Fundamentals of Modulation Techniques in Communication Systems
1. Modulation and Its Necessity
Modulation
Modulation is the process of varying a high-frequency carrier signal according to the low-frequency message (baseband) signal.
Need for Modulation
To reduce the size of the antenna (Antenna height is proportional to $\lambda/4$; without modulation, the antenna height becomes hundreds of meters).
To avoid mixing of signals (different stations use different carrier frequencies).
To increase the range of communication (High-Frequency carriers travel longer distances).
For multiplexing (many signals can be transmitted simultaneously).
To improve signal quality and reduce noise.
2. Block Diagram of a Modern Communication System
MESSAGE → TRANSDUCER → TRANSMITTER → CHANNEL → RECEIVER → DESTINATION
Explanation
Transducer: Converts physical signals to electrical signals (e.g., microphone, camera).
Transmitter: Performs modulation, amplification, and transmission.
Channel: The medium of communication (e.g., air, cable, fiber).
Noise: Unwanted disturbance added in the channel.
Receiver: Demodulates and extracts the original message.
Destination: The final user output (e.g., speaker, display).
3. Amplitude Modulation (AM)
Definition
In AM, the amplitude of a high-frequency carrier wave is varied according to the instantaneous value of the message signal.
Mathematical Expression
Carrier: $c(t)=A_c\sin(\omega_ct)$
Message: $m(t)=A_m\sin(\omega_mt)$
AM wave: $s(t)=A_c[1 + \mu\sin(\omega_mt)]\sin(\omega_ct)$
($\mu$ = modulation index)
Waveshape Characteristics
- Carrier has a constant amplitude.
- Modulating signal has a low frequency.
- The AM wave’s envelope changes according to the message signal.
4. Frequency Modulation (FM)
Definition
In FM, the frequency of the carrier is varied according to the instantaneous value of the message signal.
Mathematical Expression
$s(t)=A_c\sin[\omega_ct + k_f\int m(t)dt]$
Frequency deviation: $\Delta f = k_f A_m$
Modulation index: $\beta = \Delta f / f_m$
Waveshape Characteristics
- Amplitude remains constant.
- Frequency (spacing of cycles) changes with the message signal.
5. Comparison of AM and FM
| Parameter | AM | FM |
|---|---|---|
| Variation | Amplitude varies | Frequency varies |
| Noise Immunity | More affected | Less affected |
| Bandwidth | Low ($2f_m$) | High ($\approx 2(\Delta f + f_m)$) |
| Power | Depends on modulation index | Constant |
| Complexity | Simple | Complex |
| Applications | Broadcast radio | Mobile, TV, High-fidelity radio |
6. DSB-SC, SSB-SC, ISB, VSB Techniques
(a) DSB-SC (Double Sideband Suppressed Carrier)
- The carrier component is suppressed.
- Both Upper Sideband (USB) and Lower Sideband (LSB) are transmitted.
- Offers better power efficiency than standard AM.
(b) SSB-SC (Single Sideband Suppressed Carrier)
- Only one sideband transmitted (either USB or LSB).
- Achieves very high bandwidth efficiency.
(c) ISB (Independent Sideband)
- Both sidebands are transmitted, but each carries different information.
- Primarily used in long-distance telephony.
(d) VSB (Vestigial Sideband)
- Transmits one full sideband plus a portion (vestige) of the other sideband.
- Used effectively in TV video transmission.
7. Phase Modulation (PM)
Definition
In PM, the phase of the carrier varies according to the message signal.
Mathematical Expression
$s(t)=A_c\sin[\omega_ct + k_p m(t)]$
Waveshape Characteristics
- Amplitude remains constant.
- Phase shift increases or decreases directly with the message signal amplitude.
8. Types of AM Modulators
1. Linear Modulators
- Series Modulator
- Shunt Modulator
2. Non-linear Modulators
- Square Law Modulator
- Balanced Modulator
9. Types of FM Modulators
1. Direct FM
- Reactance Modulator
- Varactor Diode Modulator
2. Indirect FM (Armstrong Method)
- Involves initial phase modulation followed by frequency multiplication stages.
10. AM Transmitters
Essential Components
- Microphone
- Audio Amplifier
- Modulator
- RF Oscillator
- RF Power Amplifier
- Antenna
Transmitter Types
- Low-level AM transmitter (Modulation occurs at low power stages).
- High-level AM transmitter (Modulation occurs at the final power stage).
11. FM Transmitters
Essential Components
- Audio Pre-amplifier
- Frequency Modulator (Reactance or Varactor)
- Frequency Multiplier
- Power Amplifier
- Antenna
Transmitter Types
- Direct FM Transmitter
- Indirect FM Transmitter (Armstrong Method)
12. Superheterodyne AM Receiver
Block Diagram
Antenna → RF Amplifier → Mixer → IF (455 kHz) → IF Amplifier → Detector → AF Amplifier → Speaker
Working Principle
- The Mixer combines the received RF signal with a signal from the local oscillator to produce a fixed Intermediate Frequency (IF).
- The IF amplifier stage provides high selectivity and gain.
- The Detector recovers the original audio signal from the modulated carrier.
13. Performance Parameters of AM Receivers
1. Selectivity
The ability to select the desired station frequency while rejecting adjacent, unwanted stations. Achieved using sharp IF filters.
2. Sensitivity
The ability of the receiver to detect and process very weak incoming signals. Higher sensitivity means better reception of distant stations.
3. Fidelity
The faithful reproduction of the original audio signal without introducing distortion.
4. SNR (Signal-to-Noise Ratio)
The ratio of the received signal power to the noise power present in the system. A higher SNR results in better output quality.