Signals & Wireless (RF)
What is frequency-shift keying (FSK)?
Frequency-shift keying (FSK) sends digital data by switching a carrier between two frequencies: a lower one for 0 and a higher one for 1. The carrier's strength never matters, only its pitch, so FSK keeps working as a signal fades. A simple receiver has one tuned channel per frequency and asks which of them is hearing more.
Two notes
Here 10 kHz means 0 and 15 kHz means 1. The transmitter switches between them bit by bit and keeps the carrier's height fixed throughout.
A tank opens at one note
Each receiver channel has a parallel LC tank from the signal to ground. At its resonance a parallel tank is a very high impedance, so the signal reaches the diode behind it. Off resonance, either the coil or the capacitor has a small reactance and shunts the signal to ground. The diode then fills a hold capacitor, which lights that channel's LED.
With a 330 µH coil, 768 nF tunes the tank to 10 kHz and 341 nF to 15 kHz.
Live experiment
Two notes, two lights
TX sends 0, 1, 0: 10 kHz for a 0 and 15 kHz for a 1. ZERO's tank is tuned to 10 kHz. Pick ONE's capacitor: 768 nF makes both channels hear the 0s, 470 nF and 100 nF miss the 1, and 341 nF lights ONE for the 1 alone.
“Two notes for 0 and 1” is lesson 4 of 6 in Airwave Atoll, Signals & Wireless (RF) World 3. Free with an account. Lessons unlock in order, so you’ll do the 17 before it first.
Why two notes beat on and off
On-off keying sends a 1 as carrier and a 0 as silence, and judges each bit against a fixed threshold. If the signal fades to a quarter, a 1 can fall under the line and read as a 0. With FSK, both channels shrink together, so the louder channel is still the right one. Interference at one frequency hurts one channel, not the comparison.
Spacing and speed
The two tones must sit further apart than each tank's bandwidth, and a tank needs a few cycles to ring up and die down, so faster bit rates need tones further apart. Minimum-shift keying sets the gap to exactly half the bit rate, the smallest that keeps the two tones distinguishable. Modern receivers often replace the tanks with a phase-locked loop or a digital discriminator.
Where you'll meet it
The Bell 103 modems of the 1960s sent bits as 1070 and 1270 Hz in one direction and 2025 and 2225 Hz in the other. Caller ID still sends the number as 1200 and 2200 Hz FSK. Bluetooth uses Gaussian FSK, with the frequency changes smoothed to keep the signal narrow, and many 433 MHz sensors use FSK too.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Two notes for 0 and 1” is lesson 4 of 6 in Airwave Atoll, Signals & Wireless (RF) World 3. Free with an account. Lessons unlock in order, so you’ll do the 17 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: 341 nF. 1 ÷ (4π² × (15 kHz)² × 330 µH) ≈ 341 nF. 768 nF tunes it to 10 kHz.
- Why not “3.4 µF”? 3.4 µF is ten times too big and tunes it to under 5 kHz.
- Why not “100 nF”? 100 nF tunes it to about 27.7 kHz, well above the 1's note.
- Why not “768 nF”? 768 nF tunes 330 µH to 10 kHz, the 0's note.
Show the answer
Answer: The 0 channel is still louder than the 1 channel. Both channels shrink together, so the comparison stays right. On-off keying with a fixed threshold could misread a faded 1 as a 0.
- Why not “The bits get longer”? Fading changes the signal's strength, not its timing.
- Why not “It switches to AM”? The receiver doesn't change how it works. It compares the two channels, as always.
- Why not “Fading never happens”? Radio signals fade all the time, with distance, walls and movement. FSK is built to cope with it.
Show the answer
Answer: Both tones, so ONE lights for every bit. Without the tank there's nothing to pick a frequency: FEED passes everything and the diode rectifies it all.
- Why not “Only the 1s”? Picking out the 1s is the tank's job. Without it, nothing in the channel chooses a frequency.
- Why not “Nothing”? Without the tank nothing shunts any frequency away, so the diode gets the whole signal.
- Why not “Only the 0s”? Nothing in the channel prefers the 0s either. With no tank, every tone reaches the diode.
“Two notes for 0 and 1” is lesson 4 of 6 in Airwave Atoll, Signals & Wireless (RF) World 3. Free with an account. Lessons unlock in order, so you’ll do the 17 before it first.
Questions people ask
What is FSK modulation?
A way of sending digital data by changing a carrier's frequency: one frequency for a 0 and another for a 1. The receiver decides each bit by which frequency is present, not by how strong the signal is.
What is the difference between FSK and ASK?
ASK, amplitude-shift keying, changes the carrier's strength; its simplest form, on-off keying, switches it on and off. FSK keeps the strength constant and changes the frequency, which makes it far less sensitive to fading and noise.
What is the difference between FSK and FM?
FM varies the carrier's frequency smoothly with an analog signal such as audio. FSK is FM with only two, or a few, fixed frequencies, one per symbol, carrying digital data.
Related topics
- Amplitude modulation (AM) explainedTurn a carrier's height up and down with a voice: modulation depth, sidebands, and why past 100% it tears.
- LC resonant frequency explainedA coil and a capacitor swapping energy at one frequency: f₀ = 1 ÷ (2π√(LC)), tanks and tuning.
- Q factor and bandwidth: how wide a tuned circuit passesL × C sets where the band sits; L against R sets how wide. Wide enough for the voice, no wider.
- Decibels explaineddB as a ratio on a log scale: 10 log for power, 20 log for voltage, and why gains just add.