MindMap Gallery Analog modulation system
It introduces the principle of arc modulation, the anti-noise performance of linear modulation systems, the principle of nonlinear modulation, the comparison of various analog modulation systems using frequency division multiplexing, and the anti-noise performance of modulation systems.
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Analog modulation system
Introduction to modulation
definition
Load the message signal onto a certain parameter of the carrier wave
Purpose
Perform spectrum migration to match channel characteristics and reduce antenna size
Implement multiplexing and improve channel utilization
Improve system performance (effectiveness, reliability)
Implement frequency allocation
Classification
Amplitude modulation principle
General model
Conventional Amplitude Modulation (AM)
AM expression
m<1 normal amplitude modulation m>1 over-amplification m=1 critical state, full amplitude modulation (100%)
AM modulator
condition
AM waveforms and spectrum
Characteristics of AM signals
Disadvantages of AM signals
AM power utilization is low
Double sideband modulation (DSB-SC)
expression
Modulator
condition
Waveforms and Spectrum
Signal characteristics
The envelope is no longer proportional to m(t); when m(t) changes sign, the carrier phase reverses, so envelope detection cannot be used and coherent demodulation is required.
No carrier frequency component, only upper and lower sidebands
Bandwidth is the same as AM:
Modulation efficiency is 100%, that is, high power utilization
Mainly used as the technical basis for SSB, VSB, differential signal modulation in FM stereo, etc.
Single sideband modulation (SSB)
signal generation
filter method
picture
principle
The DSB signal is first formed, and the upper or lower sideband signal is obtained by sideband filtering.
Require
Filter has a steep cutoff characteristic at the carrier frequency
Characteristics of sideband filtering
phase shift method
picture
meaning
Amplitude unchanged, phase shifted by π/2
Transfer Function
Require
Signal characteristics
One of the advantages is high frequency band utilization. Its transmission bandwidth is only half of AM/DSB: Therefore, it has been widely used in communication scenarios with crowded spectrum, especially in short-wave communications and multi-carrier telephones.
The second advantage is the low power consumption feature, which saves power because there is no need to transmit the carrier wave and another sideband. This is particularly important for mobile communication systems.
shortcoming
The equipment is complex and there are technical difficulties. Coherent demodulation is also required.
Vestigial Sideband Modulation (VSB)
A compromise between SSB and DSB
produce
demodulation
picture
meaning
Has complementary symmetry characteristics at the carrier frequency
Geometric interpretation
Signal characteristics
There is only a small increase in the bandwidth required by SSB, but in exchange for a simpler circuit.
application
Television signal transmission in commercial television broadcasting, etc.
coherent demodulation
Be applicable
AM, DSB, SSB, VSB
Features
No threshold effect
Require
Carrier synchronization
envelope detection
Be applicable
AM signal
Advantage
Simple, no need for carrier synchronization
Require
insertion envelope detection method
Be applicable
Modulated signals such as DSB, SSB or VSB that suppress the carrier
principle
Insert these signals into the recovered carrier to make them become or approximate AM signals, and then use an envelope detector to recover m(t)
Require
method
Insert carrier into sender/receiver
Noise immunity performance of linear modulation systems
Models and Indicators
Analytical model
Performance
Input signal-to-noise ratio
Output signal-to-noise ratio
institutional gain
Anti-noise performance of DSB coherent demodulation system
Anti-noise performance of SSB coherent demodulation system
AM – Noise Immunity Performance of Envelope Detection Systems
Nonlinear modulation principle
Overview
Angle modulation: general term for FM and PM
The amplitude of the carrier wave is constant, but the frequency or phase is modulated
Belongs to nonlinear modulation.
Better noise immunity than amplitude modulation – Advantages
basic concept
General expression of angle modulation signal
The relationship between PM and FM
PM is the phase offset, which changes linearly with m(t); FM is the phase offset, which changes linearly with the integral of m(t).
Bandwidth FM
monotone FM
Spectrum and bandwidth of FM signal
For FM signals with multi-tone or arbitrary band-limited modulated signals:
FM signal power distribution
The total power after modulation remains unchanged, it is just redistributed, and the proportion of power distribution is related to the FM index.
Features of FM
The envelope is constant; the frequency deviation changes linearly with the modulation signal m(t); the phase deviation changes linearly with the integral of the message signal m(t); the bandwidth is related to the bandwidth of m(t) and m;, which is larger than the AM bandwidth ( m 1) times
Advantages of FM
Advantages: strong anti-noise capability. Cost: occupying a larger channel bandwidth, resulting in lower spectrum utilization.
FM applications
Application: In situations where high quality is required or the channel noise is large, such as: FM mail broadcasting, TV audio, satellite communications, mobile communications, microwave communications and cellular phone systems.
Generation and demodulation of FM signals
Generation of FM signals
direct method
principle
Frequency of modulated voltage controlled oscillator:
advantage
The circuit is simple and large frequency deviation can be obtained
shortcoming
The frequency stability is not high and can be improved by using a PLL frequency modulator.
indirect method
principle
Integral ⤑ Phase modulation (NBFM) ⤑ n times frequency multiplication ⤑ WBFM
advantage
Good frequency stability
shortcoming
It requires multiple frequency doubling and mixing, so the circuit is more complex
Demodulation of demodulated signals
Non-coherent demodulation - frequency discriminator
Be applicable
NBFM and WBFM
Ideas
Complete frequency ~ voltage conversion
principle
coherent demodulation
frequency division multiplexing
Reuse
Generate multiple signals simultaneously in one channel
Purpose
Make full use of the channel’s frequency band or time resources
Classification
FDM, TDM, SDM, CDM
frequency division
Channel multiplexing by frequency
method
Modulation ⤳ synthesis ⤳ transmission ⤳ splitting ⤳ demodulation
principle
Comparison of various analog modulation systems
Performance comparison
Anti-noise performance: FM is the best, followed by DSB/SSB and VSB, and AM is the worst;
Spectrum utilization: SSB is the highest, VSB is higher, DSB/AM is second, FM is the worst;
Power utilization: FM is the highest, followed by DSB/SSB, VSB, and AM is the worst;
Equipment complexity: AM is the simplest, followed by DSB/FM, VSB is more complex, and SSB is the most complex.
Features and Applications
AM: The advantage is that the receiving equipment is simple; the disadvantage is low power utilization and poor anti-interference ability. Mainly used in medium wave and short wave AM broadcasting
DSB: The advantage is high power utilization and the same bandwidth as AM. Mainly used for differential signal modulation in FM stereo and color difference signal modulation in color TV
SSB: The advantage is that the power utilization rate and frequency band utilization rate are high, the anti-interference ability and anti-selective fading ability are better than AM, and the bandwidth is only half of AM; the disadvantage is that the transceiver equipment is complex. Commonly used in frequency division multiplexing systems
VSB: Anti-noise performance and frequency band utilization are equivalent to SSB. It has been widely used in television broadcasting and other systems
FM: Strong anti-interference ability, widely used in long-distance and high-quality communication systems. The disadvantage is that the frequency band utilization is low and there is a threshold effect.
Noise immunity performance of FM systems
Small signal-to-noise ratio: threshold effect