Study Notes for Unit 3: Radio Transmitters, Receivers & Pulse Modulations

Unit 3: Radio Transmitters, Receivers & Pulse Modulations

Overview

  • Instructor: Dr. G. Anjaneyulu

CG1: Radio Transmitter

  • Classification of Transmitter
    • AM (Amplitude Modulation) Transmitter
    • FM (Frequency Modulation) Transmitter
    • Block diagrams for both types
  • Radio Receiver
    • Types & characteristics:
    • TRF (Tuned Radio Frequency) receiver
    • Super heterodyne receiver
    • Concepts of Intermediate Frequency, Image Frequency & its rejection
    • AGC (Automatic Gain Control)
    • FM Receivers compared with AM Receivers

CG2: Noise in AM & FM Systems

  • Noise Handling
    • Pre-emphasis & De-emphasis
    • Multiplexing Techniques
    • Time Division Multiplexing (TDM)
    • Frequency Division Multiplexing (FDM)
  • Pulse Modulation Types
    • Generation & Demodulation of PAM (Pulse Amplitude Modulation), PWM (Pulse Width Modulation), PPM (Pulse Position Modulation)

Radio Frequency Spectrum: Ranges

DesignationAbbreviationFrequenciesWavelengths
Very Low FrequencyVLF3 kHz - 30 kHz100 km - 10 km
Low FrequencyLF30 kHz - 300 kHz10 km - 1 km
Medium FrequencyMF300 kHz - 3 MHz1 km - 100 m
High FrequencyHF3 MHz - 30 MHz100 m - 10 m
Very High FrequencyVHF30 MHz - 300 MHz10 m - 1 m
Ultra High FrequencyUHF300 MHz - 3 GHz1 m - 100 mm
Super High FrequencySHF3 GHz - 30 GHz100 mm - 10 mm
Extremely High FrequencyEHF30 GHz - 300 GHz10 mm - 1 mm

Classification of Radio Transmitters

  • Based on Type of Modulation:

    • AM Transmitter: Amplitude modulates the carrier; used for radio broadcasts on long/medium/short waves and telegraphy.
    • FM Transmitter: Frequencies modulate the carrier; used for TV sound broadcasts in VHF/UHF ranges.
    • PM (Pulse Modulation) Transmitter: Alters pulse characteristics; includes:
    1. Pulse Width Modulation (PWM)
    2. Pulse Position Modulation (PPM)
    3. Pulse Amplitude Modulation (PAM)
    4. Pulse Frequency Modulation

  • Based on Type of Service:

    • Radio Broadcast Transmitters: Deliver information and entertainment.
    • Radio Telephone Transmitters: Enables long-distance telephony.
    • Radio Telegraph Transmitters: Facilitate telegraph signals across stations.
    • Television Transmitters: Require specific transmitters for picture and sound transmission over UHF and VHF.
    • Radar Transmitters: Utilize pulse modulation for various applications, including navigation.

  • Based on Carrier Frequency:

    • Long Wave Transmitters: Under 30 kHz, offers reliable broadcasting in temperate regions.
    • Medium Wave Transmitters: Operate at 535 kHz to 1650 kHz for broadcasts.
    • Short Wave Transmitters: Operate in the 3 to 30 MHz range.
    • VHF and UHF Transmitters: Used for television broadcasting.
    • Microwave Frequencies: Operate at high frequencies for relay systems.

AM Transmitter Characteristics

  • Design Requirements:

    • Generate a signal with appropriate modulation,
    • Provide sufficient output power,
    • Maintain efficiency at the correct carrier frequency.

  • Functional Components include the following stages:

    1. Master Oscillator: Generates a constant desired frequency.
    2. Buffer Amplifier: Prevents loading to the oscillator, maintaining frequency stability.
    3. Harmonic Generators: Class C tuned amplifiers distort RF voltage to produce required harmonics.
    4. Class-C Amplifiers: Increase power levels to around 70% output circuit efficiency.
    5. Modulated Amplifier: Typically push-pull configurations, ensuring high efficiency for AM.

FM Transmitter Design

  • Block Diagram Components:
    • AF Amplifier: Pre-emphasis circuit to reduce noise effects at high audio frequencies.
    • Buffer Amplifier: Stabilizes carrier frequency.
    • Multiplier Circuits: Raise the carrier frequency from a lower operational frequency.

Introduction to Radio Receivers

  • A radio receiver: device that converts radio waves into a usable form of information.

Characteristics of Radio Receiver

  • Selectivity: Ability to isolate desired signal frequency while rejecting others. Achieved through tuned circuits.

    • Definition of Selectivity:
      Q = rac{X_L}{R}
      Width Bandwidth:
      Bw = rac{1}{f_0}

  • Sensitivity: Receiver's capability to amplify weak signals, often measured in microvolts; typical sensitivity: 0.2 - 1 uV.

  • Fidelity: Ability to accurately reproduce modulating frequencies.

Image Frequency and Rejection

  • Image Frequency: Defined as a frequency that, when mixed with the local oscillator, results in the same IF as the desired signal frequency.
  • Rejection Formula:
    R = rac{Gain_{signal}}{Gain_{image}} = 1 + Q^2 * p^2

Receiver Functionality

  1. Intercept incoming modulated signals via antenna.
  2. Select desired signals while rejecting the rest.
  3. Amplify RF signals.
  4. Detect modulated signals to retrieve original baseband signal.
  5. Further amplify the modulating frequency.

Classification of Radio Receivers

  • Types based on Applications:

    • AM Broadcast Receivers: For receiving AM broadcasts.
    • FM Broadcast Receivers: For FM broadcasts over VHF/UHF.
    • Communication Receivers: For telegraph/short-wave signals.
    • Television Receivers: For television broadcasts.
    • Radar Receivers: For receiving radar signals.

  • Types based on technical aspects:

    1. TRF Receivers (Tuned Radio Frequency)
    2. Superheterodyne Receivers (most popular today).

Advantages of Superheterodyne Receivers

  1. No variation in bandwidth.
  2. High sensitivity and selectivity.
  3. High adjacent channel rejection.

AM Receiver Frequency Parameters

  • Frequency bands: Medium wave (MW) and Short wave (SW).
  • RF Carrier range (MW band): 535 kHz - 1650 kHz
  • Intermediate Frequency (IF): 455 kHz
  • IF Bandwidth: 10 kHz

Noise in Communication Systems

External Noise
  • Atmospheric noise
  • Extraterrestrial noise
  • Man-made industrial noise
Internal Noise
  • Thermal noise
  • Shot noise
  • Flicker noise
Other Important Noise Types
  • Intermodulation noise
  • Crosstalk
  • Impulse noise

Pulse Modulation Techniques

  • Pulse Amplitude Modulation (PAM): Amplitude varies proportionally to the modulating signal.
  • Types: Impulse Sampling, Natural Sampling, Flat Top Sampling.

Advantages & Disadvantages of PAM, PWM, PPM

  • PAM: High noise interference, large bandwidth.
  • PWM: Low noise, needs large bandwidth.
  • PPM: Requires synchronization, better power handling.

Time Division Multiplexing (TDM)

  • Technique for simultaneous transmission of multiple signals over one channel.
  • Applications: Digital audio mixing, optical data transmission systems, landline phone systems, etc.

Frequency Division Multiplexing (FDM)

  • Allows simultaneous transmission of several signals, commonly used in telecommunications and broadcasting, differentiating signals by frequency.

Applications of TDM vs. FDM

  • TDM relates signals by time; FDM divides by frequency.
  • TDM is used in digital systems; FDM is used for analog signals.

Conclusion

  • The study of radio transmitters, receivers, and modulation techniques is fundamental in communications technology.
  • Understanding noise and multiplexing methods enhances signal integrity in communication systems.