📻 Amplitude Modulation (AM)

In our previous blog on the Basics of Analog Modulation, we introduced the core concept of transmitting information by varying certain properties of a carrier wave. As promised, in this post, we'll dive deep into Amplitude Modulation (AM) — one of the most fundamental and historically significant types of analog modulation.

🔷 What is Amplitude Modulation?

Amplitude Modulation (AM) is a technique where the amplitude of the carrier wave is varied in proportion to the message (modulating) signal, while the frequency and phase remain constant.

Mathematically, the AM wave can be represented as:


Where:

  • AcA_c = amplitude of the carrier wave

  • fcf_c = carrier frequency

  • m(t)m(t) = message signal

🧱 Block Diagram of AM Transmitter

📌 Explanation:

  1. Message Signal (m(t)): The original information signal (voice, music, etc.)

  2. Carrier Generator: Produces a high-frequency sinusoidal carrier.

  3. Modulator: Combines the message and carrier to produce an amplitude-modulated wave.

  4. Power Amplifier: Boosts the signal for transmission over long distances.

  5. Antenna: Radiates the AM signal into space.

📉 Modulation Index in AM

The modulation index (μ) defines the extent of modulation and is given by:


Where:

  • AmA_m = amplitude of message signal

  • AcA_c = amplitude of carrier signal

✅ Types Based on Modulation Index:

  • Under-modulation (μ<1μ < 1): Signal is safe but not fully utilized.

  • Perfect modulation (μ=1μ = 1): Ideal condition.

  • Over-modulation (μ>1μ > 1): Causes distortion and loss of information.

📊 Frequency Spectrum of AM Signal

An AM signal consists of three components:

  • Carrier frequency fcf_c

  • Upper sideband (USB): fc+fmf_c + f_m

  • Lower sideband (LSB): fc−fmf_c - f_m

Thus, total bandwidth =2fm= 2f_m, where fmf_m is the highest frequency in the message signal.

📡 Power Distribution in AM

The total power in AM is distributed as:

Where PcP_c is the power of the carrier. It's important to note that a large portion of power is wasted in the carrier, which carries no information.

📥 Demodulation of AM Signal

To retrieve the original message, envelope detectors are commonly used in AM receivers. These circuits follow the envelope of the AM wave to extract the original information signal.

⚙️ Applications of AM

  • AM Radio Broadcasting (Medium Wave, Shortwave)

  • Aircraft Communication

  • Two-way Radios (Walkie-talkies)

  • Citizen Band (CB) Radio

📝 What’s Next?

In our next blog, we’ll break down Frequency Modulation (FM) in detail — how it works, why it sounds better than AM, and where it is used in real-world systems.

Stay tuned to Hobitronics as we continue this journey into the fascinating world of communication systems! Follow hobitronics.blog for more!!


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