LECTURE ON AM/FM TRANSMITTER

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LECTURE ON AM/FM TRANSMITTER

• Subject Communication Systems-I
• Class 4th, ECE
• PRESENTED BY
• DIMPLE BADHWAR
• LECTURER, GPC BATHINDA.
• Mobile No. 9855505814

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Block diagram of communication system
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TRANSMITTER
Information or Message
Communication Channel or Medium

• It takes the information to be communicated in
  electrical form and convert it into an electronic
  signal compatible with the communication medium .

Information in Electrical form
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TRANSMITTER

• In this block diagram of communication system,
  the upper section is called the transmitting
  section.

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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.

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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.

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TRANSMITTER

• An antenna is a transducer which converts
  electrical signals into electromagnetic waves.

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BASIC BLOCKS OF TRANSMITTER

• Modulator
• RF oscillator
• Power amplifier

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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 .

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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.

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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.

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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)

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CLASSIFICATION BASED ON TRANSMITTED FREQUENCY

• Very high frequency (VHF) transmitters (30MHZ-300
  MHZ)
• Ultra high frequency (UHF) transmitters (300 MHZ-
  3GHZ)
• Microwave transmitters (gt3GHZ)

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CLASSIFICATION BASED ON TYPE OF SERVICE INVOLVED

• Radio broadcast transmitters.
• Radio telephony transmitters.
• Radio telegraph transmitters.

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CLASSIFICATION BASED ON TYPE OF SERVICE INVOLVED

• Television transmitters.
• Radar transmitters.
• Navigational transmitters.

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CLASSIFICATION BASED ON TYPES OF MODULATION

• CW Transmitters
• AM Transmitters
• FM Transmitters
• SSB Transmitters

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CONTINIOUS WAVE (CW) TRANSMITTERS

• The CW Transmitter is the simplest type of
  transmitter.
• It is a simple crystal oscillator circuit.
• This oscillator generates a carrier signal of the
  desired frequency.

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CONTINIOUS WAVE (CW) TRANSMITTERS

• Information to be transmitted is expressed in a
  special form of code using dots and dashes to
  represent letters of the alphabet and numbers.
• The information transmitted in this way is called
  as continuous wave (CW) transmission.

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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.

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CONTINIOUS WAVE (CW) TRANSMITTERS

• 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.

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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.

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ADVANTAGES OF CW TRANSMITTER

• Simple to construct
• Compact and portable
• Can be operated on batteries

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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.

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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.

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AM TRANSMITTER

• In AM Transmitter, AM signal is transmitted by
  a transmitter. The information is contained in
  its amplitude variation.

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TYPES OF AM TRANSMITTERS

• Low Level modulation transmitters.
• High Level modulation transmitters.

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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).

Signal Source (Modulating Signal)
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STABILISEDRF OSCILLATOR
LOW LEVEL MODULATION TRANSMITTERS
Antenna
Class A Buffer amplifier
Power Amplifiers
Linear Amplifiers
Modulator
Audio processing filtering
Class A AF amplifier
AF modulating signal
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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.

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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.

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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.

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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.

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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.

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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.

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LOW LEVEL MODULATION TRANSMITTERS

• Overall efficiency is much lower compared to high
  level modulation . This reduce to the use of less
  efficient linear amplifiers.

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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.

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HIGH LEVEL MODULATION TRANSMITTERS

• The generation of AM wave takes place at high
  power levels.

Signal Source (Modulating Signal)
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HIGH LEVEL MODULATION TRANSMITTERS

• Highly efficient class C amplifier are used in
  high level modulation.
• Efficiency is more than low level modulation.

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

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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.

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COMPARISION BETWEEN LOW-LEVEL AND HIGH-LEVEL
MODULATION

• Power level
• Modulation is carried out at low power level.
• Amplifier stages
• Need lesser amplifier stages.

• Modulation is carried out at high power level.
• Need more amplifier stages.

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COMPARISION BETWEEN LOW-LEVEL AND HIGH-LEVEL
MODULATION

• Power efficiency
• After modulation linear amplifiers can only be
  used. This gives lower power efficiency.
• 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.

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COMPARISION BETWEEN LOW-LEVEL AND HIGH-LEVEL
MODULATION

• Applications
• Used as higher power broadcast transmitters.

• Used in TV transmitters.

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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.

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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.

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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.

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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.

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METHODS OF FM GENERATION

Methods of FM Generation
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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

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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.

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DIRECT FM

Modulating Signal
Oscillator Tank Circuit
Variable reactance device
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DIRECT FM

• When the variable reactance of the device varies
  with the modulating signal the oscillator
  generator the corresponding FM signal.

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DIRECT MODULATORS

• Few other direct modulators are
• Varactor diode modulator.
• Reactance modulator.
• V.C.O. modulator.
• Stabilized reactance modulator.
• Cross by direct FM transmitters.

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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.

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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.

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ADVANTAGES OF DIRECT FM MODULATION

• The main advantage of direct FM generation is the
  simplicity of the modulators and their low cost.

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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.

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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.

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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.

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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.

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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.

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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.

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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