Oscillators and Modulation Techniques in Radio Communication
Item 19: Oscillator Components
Oscillator Types
Oscillators can be formed by different components:
- RC oscillators: Composed of resistors and capacitors.
- LC oscillators: Made up of inductors and capacitors.
- Crystal oscillators: Composed of quartz crystal.
Oscillator Types, Including 555
Multivibrator
A multivibrator oscillator circuit generates a square wave. Multivibrators can be divided into two classes:
- Continuous operation (astable or free oscillation): Waves generated from the power supply itself.
- Driven operation: Triggered by a signal or pulse.
A multivibrator with two states is called bistable. One with a single state is called monostable. In its simplest form, two transistors are fed back to each other. Networks using resistors and capacitors in the feedback define the periods of instability.
555 Timer
The 555 timer can be connected to work in different ways, most importantly as an astable or monostable multivibrator.
- Astable Multivibrator: This configuration outputs a continuous square (or rectangular) wave.
- Monostable Multivibrator: This configuration outputs a single pulse.
RC Oscillator
Built from a phase-inverting amplifier and a feedback network that offsets the output signal by 180 degrees and reintroduces it at the input.
Wien-bridge Oscillator
Generates sine waves without any input signal across a wide range of frequencies. The bridge consists of four resistors and two capacitors.
LC Oscillator
Consists of a coil and a capacitor in parallel. It stores energy as electric charge in the capacitor and a magnetic field in the coil.
Colpitts Oscillator
A high-frequency oscillator that produces a fixed-frequency output signal without an input.
Hartley Oscillator
A high-frequency oscillator that produces a fixed-frequency output signal without an input.
Vackar Oscillator
A stable, high-bandwidth LC oscillator similar to the Colpitts or Clapp oscillator, but with a more stable output level over the frequency range.
Seiler Oscillator
An improvement on the Colpitts oscillator, offering better stability than the Clapp oscillator when using varactors.
Clapp Oscillator
Similar to the Seiler oscillator, but with a capacitor in series with the coil resonant circuit.
Crystal Oscillator
Includes a piezoelectric resonator in its feedback, characterized by frequency stability and phase purity. Supports minor frequency adjustments.
Pierce Oscillator
Replaces the LC resonant circuit with a quartz crystal, offering high-frequency operation and stable frequency.
Determined and Assessment Act
Astable Multivibrator (Self Assessment Exercise 19.11)
An astable multivibrator has two quasi-stable states, switching between them at a frequency determined by the charging and discharging of capacitors. It’s commonly used as a clock generator or to produce pulse trains.
Item 20: Modulation
Modulation adapts a signal to the transmission medium. It involves techniques for conveying information on a carrier wave, typically sinusoidal. This allows for efficient channel use, transmitting more data simultaneously while protecting it from interference and noise.
Modulation changes a carrier wave parameter based on the modulating signal (the information being transmitted). In radio, the message is “mixed” with the radio signal (carrier) to be transmitted.
Types of Radio Modulation: AM and FM
- AM (Amplitude Modulation): Varies the carrier wave’s amplitude according to the modulating signal. Used for transmitting voice, music, etc.
- FM (Frequency Modulation): Varies the carrier wave’s frequency according to the modulating signal.
Other Modulation Types
- PM (Phase Modulation): Varies the carrier wave’s phase.
- DSB (Double Sideband Modulation): Changes the carrier signal’s amplitude based on the modulating signal, similar to AM but without carrier reinsertion.
- SSB (Single Sideband Modulation): An evolution of AM, enabling transmission of signals that other modulations can’t handle.