequencyandphaseconstant.
Usedinradio&TVbroadcasting.
24 SESSION-1

25. AM TRANSMITTER
InAMTransmitter,AMsignalistransmitted
byatransmitter.Theinformationiscontained
initsamplitudevariation.
25 SESSION-1

26. TYPES OF AM TRANSMITTERS
LowLevelmodulationtransmitters.
HighLevelmodulationtransmitters.
26 SESSION-1

27. LOW LEVEL MODULATION
TRANSMITTERS
The generation of AM wave takes place at a low power
level.
The generated AM signal is then amplified using a chain
of linear amplifier ( A , AB or B).
Low Level
Modulator
Power
Amplifier
(Linear)
RF Carrier
Oscillator
Signal
Source
(Modulating
Signal)
27 SESSION-1

28. Class A
Buffer
amplifier
Modulator
Linear
Amplifiers
STABILISED
RF
OSCILLATOR
Power
Amplifiers
Antenna
Class A AF
amplifier
Audio
processing &
filtering
AF
modulating
signal
LOW LEVEL MODULATION TRANSMITTERS
28 SESSION-1

29. LOW LEVEL MODULATION
TRANSMITTERS
The RF oscillator produces the carrier signal.
The RF oscillator is stabilized in order to
maintain the frequency deviation within the
prescribed limit. The carrier frequency is
equal to the transmitter frequency.
Usually the transmitter operates on assigned
frequencies or channels. Crystal provides the
best way to obtain the described frequency
with good stability.
29 SESSION-1

30. LOW LEVEL MODULATION
TRANSMITTERS
We cannot use the LC oscillator because
they have low frequency stability.
The carrier signal from the crystal oscillator is
applied to the modulator with a modulating
signal. At the output of the modulator we get
the AM wave.
30 SESSION-1

31. LOW LEVEL MODULATION
TRANSMITTERS
The modulating signal is obtained from a
source such as a microphone and applied to
a buffer processing unit.
The buffer is a class A amplifier which
isolates the AF source from the rest of high
power circuit and amplifies it to an adequate
level.
31 SESSION-1

32. LOW LEVEL MODULATION
TRANSMITTERS
The amplified modulating signal is applied to
the modulator along with the carrier. At the
output of the modulator we get the AM wave.
The AM signal is then amplified using a chain
of linear amplifier to raise the power level.
32 SESSION-1

33. LOW LEVEL MODULATION
TRANSMITTERS
The linear amplifier can be class A, AB or B
type amplifiers. The linear amplifier are used
in order to avoid the wave form distortion in
AM wave.
The amplitude modulated signal is then
transmitted using transmitted antenna.
33 SESSION-1

34. LOW LEVEL MODULATION
TRANSMITTERS
The transistorized modulator circuits can be
used for low level modulator due to the low
power which is to be handled.
The low level transmitter does not require a
large AF modulator power so its design is
simplified.
34 SESSION-1

35. LOW LEVEL MODULATION
TRANSMITTERS
Overall efficiency is much lower compared to
high level modulation . This reduce to the
use of less efficient linear amplifiers.
35 SESSION-1

36. AUDIO PROCESSING
TheAFmodulatingsignalispassed
throughanaudioprocessingunitbefore
applyingittothemodulator.
Thisblockcarriesoutsomeformof
“speechprocessing”intheformoffiltering
andamplitudecontrol.
Theweaksignalsamplifiedautomatically
withahighergainandstrongsignalsare
amplifiedwithsmallergain.Thiswillbring
allthesignalstoasufficientlevel.36 SESSION-1

37. HIGH LEVEL MODULATION
TRANSMITTERS
ThegenerationofAMwavetakesplaceathighpower
levels.
RF Carrier
Oscillator
Narrow Band
Power Amplifier
High Level
Modulator
Wide Band
Power
Amplifier
Signal
Source
(Modulating
Signal)
37 SESSION-1

38. HIGH LEVEL MODULATION
TRANSMITTERS
HighlyefficientclassCamplifierareusedin
highlevelmodulation.
Efficiencyismorethanlowlevelmodulation.
38 SESSION-1

39. Class A RF
amplifier
Stabilized RF
crystal
oscillator
Antenna
Class A AF
amplifier
Audio
processing &
filtering
AF
modulating
signal
Class C RF
power
amplifier
High Level
Modulator
Class B AF
power
amplifier
HIGH LEVEL MODULATION TRANSMITTERS
39 SESSION-1

40. HIGH LEVEL MODULATION
TRANSMITTERS
ManyoftheAMtransmittersusethehighlevel
modulationtechnique.
Thecrystaloscillatorproducestherequired
carriersignal.TheclassAamplifierfollowingthe
oscillatoractsasabufferwhichisolatesthe
oscillatorfromthehighpowercircuit.
40 SESSION-1

41. HIGH LEVEL MODULATION
TRANSMITTERS
TheoutputofthisclassAamplifierisapplied
toaclassCpoweramplifier.Itraisesthe
powerlevelofthecarriertoanintermediately
highvalue.
TheAFmodulatingsignalisappliedtothe
audioprocessingunitwhichprocessesthis
signalasdiscussedintheprevioussection.
41 SESSION-1

42. COMPARISION BETWEEN LOW -LEVEL
AND HIGH-LEVEL MODULATION
1.Power level :
Modulation is carried
out at low power level.
2.Amplifier stages:
Need lesser amplifier
stages.
Modulation is carried
out at high power
level.
Need more amplifier
stages.
42 SESSION-1

43. COMPARISION BETWEEN LOW -LEVEL
AND HIGH-LEVEL MODULATION
3.Power efficiency :
After modulation linear
amplifiers can only be
used. This gives lower
power efficiency.
4.Power losses :
Power losses in
amplifiers is higher,
the cooling problem is
severe.
Non linear amplifiers
can also be used. This
leads to higher power
efficiency.
Power losses is less,
the cooling problem is
not severe.
43 SESSION-1

44. COMPARISION BETWEEN LOW -LEVEL
AND HIGH-LEVEL MODULATION
5.Applications:
Used as higher power
broadcast transmitters.
Used in TV
transmitters.
44 SESSION-1

45. SSB TRANSMITTERS
PILOT CARRIER SYSTEM
INDEPENDENT SIDEBAND SYSTEM(ISB)
45 SESSION-1

46. PILOT CARRIER SSB TRANSMITTERS
46 SESSION-1

47. ISB
47 SESSION-1

48. STANDARD BROADCAST BANDS
AM(MEDIUM WAVE) : 550KHz-1650KHZ
AM(SHORT WAVE) : 3-30MHz
FM : 88MHz-108MHz
TV : 54MHz-88MHz
174MHz-216MHz VHF
470MHz-890MHz(UHF)
48 SESSION-1

49. FM TRANSMITTERS
Frequencymodulationtechniqueisused.
InFMfrequencyofthecarrierisvariedin
proportionwiththeamplitudeofthemodulating
signalkeepingitsamplitudeconstant.
49 SESSION-1

50. FM TRANSMITTERS
Usedinradio,TVsoundbroadcasting&
policewirelesstransmission.
InFMtransmittertheFMsignalistransmitted
byatransmitter.Theinformationiscontained
initsfrequencyvariation.
50 SESSION-1

51. FM TRANSMITTERS
TheFCChasassignedabandof20MHzto
thecommercialFMbroadcastservice.
Thisbandextendsfrom88MHzto108MHz.
51 SESSION-1

52. FM TRANSMITTERS
This20MHzbandisdividedin100channels,
eachhavingabandwidthof200KHz.
Forprovidinghighqualityreliablemusicthe
maximumfrequencydeviationallowedis75
KHz,withamaximummodulatingsignal
frequencyof15KHz.
52 SESSION-1

53. METHODS OF FM GENERATION
Indirect MethodsDirect Methods
Methods of FM Generation
53 SESSION-1

54. DIRECT FM
IndirectFMgenerationthefrequencyofthe
carrierischangeddirectlyinproportionwiththe
modulatingsignalamplitude.
Types of Direct FM
Reactance modulator
Varactor diode modulator
54 SESSION-1

55. DIRECT FM
Thesemethodsuseavaractordiodeora
reactancetransistorforpresentingavariable
reactanceacrossthefrequencydetermining
circuitofanoscillator.
55 SESSION-1

56. DIRECT FM
Oscillator Tank
Circuit
Modulating
Signal
Variable reactance device
56 SESSION-1

57. DIRECT FM
When the variable reactance of the device
varies with the modulating signal the
oscillator generator the corresponding FM
signal.
57 SESSION-1

58. DIRECT MODULATORS
Few other direct modulators are:
Varactor diode modulator.
Reactance modulator.
V.C.O. modulator.
Stabilized reactance modulator.
Cross by direct FM transmitters.
58 SESSION-1

59. REACTANCE MODULATOR
In the reactance modulator, a transistor or
FET is operated as a variable reactance
(inductive or capacitive) device.
This device is connected across the tuned
circuit of an oscillator.
As the instantaneous value of modulating
voltage changes, the reactance offered by
the transistor or FET will change
proportionally.
59 SESSION-1

60. VARACTOR DIODE
Herethevoltageappliedacrossthevaractordiode
variesinproportionwiththemodulatingvoltage.
Thiswillvarythejunctioncapacitanceofthevaractor
diode.Thevaractordiodeappearsinparallelwiththe
oscillatortunedcircuit.Hencetheoscillatorfrequency
willchangewithchangeinvaractordiodecapacitance
andFMwaveisproduced.
60 SESSION-1

61. ADVANTAGES OF DIRECT FM
MODULATION
ThemainadvantageofdirectFMgenerationis
thesimplicityofthemodulatorsandtheirlow
cost.
61 SESSION-1

62. DISADVANTAGES OF DIRECT
METHOD
InthedirectmethodofFMgenerationwehave
tousetheLCoscillator.TheLCoscillator
frequencyisnotstable.
Thereforeitsnotpossibletousesuchoscillator
forcommunicationorbroadcastpurpose.
62 SESSION-1

63. DIRECT FM
Thereforewehavetouseaschemeinwhich
wecanusethecrystaloscillatortocontrol
thecarrierfrequency.
Thereforewehavetousetheautomatic
frequencycontrolsystem.
63 SESSION-1

64. INDIRECT METHOD ( ARMSTRONG
METHOD)
Here FM is obtained through phase modulation.
ACrystaloscillatorisusedandhencethe
frequencystabilityisveryhigh.
AnarrowbandPMsignalisgeneratedviaDSB-
SCsignalanda90
°
phaseshiftedsubcarrier
signalfromacrystaloscillator.
64 SESSION-1

65. INDIRECT METHOD ( ARMSTRONG
METHOD)
Theintegrationofthemodulatingsignal
makestheNBPMsignaltoNBFMsignal.
ThisNBFMsignalisappliedtoaharmonic
generator(frequencymultiplier)whichwill
increaseboththecarrierfrequencyandthe
maximumdeviationtotherequired.
65 SESSION-1

66. COMPARISION OF AM AND FM
BROADCASTING
AM Broadcasting
It requires smaller
transmission
bandwidth.
It can be operated in
low, medium and high
frequency bands.
It has wider coverage.
FM Broadcasting
It requires larger bandwidth.
It needs to be operated in very
high and frequency bands.
Its range is restricted to 50 km.
66 SESSION-1

67. COMPARISION OF AM AND FM
BROADCASTING
The demodulation is
simple.
The stereophonic
transmission is not
possible.
The system has poor
noise performance.
The process of
demodulation is
complex.
In this, stereophonic
transmission is
possible.
It has an improved
noise performance.
67 SESSION-1

68. REVISION
What is Transmitter
Classification of Transmitters
Based on Type of modulation
Based on Transmitted frequency
AM Transmitters
Types of AM transmitters
Low Level modulation transmitter
High Level modulation transmitter
FM Transmitters
Types of FM generation
Direct method of FM generation
Indirect method of FM generation68 SESSION-1