Type of Modulation

Modulation is the process of varying a property of a high-frequency carrier wave according to a low-frequency information (message) signal so the signal can be transmitted efficiently.

The main types of modulation are:

1. Analog Modulation

Used for analog signals.

  • Amplitude Modulation (AM): The amplitude of the carrier wave changes while frequency and phase remain constant.

  • Frequency Modulation (FM): The frequency of the carrier wave changes while amplitude remains constant.

  • Phase Modulation (PM): The phase of the carrier wave changes according to the message signal.

2. Digital Modulation

Used for digital (binary) data.

  • Amplitude Shift Keying (ASK): Carrier amplitude changes between discrete levels.

  • Frequency Shift Keying (FSK): Carrier frequency changes between discrete frequencies.

  • Phase Shift Keying (PSK): Carrier phase changes to represent digital data.

  • Quadrature Amplitude Modulation (QAM): Both amplitude and phase are varied to transmit more bits per symbol.

Summary Table

Modulation TypeCarrier Property ChangedTypical Use
AMAmplitudeAM radio broadcasting
FMFrequencyFM radio, TV sound
PMPhaseDigital communication systems
ASKAmplitudeOptical communication, RFID
FSKFrequencyModems, telemetry
PSKPhaseWi-Fi, satellite communication
QAMAmplitude & PhaseCable TV, LTE, 5G, Wi-Fi

In most basic communication engineering courses, the three primary types of analog modulation are AM, FM, and PM, while ASK, FSK, PSK, and QAM are the most common digital modulation techniques.


If you mean how modulation is controlled, it depends on the type of modulation. In every case, the message (modulating) signal controls the carrier wave using a device called a modulator.

1. Amplitude Modulation (AM)

  • What is controlled? Amplitude (height) of the carrier wave.

  • How is it controlled? The audio or information signal increases and decreases the carrier's amplitude.

  • Frequency and phase remain constant.

2. Frequency Modulation (FM)

  • What is controlled? Frequency of the carrier wave.

  • How is it controlled? The voltage of the message signal causes the carrier frequency to increase or decrease.

  • Amplitude remains constant.

3. Phase Modulation (PM)

  • What is controlled? Phase of the carrier wave.

  • How is it controlled? The message signal shifts the phase of the carrier according to its instantaneous value.

  • Amplitude remains constant.

Practical control methods

In modern communication systems, modulation is controlled by:

  • An electronic modulator circuit.

  • A microcontroller or DSP (Digital Signal Processor) in digital radios.

  • Software-defined radio (SDR), where software controls the modulation type and parameters.

Example

Suppose you speak into a microphone:

  1. The microphone converts your voice into an electrical signal.

  2. A modulator uses this signal to control the carrier wave:

    • In AM, it changes the carrier's amplitude.

    • In FM, it changes the carrier's frequency.

    • In PM, it changes the carrier's phase.

  3. The modulated signal is then transmitted by the antenna.

If you're studying wireless communication, a simple way to remember it is:

  • AM → Control the amplitude

  • FM → Control the frequency

  • PM → Control the phase



Deviation in FM (Frequency Modulation) is the maximum change in the carrier frequency from its original (center) frequency due to the modulating signal.

Definition

Frequency deviation ((\Delta f)) is the maximum amount by which the carrier frequency shifts above or below its unmodulated value.

Formula

[
\Delta f = k_f \times A_m
]

Where:

  • (\Delta f) = Frequency deviation (Hz)

  • (k_f) = Frequency sensitivity of the modulator (Hz/V)

  • (A_m) = Amplitude of the modulating signal (V)

Example

  • Carrier frequency = 100 MHz

  • Maximum deviation = ±75 kHz

Then:

  • Highest transmitted frequency = 100.075 MHz

  • Lowest transmitted frequency = 99.925 MHz

Key Points

  • A larger modulating signal amplitude produces a larger frequency deviation.

  • The modulating signal frequency does not change the deviation; it affects how fast the carrier frequency swings.

  • In commercial FM broadcasting, the maximum allowed deviation is ±75 kHz.

Importance of Frequency Deviation

  • Determines the bandwidth of the FM signal.

  • Improves noise immunity when increased (within system limits).

  • Used to calculate the modulation index.

The modulation index for FM is:

[
\beta = \frac{\Delta f}{f_m}
]

Where:

  • (\beta) = Modulation index

  • (\Delta f) = Frequency deviation

  • (f_m) = Modulating signal frequency

For exams, remember:

  • Frequency deviation = Maximum change in carrier frequency.

  • Unit = Hertz (Hz).

  • Controlled by = Amplitude of the modulating signal.

  • Higher deviation = Wider bandwidth and generally better noise performance.

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