Why We Modulate
Every wireless link you use — AM and FM broadcast radio, television, Wi-Fi, Bluetooth, your phone, a garage-door remote, a contactless payment card — shares one fundamental property: information has been impressed onto a high-frequency carrier wave before being sent. That process is called modulation, and this lesson explains from first principles why it is necessary, what it achieves, and why different systems use different kinds of modulation.
The Problem with Audio Frequencies
Speech occupies roughly 300 to 3,400 Hz, music 20 Hz to 20 kHz, and a slow data signal a similar range. Could we simply connect a microphone amplifier to an antenna and radiate the audio directly? Two problems make this impossible in practice.
The first is antenna size. An antenna radiates efficiently only when its length is a reasonable fraction of the wavelength — typically a quarter or a half wavelength. The wavelength is set by the frequency:
λ = wavelength in meters
c = speed of light = 300,000,000 m/s (3 × 10⁸ m/s)
f = frequency in hertz
At 1 kHz, the wavelength is 300 km, and a quarter-wave antenna would be 75 km tall. Even at 20 kHz, the top of the audio range, it would still be 3.75 km. No practical antenna can be built for audio frequencies.
The second problem is sharing. If every station radiated its audio directly, they would all occupy the same narrow slice of spectrum at the same time, and no receiver could separate one from another. There would be room for exactly one signal in any area.
Antenna size depends on wavelength: a quarter-wave antenna for a 1 kHz tone would be 75 km tall, while at 2.4 GHz it fits on a circuit board. Modulation moves information to frequencies where antennas are practical.
View LargerAntenna Size and Wavelength
At radio frequencies, wavelengths become physically manageable. The higher the frequency, the smaller the antenna:
| Example | Frequency | Wavelength | Quarter-wave antenna |
|---|---|---|---|
| Audio tone | 1 kHz | 300 km | 75 km (impossible) |
| AM broadcast station | 1 MHz | 300 m | 75 m (a tall mast) |
| Shortwave broadcast | 10 MHz | 30 m | 7.5 m |
| FM broadcast station | 100 MHz | 3 m | 75 cm (a car whip) |
| Key fob / sensor link | 433 MHz | 69 cm | 17 cm |
| Mobile phone | 900 MHz | 33 cm | 8.3 cm |
| Wi-Fi / Bluetooth | 2.4 GHz | 12.5 cm | 3.1 cm (fits on a circuit board) |
An antenna much shorter than a quarter wavelength looks almost entirely like a capacitor: it has high reactance and very low radiation resistance, so most of the signal energy is reflected back instead of radiated. You can force a short antenna to work by adding an inductor to cancel the reactance (called loading) — which is how AM broadcast receivers manage with a small ferrite rod — but its efficiency remains low. The key insight is: by moving information to a higher frequency, we get antennas that are buildable.
Speech at 1 kHz: λ = 300,000,000 / 1,000 = 300,000 m; quarter-wave antenna = 75 km.
Speech carried on a 900 MHz carrier: λ = 300 / 900 = 0.333 m; quarter-wave antenna = 8.3 cm — small enough to fit inside the phone.
The ratio is 900,000 to 1. The information (the voice) is unchanged; only the frequency of the vehicle carrying it has changed. That vehicle is the carrier wave.
The Carrier Wave
A carrier is a steady, high-frequency sine wave produced by an oscillator. On its own it carries no information — it is a pure tone at a single frequency. Modulation varies some property of the carrier in step with the information signal, so that the information travels with the carrier and can be recovered by a receiver tuned to the carrier's frequency.
The carrier frequency is chosen to suit the job: a frequency the system is permitted to use, an antenna of practical size, and a range that fits the application — low frequencies that follow the curve of the earth for long-distance broadcasting, very high frequencies that travel in straight lines for local links, and microwave frequencies that carry enormous data rates over short distances. Because each signal sits on its own carrier frequency, many signals can share the same space without interfering, and a receiver selects one by tuning to its frequency.
The three elements of modulation: a low-frequency audio signal (top) combines with a high-frequency carrier (middle) to produce a modulated signal (bottom) that can be radiated efficiently from a practical antenna.
View LargerWhat Modulation Does
When a carrier is modulated, new frequency components appear on either side of it, called sidebands. A carrier at fc modulated by a tone at fm produces components at fc + fm and fc − fm. A voice signal spanning 300 to 3,000 Hz therefore produces sidebands spanning up to 3 kHz on each side of the carrier.
This is why every modulated signal occupies a range of frequencies — a bandwidth — rather than a single point on the dial, and why channels must be spaced apart. AM broadcast stations, each carrying audio up to about 4.5–5 kHz, are spaced 9 or 10 kHz apart; FM broadcast stations, with a much wider signal, 100 or 200 kHz apart; and a Wi-Fi channel carrying hundreds of megabits per second needs 20 to 160 MHz.
Three Parameters You Can Vary: Amplitude, Frequency, Phase
A sine wave has only three properties that can be changed: its amplitude, its frequency and its phase. Every form of modulation — from the earliest broadcast stations to the most sophisticated modern data link — works by varying one or more of them.
Amplitude Modulation (AM)
The strength of the carrier rises and falls with the information signal. AM is the oldest voice modulation and is still used for medium-wave and shortwave broadcasting and for aircraft voice radio. Its simple receiver (a diode detector) is its great advantage; its weakness is that most noise and interference also affect the amplitude, so AM is noisy.
Frequency Modulation (FM)
The frequency of the carrier moves up and down with the information signal while its amplitude stays constant. FM is used for VHF broadcasting (88–108 MHz), two-way radios, wireless microphones and many data links. Because the information is in the frequency, a receiver can ignore amplitude noise, and FM gives much better audio quality than AM when signals are reasonably strong — a property called the capture effect also lets it reject weaker interfering signals.
Phase Modulation (PM)
The phase of the carrier is advanced or retarded by the information signal. Continuous phase modulation is closely related to FM. Digital phase modulation — where the phase is switched between a few fixed states to represent bits — underlies most modern digital communications: Binary and Quadrature Phase Shift Keying (BPSK, QPSK) and Quadrature Amplitude Modulation (QAM, which varies both amplitude and phase) are used in Wi-Fi, mobile phones, satellite links, digital TV and cable modems.
The three fundamental modulation types compared: AM varies the carrier amplitude, FM varies the carrier frequency, and PM varies the carrier phase. All three use the same carrier frequency but encode information differently. The top trace is the original audio modulating signal in all three cases.
View LargerModulation Types in Everyday Use
The following lessons cover AM, FM and digital modulation in detail. This table shows where the main types are found:
| Modulation | What varies | Typical bandwidth | Where you find it |
|---|---|---|---|
| AM (double sideband with carrier) | Amplitude | 9–10 kHz | Medium-wave and shortwave broadcasting, aircraft voice radio |
| SSB (single sideband) | Amplitude, one sideband only | 2.4–3 kHz | Long-distance HF voice: marine, aviation, military |
| FM (narrow-band) | Frequency | 11–16 kHz | Two-way and business radio, walkie-talkies |
| FM (wide-band, stereo) | Frequency | about 200 kHz | FM broadcasting, wireless microphones |
| OOK / ASK | Amplitude switched on and off | A few kHz | Key fobs, cheap 433 MHz remotes, RFID and contactless cards |
| FSK / GFSK | Frequency switched between values | Tens of kHz to 1–2 MHz | Bluetooth, many sensor links, older modems |
| PSK / QAM | Phase, or phase and amplitude | Depends on data rate | Wi-Fi, mobile phones, satellite and digital TV, cable modems |
| OFDM | Many PSK/QAM carriers side by side | 1–160 MHz | Wi-Fi, 4G/5G, DAB radio, digital TV |
| Chirp spread spectrum | Frequency swept up and down | 125–500 kHz | LoRa long-range low-power sensor networks |
Notice the enormous range of bandwidths — from a few kilohertz for a key fob to 160 MHz for the fastest Wi-Fi. Bandwidth is not an accident: it is set by the modulation method and by how much information must be carried each second.
How the Radio Spectrum Is Shared
Radio waves occupy frequencies from roughly 3 kHz to 300 GHz. The International Telecommunication Union (ITU) divides this range into named bands — LF, MF, HF, VHF, UHF, SHF — and allocates parts of it to services such as broadcasting, mobile phones, aviation, marine, satellites, radar and scientific research. National regulators (the FCC in the United States, Ofcom in the UK and their equivalents elsewhere) then license users within those allocations.
Some bands are set aside for licence-exempt use by low-power devices, provided they meet technical limits on power, bandwidth and spurious emissions. These are the bands used by the wireless gadgets you can build or buy: 433 MHz in Europe and many other regions, 868 MHz in Europe, 902–928 MHz in the Americas, 2.4 GHz (Wi-Fi, Bluetooth, microwave ovens) worldwide, and 5–6 GHz for Wi-Fi. Any product sold that uses them must be tested and certified to the local rules, which is one reason designers usually use pre-certified radio modules rather than designing their own transmitters.
Why Different Systems Use Different Modulation
Power Efficiency
An AM transmitter puts most of its power into the carrier, which carries no information; only a fraction goes into the sidebands. SSB transmits only one sideband, giving about a 9 dB advantage for long-distance voice. Constant-amplitude modulations (FM, FSK, GFSK) let the transmitter use an efficient non-linear amplifier, which is why battery-powered devices such as Bluetooth earbuds use them.
Noise Immunity
Most natural and electrical noise affects amplitude. Modulations that carry information in frequency or phase can ignore amplitude noise, and digital modulation with error correction can recover data perfectly from a noisy signal, up to a limit.
Bandwidth and Spectrum Efficiency
Available spectrum is finite and shared. A modulation that uses less bandwidth lets more signals share a given band. QAM packs several bits into each symbol — 256-QAM carries 8 bits per symbol — which is how Wi-Fi achieves hundreds of megabits per second in a limited channel.
Equipment Complexity and Cost
An AM receiver can be a diode, a capacitor and a pair of headphones. An OOK key-fob transmitter is a single oscillator transistor switched on and off. FM needs a discriminator or PLL; QAM and OFDM need a powerful digital signal processor. Every system chooses the simplest modulation that meets its range, data-rate and battery-life needs.
The Information Bandwidth Match
Shannon's theorem links the maximum information rate a channel can carry to its bandwidth and signal-to-noise ratio. A system that must work at very low signal levels — LoRa sensors reaching several kilometers on a coin cell, GPS signals arriving from orbit below the noise floor — trades data rate for range, sending only a few bits per second or spreading them over a wide band. A system with strong signals and plenty of bandwidth, such as Wi-Fi in the same room, uses dense modulation to carry as many bits as possible.
- Modulation moves information onto a high-frequency carrier so that antennas are practical and many signals can share the spectrum
- Modulation creates sidebands, so every signal occupies a bandwidth
- A carrier can be varied in amplitude, frequency or phase — every modulation uses one or more of these
- The choice of modulation trades power efficiency, noise immunity, bandwidth and complexity
Frequently Asked Questions
If we can't transmit audio frequencies directly, how do earphones and speakers work?
Earphones and speakers are not radiating radio waves; they move air. The audio signal travels to them along a wire, and the transducer converts it into sound. Wires carry audio frequencies perfectly well, because a wire guides the energy instead of trying to radiate it into space. The problem modulation solves is radiating the signal through space with no wire, which requires an antenna of a practical size and a way to keep different signals apart.
What does a receiver do to recover the original information?
It does the reverse of the transmitter, a process called demodulation. First it selects the wanted signal from all the others with filters, usually after converting it to a fixed intermediate frequency. Then a demodulator matched to the modulation extracts the information: a diode envelope detector for AM, a discriminator or PLL for FM, or a digital signal processor that measures amplitude and phase to recover the bits of a digital signal. The following lessons cover each in detail.
Why do some signals (like AM broadcasts) have a carrier in the middle, while others (like SSB) do not?
Standard AM transmits the carrier along with both sidebands so that the simplest possible receiver — a diode detector — can recover the audio. That made sense when broadcast receivers had to be cheap and simple. The carrier carries no information, though, and wastes most of the transmitter power. Single sideband (SSB) removes the carrier and one sideband, saving power and bandwidth, but the receiver must re-insert a carrier at exactly the right frequency to recover the audio, which makes it more complex.
Is there any modulation that uses zero bandwidth — a single frequency?
No. An unmodulated carrier occupies a single frequency with zero bandwidth — but it carries no information. The moment you begin varying any property of the carrier to encode information, sidebands appear and the signal occupies a bandwidth. This is a fundamental result of Fourier analysis: any signal that changes with time contains multiple frequency components. Even switching a carrier on and off creates sidebands, and switching it abruptly creates wide ones — which is why the on-off transitions of a transmitter are deliberately shaped.
Why does Wi-Fi use thousands of carriers instead of one?
Indoors, a signal reaches the receiver by many paths at once — directly and by reflections from walls, floors and furniture — each arriving a few nanoseconds later than the last. On a single very fast carrier, those echoes would smear each symbol into the next. OFDM splits the data across hundreds or thousands of slow carriers side by side, each with symbols long enough that the echoes have died away. Any carriers that are wiped out by a reflection null can be recovered by error correction. The Digital Modulation Overview lesson explains OFDM in more detail.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.