Analog Communications
What is Analog Communications?
The process of varying a characteristic (amplitude, frequency, or phase) of a high-frequency carrier wave in accordance with the instantaneous value of a low-frequency message signal.
Key formula / rule: AM (Conventional) Modulation Index
Key points
- Understand the fundamental principles of amplitude, frequency, and phase modulation.
- Analyze the time-domain and frequency-domain representations of modulated signals.
- Calculate bandwidth and power for various analog modulation schemes.
- Describe different modulation and demodulation techniques (e.g., envelope detector, coherent detector, FM discriminator).
Common exam trap
Confusing modulation index definitions for AM, FM, and PM.
Definitions
- Term
Modulation
- Meaning
The process of varying a characteristic (amplitude, frequency, or phase) of a high-frequency carrier wave in accordance with the instantaneous value of a low-frequency message signal.
- Term
Demodulation
- Meaning
The process of extracting the original message signal from the modulated carrier wave.
- Term
Carrier Wave
- Meaning
A high-frequency sinusoidal wave that is modulated to carry information.
- Term
Baseband Signal
- Meaning
The original message signal, typically at low frequencies, before modulation.
- Term
Modulation Index
- Meaning
A dimensionless quantity that describes the extent of modulation of a carrier wave by a message signal.
- Term
Bandwidth
- Meaning
The range of frequencies occupied by a signal or a communication channel.
- Term
Sidebands
- Meaning
The frequency components generated on either side of the carrier frequency due to the modulation process, containing the information.
- Term
Noise
- Meaning
Unwanted electrical signals that interfere with the transmission and reception of desired signals, degrading communication quality.
Learning objectives
Understand the fundamental principles of amplitude, frequency, and phase modulation.
Analyze the time-domain and frequency-domain representations of modulated signals.
Calculate bandwidth and power for various analog modulation schemes.
Describe different modulation and demodulation techniques (e.g., envelope detector, coherent detector, FM discriminator).
Compare the advantages and disadvantages of AM, FM, and PM in terms of bandwidth, power, and noise performance.
Formulae
- Name
AM (Conventional) Modulation Index
- Note
For sinusoidal message m = Am/Ac.
- Expression
m = (Amax - Amin) / (Amax + Amin)
- Name
AM (Conventional) Total Transmitted Power
- Note
Pc is carrier power.
- Expression
Pt = Pc (1 + m2/2)
- Name
AM (Conventional) Bandwidth
- Note
fm is the maximum frequency component of the message signal.
- Expression
BW = 2fm
- Name
DSB-SC Transmitted Power
- Note
Ps is sideband power. For sinusoidal message Ps = Pc m2/2.
- Expression
Pt = Ps
- Name
DSB-SC Bandwidth
- Note
- Expression
BW = 2fm
- Name
SSB Transmitted Power
- Note
Half of DSB-SC power.
- Expression
Pt = Ps/2
- Name
SSB Bandwidth
- Note
- Expression
BW = fm
- Name
FM Frequency Deviation
- Note
kf is frequency sensitivity, Am is peak amplitude of message.
- Expression
Δf = kf Am
- Name
FM Modulation Index
- Note
fm is the maximum frequency component of the message signal.
- Expression
β = Δf / fm
- Name
FM Carson's Rule Bandwidth
- Note
- Expression
BW ≈ 2(Δf + fm) = 2fm(1+β)
- Name
PM Phase Deviation
- Note
kp is phase sensitivity, Am is peak amplitude of message.
- Expression
Δφ = kp Am
- Name
PM Modulation Index
- Note
- Expression
β = Δφ
- Name
PM Bandwidth (for sinusoidal message)
- Note
Where Δf = β fm.
- Expression
BW ≈ 2(Δf + fm)
Prerequisites
Signals and Systems (Fourier Transform, convolution, spectral analysis).
Basic Electronic Circuits (filters, amplifiers, oscillators).
Basic knowledge of trigonometry and calculus.
Common mistakes
Confusing modulation index definitions for AM, FM, and PM.
Incorrectly calculating bandwidth for different AM schemes (DSB-SC vs. SSB vs. conventional AM).
Misapplying Carson's Rule for FM bandwidth, especially for narrow-band vs. wide-band FM.
Ignoring the effect of noise and assuming ideal conditions in calculations.
Not understanding the difference between instantaneous frequency and carrier frequency in angle modulation.
Keywords
Modulation
Demodulation
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
Carrier Wave
Baseband Signal
Modulation Index
Bandwidth
Sidebands
Noise
DSB-SC
SSB
VSB
Carson's Rule
Envelope Detector
Coherent Detector
Discriminator
Practice preview
A sinusoidal message signal m(t) = Am cos(2*pi*fm*t) is used to generate a VSB signal. If the VSB filter has a transition width of 2*fv, what is the exact bandwidth required for transmission?…
hard
In a coherent detector for DSB-SC, there is a phase error of 'phi' between the received carrier and the local oscillator. The output signal amplitude will be proportional to:…
hard
In a standard Amplitude Modulation (AM) system, if the modulation index is 'm', what is the ratio of the power in the sidebands to the total transmitted power?…
easy