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-1
2
Transmitters and Receivers:
(i)Classification of transmitters. High level and low level
AM transmitters.
(ii)FM transmitters.
(iii) Principle of operation of Tuned radio frequency
(TRF) and super heterodyne receivers.
(iv)Selection of RF amplifier. Choice of Intermediate
frequency. Image frequency and its rejection ratio
(v)Receiver characteristics: Sensitivity, Selectivity,
Fidelity,Double spotting, Automatic Gain Control.

3. Block diagram of communication
system
3 SESSION-1

4. Information or
Message
Transducer Transmitter Communication
Channel or
Medium
Information
in Electrical form
•It takes the information to be communicated in electrical
form and convert it into an electronic signal compatible
with the communication medium .
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)
 It is an electronic unit which accepts the information
signal to be transmitted and converts it into an RF signal
capable of being transmitted over long distances .
10 SESSION-1

11. BASIC FUNCTIONS OF TRANSMITTER
Every transmitter has three basic functions as follows:
 The transmitter must generate a signal of correct
frequency at a desired point in the spectrum.
 Secondly it must provide some form of modulation to
modulate the carrier.
 Third it must provide sufficient power amplification in
order to carry the modulated signal to a long distance.
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)
 Medium frequency (MF) transmitters ( 300 KHZ-
3 MHZ)
 High frequency (HF) transmitters (3 MHZ-
30MHZ)
13 SESSION-1

14. CLASSIFICATION BASED ON
TRANSMITTED FREQUENCY
 Very high frequency (VHF) transmitters
(30MHZ-300 MHZ)
 Ultra high frequency (UHF) transmitters
(300 MHZ- 3GHZ)
 Microwave transmitters (>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. Txd power is order of few KW
 Television transmitters.
i. Txmitting of picture/video signal—VSB
ii. Txmitting of 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 (doppler effect)
 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
 It is a simple crystal oscillator circuit which generates a carrier signal of the desired
frequency.
 Information to be transmitted is expressed in a special form of code using dots and
dashes to represent letters of the alphabet and numbers.
18 SESSION-1

19. CONTINIOUS WAVE (CW)
TRANSMITTERS
 The key is a simple hand operated switch connected in emitter of the transistor.
 By closing the key we can turn on the crystal oscillator on and by opening the
key the oscillator is turned off.
 When the key is closed, the oscillator produces a sinusoidal signal at a
frequency equal to the crystal frequency, whereas with the key open, the
output of oscillator is zero.
 The key is opened and closed in order to produce zero output and dots or
dashes.
19 SESSION-1

20. CONTINIOUS WAVE (CW)
TRANSMITTERS
 Dots correspond to the short duration output whereas a dash corresponds to a
long duration output.
 The required messages can be transmitted using different combinations of
dots and dashes for different alphabets and letters.
 Length of dash is 3times the length of dots
 Spacing between letters in a word is equal to 3dots or 1 dash
20 SESSION-1

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

22. DISADVANTAGES
 A skilled operator is required to convert the
message to be sent into a coded form of dots
and dashes.
 Long distance communication is not possible.
 Voice or picture can not be sent.
22 SESSION-1

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

24. AM TRANSMITTER
 Amplitude modulation technique is used in AM
transmitters, here the amplitude of carrier is varied
in proportion with the amplitude of the modulating
signal, keeping its frequency and phase constant.
 Used in radio & TV broadcasting.
24 SESSION-1

25. AM TRANSMITTER
 In AM Transmitter, AM signal is transmitted
by a transmitter. The information is contained
in its amplitude variation.
25 SESSION-1

26. TYPES OF AM TRANSMITTERS
 Low Level modulation transmitters.
 High Level modulation transmitters.
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
The AF modulating signal is passed
through an audio processing unit before
applying it to the modulator.
This block carries out some form of
“speech processing” in the form of filtering
and amplitude control.
The weak signals amplified automatically
with a higher gain and strong signals are
amplified with smaller gain. This will bring
all the signals to a sufficient level.
36 SESSION-1

37. HIGH LEVEL MODULATION
TRANSMITTERS
 The generation of AM wave takes place at high power
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
 Highly efficient class C amplifier are used in
high level modulation.
 Efficiency is more than low level modulation.
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
 Many of the AM transmitters use the high level
modulation technique.
 The crystal oscillator produces the required
carrier signal. The class A amplifier following the
oscillator acts as a buffer which isolates the
oscillator from the high power circuit.
40 SESSION-1

41. HIGH LEVEL MODULATION
TRANSMITTERS
 The output of this class A amplifier is applied
to a class C power amplifier. It raises the
power level of the carrier to an intermediately
high value.
 The AF modulating signal is applied to the
audio processing unit which processes this
signal as discussed in the previous section.
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
 Frequency modulation technique is used.
 In FM frequency of the carrier is varied in
proportion with the amplitude of the modulating
signal keeping its amplitude constant.
49 SESSION-1

50. FM TRANSMITTERS
 Used in radio, TV sound broadcasting &
police wireless transmission.
 In FM transmitter the FM signal is transmitted
by a transmitter. The information is contained
in its frequency variation.
50 SESSION-1

51. FM TRANSMITTERS
 The FCC has assigned a band of 20 MHz to
the commercial FM broad cast service.
 This band extends from 88 MHz to 108 MHz.
51 SESSION-1

52. FM TRANSMITTERS
 This 20 MHz band is divided in 100 channels,
each having a bandwidth of 200 KHz.
 For providing high quality reliable music the
maximum frequency deviation allowed is 75
KHz, with a maximum modulating signal
frequency of 15 KHz.
52 SESSION-1

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

54. DIRECT FM
 In direct FM generation the frequency of the
carrier is changed directly in proportion with the
modulating signal amplitude.
Types of Direct FM
 Reactance modulator
 Varactor diode modulator
54 SESSION-1

55. DIRECT FM
 These methods use a varactor diode or a
reactance transistor for presenting a variable
reactance across the frequency determining
circuit of an oscillator.
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
 Here the voltage applied across the varactor diode
varies in proportion with the modulating voltage.
 This will vary the junction capacitance of the varactor
diode. The varactor diode appears in parallel with the
oscillator tuned circuit. Hence the oscillator frequency
will change with change in varactor diode capacitance
and FM wave is produced.
60 SESSION-1

61. ADVANTAGES OF DIRECT FM
MODULATION
 The main advantage of direct FM generation is
the simplicity of the modulators and their low
cost.
61 SESSION-1

62. DISADVANTAGES OF DIRECT
METHOD
 In the direct method of FM generation we have
to use the LC oscillator. The LC oscillator
frequency is not stable.
 Therefore its not possible to use such oscillator
for communication or broadcast purpose.
62 SESSION-1

63. DIRECT FM
 Therefore we have to use a scheme in which
we can use the crystal oscillator to control
the carrier frequency.
 Therefore we have to use the automatic
frequency control system.
63 SESSION-1

64. INDIRECT METHOD ( ARMSTRONG
METHOD)
 Here FM is obtained through phase modulation.
 A Crystal oscillator is used and hence the
frequency stability is very high.
 A narrow band PM signal is generated via DSB-
SC signal and a 90° phase shifted sub carrier
signal from a crystal oscillator.
64 SESSION-1

65. INDIRECT METHOD ( ARMSTRONG
METHOD)
 The integration of the modulating signal
makes the NBPM signal to NBFM signal.
 This NBFM signal is applied to a harmonic
generator (frequency multiplier) which will
increase both the carrier frequency and the
maximum deviation to the required.
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 generation
68 SESSION-1