Download Amplitude Modulation (AM) and Frequency Modulation (FM): Principles and Applications and more Summaries Engineering Mathematics in PDF only on Docsity!
AMPLITUDE MODULATION (AM)
- A type of modulation wherein the information signal is modulated by
varying its amplitude (e.g., peak value)
- The instantaneous value of the carrier amplitude changes in
accordance with the amplitude and frequency variations of the
modulating signal.
MATHEMATICAL REPRESENTATION
1
= ๐๐ + ๐๐ sin
๐
2
1
sin( 2 ๐๐
๐
๐ก) = [๐๐ + ๐๐ sin( 2 ๐๐
๐
๐ก)] sin( 2 ๐๐
๐
ENVELOPE
- An imaginary line that connects the positive and negative peaks of the
carrier waveform.
MODULATING INDEX
Other terms:
- Modulating index
- Modulating factor
- Modulating coefficient
In AM, it is particularly important that the peak value of the modulating
signal (Vm) should be less than the peak value of the carrier (Vc), otherwise, it
would result to overmodulation (ideal modulating index in AM should be
between 0 and 1).
Formulas:
SIDEBANDS/SIDEFREQUENCIES
For Fundamental/Far-most Sidebands:
๐ณ๐บ๐ฉ
๐ช
๐
๐ผ๐บ๐ฉ
๐ช
๐
For Sidebands within a certain harmonic:
๐๐๐๐๐ ๐๐๐๐๐๐๐๐
๐ช
๐
๐๐๐๐๐ ๐๐๐๐๐๐๐๐
๐ช
๐
Bandwidth:
๐ผ๐บ๐ฉ
๐ณ๐บ๐ฉ
Where:
AM POWER (ampu-)
๐
( ๐น๐ด๐บ
)
๐
๐
๐บ๐ฉ
๐บ๐ฉ
( ๐น๐ด๐บ
)
๐
๐
๐
๐
๐ป
๐บ๐ฉ
๐
๐
RADIO EMISSION DESIGNATION
AMPLITUDE MODULATORS
- Modulator circuits cause carrier amplitude to be varied in accordance
with modulating signals.
Equation:
๐ด๐
= ๐๐ sin( 2 ๐๐
๐
๐ก) + ๐๐ sin( 2 ๐๐
๐
๐ก) sin( 2 ๐๐
๐ถ
* Carrier Wave
* Product of Carrier and Modulating Wave
Low-Level Modulators:
- In low-level modulation, the modulation occurs at low power levels,
typically at the early stages of the transmitter.
- The modulated signal is amplified after modulation to reach the
required transmission power.
- Uses transistors or operational amplifiers to perform the modulation,
where both the carrier and message signals are at low power.
High-Level Modulators:
- In high-level modulation, the modulation occurs at high power levels,
typically at the final stage of the transmitter, just before the antenna.
- The carrier and message signal are both amplified to high power
before modulation.
- Uses high-power amplifiers, such as vacuum tubes or transistors in
the final stage.
AMPLITUDE DEMODULATORS
Diode Detector
- Works by detecting the envelope of the AM signal, which corresponds
to the original message signal.
- Typically consists of a diode, a capacitor, and a resistor.
- The diode rectifies the AM signal, allowing only one half of the
waveform (either positive or negative) to pass through.
- The capacitor smooths the rectified signal by charging and
discharging in response to the envelope of the modulated signal.
- The resistor controls the discharge rate of the capacitor, helping to
track the signal changes.
Synchronous Detectors
- A more advanced demodulation technique used to accurately recover
the message signal from the modulated carrier by multiplying the
incoming AM signal with a locally generated carrier that is
synchronized with the original transmitted carrier.
BALANCED MODULATORS
- A balanced modulator is a circuit used in AM systems to produce a
modulated signal where the carrier is suppressed, leaving only the
upper and lower sidebands.
- A lattice modulator is a type of balanced modulator that uses a lattice
structure of components (usually diodes or transistors) to perform
modulation. It is widely used in SSB (Single Sideband) systems.
FREQUENCY MODULATION
- A modulation technique where the frequency of the carrier wave is
varied in proportion to the amplitude of the input signal (message
signal), while the carrier's amplitude remains constant.
- The greater the amplitude of the message signal, the more the carrier
frequency deviates from its resting or center frequency.
- FM provides better noise immunity and higher sound quality
compared to amplitude modulation (AM), making it widely used in
radio broadcasting, audio transmission, and two-way radio
communication.
