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Chapter 4 - Analog Modulation
Chapter 4 - Mind map of analog modulation. Modulation is to change certain parameters of the carrier according to the changing rules of the modulated signal (original baseband signal). The functions are: frequency conversion,...; realizing channel multiplexing and improving the channel Utilization; improve signal transmission performance, such as improving anti-interference ability
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Chapter 4 - Analog Modulation
- bacteria
This is a mind map about bacteria, and its main contents include: overview, morphology, types, structure, reproduction, distribution, application, and expansion. The summary is comprehensive and meticulous, suitable as review materials.
- Plant asexual reproduction
This is a mind map about plant asexual reproduction, and its main contents include: concept, spore reproduction, vegetative reproduction, tissue culture, and buds. The summary is comprehensive and meticulous, suitable as review materials.
- Reproductive development of animals
This is a mind map about the reproductive development of animals, and its main contents include: insects, frogs, birds, sexual reproduction, and asexual reproduction. The summary is comprehensive and meticulous, suitable as review materials.
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- Outline
Chapter 4 - Analog Modulation
The concept and classification of modulation
Modulation is to change certain parameters of the carrier according to the changing rules of the modulated signal (original baseband signal)
Function: perform frequency conversion,...; realize channel multiplexing and improve channel utilization; improve signal transmission performance, such as improving anti-interference ability
Classification
Classification according to modulation signal m: analog modulation and digital modulation
Classification according to carrier signal c: continuous wave modulation and pulse wave modulation
Control of different parameters of c according to m: amplitude modulation (AM ASK) frequency modulation (FM FSK) phase modulation (PM PSK DPSK)
linear modulation
AM FM
A0 m(t)>=0 envelope detection non-distortion condition
Bandwidth: 2fm; coherent demodulation and envelope detection
DSB suppresses carrier double sideband modulation
Bandwidth: 2fm
SSB single sideband modulation
Generation method
Filtering method: The difficulty is the production of sidebands. Multi-stage DSB modulation sideband filters can be used.
phase shift method
Modulated signal S(t)=0.5m(t)cos(wct) -/ Hilbert transform sin(wct) of 0.5m(t)
Hilbert Transform P69
Technical Difficulties: Production of Broadband Phase Shift Network
Weaver's method: w2>>w1
Bandwidth: fm; coherent demodulation
VSB vestigial sideband modulation
It overcomes the shortcoming of DSB signal occupying frequency bandwidth and solves the difficulties in realizing SSB signal.
fm<bandwidth<2fm; the low-pass filter must have complementary symmetry at wc (odd symmetry, roll-off)
Noise immunity performance of linear modulation systems
Anti-noise performance index G (modulation system gain) = output signal-to-noise ratio/output signal-to-noise ratio
AM
Envelope detection performance
Envelope E=[(A0 m(t) nc)^2 ns^2]^0.5
Large signal-to-noise ratio E=A0 m(t) nc, G increases as A0 decreases. In order to prevent over-modulation, A>=|m|, so G is always less than 1
Small signal-to-noise ratio: The output SNR does not decrease proportionally with the input SNR, but deteriorates sharply. This is called the threshold effect (caused by the nonlinear demodulation effect of the envelope detector). Coherent demodulation has no threshold effect.
Coherent demodulation performance: The AM modulation system has almost the same demodulation performance at a large signal-to-noise ratio.
DSB
G=2
SSB
G=1
Note: This does not mean that the anti-noise performance of the DSB system is better than that of SSB P76
VSB
nonlinear modulation
FM frequency modulation
The phase offset changes linearly with the integral of m(t)
FM index: mf=KfAm/wm=△w/wm=△f/fm; maximum angular frequency deviation △w △f maximum frequency deviation
PM phase modulation
Phase offset varies linearly with m(t)
Phase modulation index: mp=KpAm represents the maximum phase deviation
Advantages: Compared with AM, the most prominent advantage of angle modulation is its higher anti-noise performance, at the cost of occupying a higher bandwidth than AM
The relationship between PM and FM: m--integrator--PM=FM; m--differentiator--FM=PM
NBFM narrowband frequency modulation: |Kf∫m(t)dt|<<Π/6. When this condition is not met, the FM signal spectrum width is relatively wide and is called WBFM; the maximum frequency deviation of the NBFM signal is small and the occupied bandwidth is narrow, but Its anti-interference performance is better than AM. However, for high quality communication WBFM is required
WBFM broadband frequency modulation
B=2(mf 1)fm=2(△f fm)
When mf<<1 B=2fm
When mf>>1 B=2△f
Noise immunity performance of FM systems
Great signal-to-noise ratio
The power spectral density of the frequency discriminator output noise nd is no longer uniformly distributed, but is proportional to f^2
Under a large signal-to-noise ratio, the FM modulation system has a high gain,
Under the condition of large signal-to-noise ratio, the anti-noise performance of FM system is better than AM, and its superiority will increase with the increase of transmission bandwidth. However, the FM system's improvement in output signal-to-noise ratio in exchange for bandwidth is not endless. As B(mf) increases, the input noise power increases. When the input signal probability remains unchanged, the input signal-to-noise ratio decreases. When the input signal-to-noise ratio drops to a certain level, a threshold effect will occur, and the output signal-to-noise ratio sharp deterioration
Threshold effect at small signal-to-noise ratio
The threshold value is related to the modulation index mf. The larger the mf, the higher the threshold value. However, when mf is different, the threshold value changes within 8~10dB. It is generally believed that the threshold value is about 10dB.
Above the threshold value, the output SNR has a linear relationship with the input ANR. The larger the mf, the more obvious the improvement in the output SNR.
When below the threshold value, the output SNR decreases sharply as the input SNR decreases. The larger mf is, the faster the output SNR decreases.
Methods to lower the threshold: phase-locked loop demodulator and negative feedback demodulator
Comparison of various analog modulation systems
effectiveness
Metric: Bandwidth
SSB>VSB>DSB?AM>FM
reliability
Metric: output signal-to-noise ratio or modulation system gain
FM>SSBDSB>AM
Ease of implementation
AM can be demodulated using envelope detection, and the equipment is simple
DSB FM system drama
SSB VSB The equipment is the most complex due to the difficulty of filter implementation