Am transmitter

RF Carrier oscillator
To generate the carrier signal.
Usually a crystal-controlled oscillator is used.
Buffer amplifier
Low gain, high input impedance linear amplifier.
To isolate the oscillator from the high power amplifiers.
Modulator : can use either emitter collector modulation
Intermediate and final power amplifiers (pull-push modulators)
Required with low-level transmitters to maintain symmetry in the AM envelope
Coupling network
Matches output impedance of the final amplifier to the transmission line/antenn

Applications are in low-power, low-capacity systems : wireless intercoms, remote control units, pagers and short-range walkie-talkie
Modulating signal is processed similarly as in low-level transmitter except for the addition of power amplifier
Power amplifier
To provide higher power modulating signal necessary to achieve 100% modulation (carrier power is maximum at the high-level modulation point).
Same circuit as low-level transmitter for carrier oscillator, buffer and driver but with addition of power amplifier

1. AM AM
TRANSMITTERTRANSMITTER

2. Amplitude
modulation
Frequncy
modulation
WHAT IS MODULATION?
It is modulation of
amplitude of the wave.
This type of
transmission is simple
and easily disturbed.
AM radio ranges from
535 to 1705 kilohertz
(OR) Up to
1200 Bits per second.
Can send only one
channel at a time.
It is modulation
frequency of the wave.
This type of
transmission is
complex.
FM radio ranges in a
higher spectrum from
88 to 108 megahertz.
(OR) 1200 to
2400 bitsper second.
Can send two
channels at a time.
•Modulation is process of varying one or more properties like frequency , amplitude,
phase of periodic waves.
•AM and FM are two very popular and very different methods of sending information 
over the airwaves.

3. Amplitude modulation
Amplitude modulation is a technique used in electronic
communication, most commonly for transmitting information
via a radio carrier wave.
AM works by varying the strength of the transmitted signal in
relation to the information being sent.
In AM, a radio wave known as the "carrier" or "carrier wave"
is modulated in amplitude by the signal that is to be
transmitted.
It is used in both analog and digital communication
and telemetry.
Range of modulator is small as compared to FM.

4. How does the signal
transmitted?
•This circuit designed to transmit the input signal with the
help of a carrier wave.
•The input signal is too weak to travel long distance , so
we assist a carrier wave to the wave.
•Now here we create carrier wave
by the help of RC oscillator in 555
timer chip.
•The electrical signals are change
to electromagnetic wave with the
help of antenna.

5. 5
Principles of AM
Amplitude Modulation – is a process of changing the amplitude of a
relatively high frequency carrier signal with the instantaneous value of
the modulating signal (information signal)
2 inputs to the modulation devise (modulator)
A single, high frequency RF carrier signal of constant amplitude
Low frequency information signals that maybe a single frequency or a
complex waveform made up of many frequencies
In the modulator, the information signal modulates the RF carrier
signal to produce a modulated waveform made up of many
frequencies
This modulated waveform also called as AM envelope

6. Transmitters
•Transmitter concept is in the block diagram below:-
•Foundation Licence only permits use of commercial equipment to
minimise the risk of interference and/or out-of-band operation.
•Avoid over-deviating, and operating PAs into poor matches !!
1
3
2 4
Mic
1 - Audio Stage
2 - Modulator eg AM, FM, SSB
3 - RF Frequency Generator
4 - RF Power Amplifier

7. Transmitter
Modulator
Power Amplifier
Driver Stages
Power Stage
Antenna
Oscillator
Signal

8. Receiver
Low Noise Amplifier
Low Noise Amplifier
Gain-Stage Amplifier
Filter
Antenna Oscillator
Mixer
Filter

9. Transceiver = Transmitter + Receiver
Modulator
Power Amplifier
Antenna
Oscillator
Signal
Low Noise Amplifier
Filter
Mixer
Filter

10. The AM Radio “System”
Transmitter Receiver
AM is a nonlinear process
Sum and difference frequencies are created that carry the information

11. Basic analog communications system
Modulator
Demodulator
Transmission
Channel
Input
transducer
Transmitter
Receiver
Output
transducer
Carrier
EM waves (modulated
signal)
EM waves (modulated
signal)
Baseband signal
(electrical signal)
Baseband signal
(electrical signal)

12. Amplitude Modulation
Carrier wave
Baseband signal
Modulated wave
Amplitude varying-
frequency constant

13. SIMPLE AM TRANSMITTER
CIRCUIT

14. ABOUT THE CIRCUIT
Potentiometers are commonly used to control electrical devices such
as volume controls on audio equipment.
The 555 timer chip makes easy to connect the circuit.
This chip creates the carrier wave of higher frequency.
The audio signal is weak signal which cannot travel
long distances . So we assist carrier wave to carry
the our to the destination.

15. AM Transmitters
AM transmitters are a “mature” technology, but are
still in widespread use
Examples include:
Standard AM broadcast stations
CB radio
VHF aircraft radio

16. AM Transmitter Stages
All of the stages of a transmitter (except the power
amplifier and possibly the driver) operate at low power
levels
This part of the transmitter, exclusive of the power-
handling stages, is called the exciter
Other transmitter components include:
The oscillator stage
The buffer and multiplier stages
The driver stage
The power amplifier/modulator

17. AM Transmitter
Each AM station is allocated a frequency
band of 10kHz in which to transmit its
signal.
This frequency band is centered around the
carrier frequency of the station
A station at 610 on your dial transmits at a carrier frequency of
610kHz
The signal that is broadcast occupies the frequency range from
605kHz to 615kHz

18. AM Transmitter
Transmitter input (signal source) is an
audio signal.
Speech, music, advertisements
The input is modulated to the proper carrier
frequency.
Modulated signal is amplified and
broadcast

19. Transmitter Block Diagram
Signal
Sourc
e
Modulato
r
Power
Amplifier
Antenna

20. In the block diagram of the a-m transmitter , the r-f
section consists of an oscillator feeding a buffer,
which in turn feeds a system of frequency
multipliers and/or intermediate power amplifiers. If
frequency multiplication is unneccessary, the
buffer feeds directly into the intermediate power
amplifiers which, in turn, drive the final power
amplifier. The input to the antenna is taken from
the final power amplifier.
RF SECTION

21. Frequency domain
representation of
output

22. Modulator-Time Domain
Input Signal
Output Signal

23. Antenna
The antenna converts a current or a voltage signal
to an electromagnetic signal which is radiated
throughout space.
In the frequency domain, the antenna output is
0
frequency
Undesired
Signals
Desired Signal
Carrier
Frequency
of desired station

28. 29
AM Transmitters
1 : Low-level Transmitters
Block diagram for a low-level AM DSBFC transmitter :
Preamplifier
Linear voltage amplifier with high input impedance.
To raise source signal amplitude to a usable level with minimum nonlinear
distortion and as little thermal noise as possible.
Modulating signal driver
Amplifies the information signal to an adequate level to sufficiently drive the
modulator.

29. 30
Low-level Transmitters
Block diagram for a low-level AM DSBFC transmitter :
RF Carrier oscillator
To generate the carrier signal.
Usually a crystal-controlled oscillator is used.
Buffer amplifier
Low gain, high input impedance linear amplifier.
To isolate the oscillator from the high power amplifiers.
Modulator : can use either emitter collector modulation
Intermediate and final power amplifiers (pull-push modulators)
Required with low-level transmitters to maintain symmetry in the AM envelope

30. 31
Low-level Transmitters
Coupling network
Matches output impedance of the final amplifier to the transmission line/antenn
Applications are in low-power, low-capacity systems : wireless intercoms,
remote control units, pagers and short-range walkie-talkie

31. 32
High-level Transmitters
Block diagram for a high-level AM DSBFC transmitter
Modulating signal is processed similarly as in low-level transmitter except for the
addition of power amplifier
Power amplifier
To provide higher power modulating signal necessary to achieve 100% modulation (carrier power is
maximum at the high-level modulation point).
Same circuit as low-level transmitter for carrier oscillator, buffer and driver but with
addition of power amplifier

32. 33
High-level Transmitters
Primary functions of modulator circuit
Provide the necessary circuitry for the modulation to occur
The final power amplifier
Frequency-up converter : translates low-frequency information signals to radio-frequency
signals that can be efficiently radiated from the antenna and propagates through the free
space