/ˈfriːkwənsi ʃɪft ˈkiːɪŋ/
noun — "a modulation technique that encodes data by shifting the carrier frequency."
Frequency Shift Keying (FSK) is a digital modulation method in which the frequency of a carrier signal is changed to represent binary information. Unlike amplitude modulation, FSK varies only the frequency, making it more robust to amplitude noise and interference. It is widely used in low-bandwidth communication systems such as IoT devices, telemetry, caller ID transmission, and early modem technologies.
Technically, FSK assigns discrete frequencies to represent binary values: typically, one frequency (f0) represents a logical 0, and another frequency (f1) represents a logical 1. The modulating signal switches the carrier between these two frequencies in sync with the digital data. Advanced variants include Multiple Frequency Shift Keying (MFSK), where more than two frequencies encode multiple bits per symbol, increasing data throughput while maintaining error resistance. FSK can be transmitted over wired channels, RF links, or optical mediums and is often paired with error detection codes to ensure reliable reception.
Key characteristics of FSK include:
- Frequency-based encoding: information is conveyed through frequency shifts rather than amplitude changes.
- Robustness to amplitude noise: less sensitive to signal fading and interference.
- Simple demodulation: receivers detect frequency transitions to recover the digital data.
- Bandwidth requirement: determined by frequency deviation and symbol rate; wider deviations allow clearer distinction between logical states.
- Variants: Binary FSK (BFSK) for two frequencies, Multiple FSK (MFSK) for higher data rates, and Gaussian FSK (GFSK) for spectral efficiency.
In practical applications, FSK is commonly used in radio control systems, remote keyless entry, low-power sensor networks, and legacy telephone modems. For example, a remote IoT sensor may use BFSK to transmit temperature readings over a narrow RF channel: a low frequency for 0 and a higher frequency for 1. The receiver detects the frequency shifts, reconstructs the binary data, and forwards it to a processing system or cloud service.
Conceptually, FSK is like sending Morse code by switching between two tuning forks: one pitch signals a dot (0), the other a dash (1), and the listener decodes the message by recognizing the frequency changes rather than loudness.
Intuition anchor: FSK acts as a digital storyteller using frequency shifts—encoding information in “which note is played” rather than “how loud it is,” enabling reliable communication in noisy channels.