Amplitude Modulation (Part2)
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Transcript of Amplitude Modulation (Part2)
Block Diagram of a Low Level AM Transmitter
LOW LEVEL AM TRANSMITTER
- used for low power, low capacity system such as wireless intercoms, remote
control units, pager and short range walkie-talkies.
ELEMENTS OF A LOW-LEVEL AM TRANSMITTER
MODULATING SIGNAL
SOURCE – an acoustic transducer, such
as microphone, magnetic tape
or CD disk, that is need for
voice or music transmission.
PREAMLIFIER – a sensitive, CLASS A
linear voltage amplifier with
high input impedance. Its
function is to raise the
amplitude of the source
signal to a usable level while
producing minimum nonlinear
distortion and adding as little
thermal noise as possible.
MODULATING
SIGNAL DRIVER – also a linear amplifier
that amplifies the information
signal to an adequate level to
sufficiently drive the modulator.
To Modulator
RF CARRIER
OSCILLATOR – crystal controlled
oscillators that will produce
the high frequency carrier
signal.
BUFFER AMPLIFIER – a low gain, high input
impedance linear amplifier (EF
amplifier). Its function is to isolate
the oscillator from high power
amplifiers. The buffer provides a
relatively constant load to the
oscillator, which helps to reduce the
occurrence and magnitude of short-
term frequency variations.
CARRIER DRIVER – a linear amplifier that
amplifies the carrier signal to
an adequate level to sufficiently
drive the modulator.
To Modulator
MODULATOR – collector or emitter
modulators where AM
modulation takes place. A
circuit that changes the
amplitude of the carrier signal
in respond to the variations of
the modulating signal.
INTERMEDIATE AND
FINAL STAGE – either a linear CLASS A or
CLASS B push-pull amplifier. This is
required with low level transmitter to
maintain symmetry in the AM
envelope.
ANTENNA
COUPLING NETWORK – matches the output
impedance of the final
power amplifier to the
transmission line and
antenna.
Block Diagram of a High Level AM Transmitter
MODULATING SIGNAL
- With high-level transmitter, the power of the
modulating signal must be high that is why additional
power amplifier is used. Note that the carrier is at full
power at the point in the transmitter where
modulation occurs, therefore, it is required to have a
high amplitude-modulating signal to produce 100%
modulation.
RF CARRIER - With high-level transmitter, the RF carrier
undergoes additional power amplification prior to the
modulation stage.
MODULATOR – drain-, plate- or collector-modulated class C
amplifier. It provides the circuitry necessary for
modulation to occur. It is the final power amplifier
stage and it is a frequency up converter.
TRAPEZOIDAL PATTERN
– used for observing the modulation characteristics of AM transmitters.
Parameters used to evaluate the ability of a receiver to successfully
demodulate an RF signal
a) Selectivity is the ability of the receiver to accept a given band of frequencies
and reject all others.
b) Sensitivity is the minimum RF signal level that can be detected at the input
to the receiver and still produce a usable demodulated information signal.
c) Dynamic Range is the input power range over which the receiver is useful.
d) Bandwidth Improvement (BI) is the noise ratio achieved by reducing the
bandwidth
)(
)(
IFBW
RFBWBI
e) Insertion Loss (IL) is the ratio of the output power of a filter to the input
power for frequencies that fall within the filter’s passband.
Po
PidBIL log10)(
f) Equivalent Noise Temperature (Te) is an indication of reduction in the
signal to noise ratio as signal propagates through a receiver.
)1( FKB
PnTe
AM SUPERHETERODYNE RECEIVER
-It provides a greater gain , better selectivity and better sensitivity than
other receiver configurations.
-Heterodyne means to mix two frequencies together in a nonlinear device
or to translate one frequency to another using nonlinear mixing.
- Major Edwin Howard Armstrong, a US radio engineer, developed
superheterodyne tuning for reception over a very wide spectrum of radio
frequency in 1936.
Block Diagram of an AM
Superheterodyne Receiver
ELEMENTS OF AN AM SUPERHETERODYNE RECEIVER
PRESELECTOR - a broad-tuned bandpass filter with an
adjustable center frequency that is tuned to the
desired carrier frequency.
- provides enough initial bandlimiting to
prevent a specific unwanted radio frequency called
image frequency from entering the receiver.
- reduces noise bandwidth of the receiver
and provides initial step toward reducing the
overall receiver bandwidth to the minimum
bandwidth requirement to pass the information
signal.
RF AMPLIFIER - determines the sensitivity of the
receiver. It sets the signal threshold.
- provide a greater gain, improved
image-frequency rejection, better signal to
noise ratio and better selectivity.
LOCAL OSCILLATOR - oscillator circuit that provides a local oscillator frequency.
When the fLO is tuned above the RF, it is called high side
injection or high beat injection. When the fLO is tuned below
RF, it is called low-side injection of low-beat injection. Tracking
is the ability of the local oscillator to oscillated above or below
the selected RF carrier by an amount equal to the intermediate
frequency throughout the entire radio-frequency band.
MIXER/ CONVERTER SECTION - a nonlinear device whose purpose is to convert radio
frequencies to intermediate frequencies. Heterodyning takes
place in this stage, and RF is down converted to intermediate
frequency. Although carrier and sidebands frequencies are
translated to IF, the shape of the envelope and the original
information signal remains unchanged. The most common IF
used in AM broadcast band receivers is 455KHz.
IF SECTION
- consists of five or six IF amplifiers in series and bandpass
filters. Most of the receiver gain and selectivity is achieved in the IF
section.
DETECTOR - convert the IF signals
back to the original source
information. It can be a
single diode demodulator or
as complex a phase-locked
loop or balanced modulators.
AUDIO SECTION - comprises of several
cascaded audio amplifiers
and one or more speakers…
The number of audio
amplifiers depends on the
audio signal power desired.
Electronic Communication Systems Through Advanced by W. Tomasi
Communication Electronics by L. Frenzel
Lecture Notes in Principles of Communication by A.H. Ballado and M.M. Sejera
Electronic Communication Systems by
G. Kennedy