Demodulation Basics: What It Is and How Radios Do It

Demodulation is the reverse of modulation. It is the process of extracting the original message signal, such as voice or data, from a modulated carrier wave received by an antenna. Modulation puts information onto a high frequency carrier for travel, demodulation removes the carrier and recovers the information for a speaker or computer.

Without demodulation, a radio receiver would only produce a high frequency tone, not music or speech. Every radio, TV, phone and WiFi device must demodulate correctly to be useful. Understanding simple detectors explains how early crystal radios worked with almost no power.

Key Concepts

Modulated wave: Carrier whose amplitude, frequency or phase carries the message. Detector: Circuit that extracts message, often a diode for AM. Envelope: Outline joining peaks of AM wave, same shape as message when modulation index is proper. Low pass filter: Circuit allowing low frequencies to pass and blocking high carrier frequency, built with resistor and capacitor. Intermediate frequency: Fixed frequency converted inside superheterodyne receivers for easier filtering.

How It Works

1. Antenna receives many stations, tuner selects desired carrier frequency.
2. Amplifier raises weak signal strength without changing its shape much.
3. For AM, diode rectifies signal, allowing only positive halves, following the envelope.
4. Resistor capacitor filter removes carrier ripples and keeps only audio frequencies.
5. Audio amplifier drives speaker, converting electrical signal back to sound.

For FM, a discriminator or phase locked loop converts frequency changes into voltage changes. Digital systems sample the wave and decode bits by phase or frequency shifts.

Worked Example

An AM receiver picks a station at 1000 kHz modulated by 5 kHz audio. The diode detector uses R equals 10 kilo ohm and C equals 0.001 microfarad. Check if filter works. Step 1: Time constant RC equals 10000 times 1 times 10 to power minus 9 equals 1 times 10 to power minus 5 seconds or 10 microseconds. Step 2: Carrier period is 1 divided by 1000000 equals 1 microsecond, audio period is 1 divided by 5000 equals 200 microseconds. RC is between them, larger than carrier period to smooth ripples, smaller than audio period to follow envelope. So values are suitable for clear detection.

Common Misconceptions

• Demodulation amplifies the message: Wrong. Detection extracts it, separate amplifiers provide power gain.
• Any diode circuit demodulates FM also: Wrong. Simple envelope detector works for AM, FM needs frequency to voltage conversion first.
• Carrier is heard in speaker: Wrong. Filter removes carrier, only message frequencies reach speaker, carrier is far above hearing range anyway.

Real-World Applications

AM radios use envelope detectors, FM radios use discriminator or PLL detectors. Mobile phones demodulate digital signals millions of times per second. WiFi routers, satellite TV boxes and RFID readers all include demodulators. Software defined radios perform demodulation in code on a computer.

Demodulation recovers the original message from a modulated carrier using rectification and filtering for AM, and frequency conversion for FM. It is the essential final step that turns invisible radio waves into sound, pictures and data we use.