Modulation (ASK, FSK, PSK)
A carrier is a featureless sine wave until you disturb it, and there are exactly three things you can disturb: amplitude, frequency, and phase. Every scheme from a garage remote to a satellite downlink is one of those three or a mix.
Modulation is how data is written onto a radio carrier. An unmodulated carrier is a perfectly regular sine wave and carries no information at all; to send anything you must disturb one of its three properties, its amplitude, its frequency, or its phase. Every scheme you will meet is one of those, or a combination of two.
What it is
Take a carrier A * cos(2 pi f t + phi). Vary A in step with the data and you have amplitude keying: ASK in general, and OOK (on-off keying) in the degenerate two-level case where the transmitter is simply switched on and off. Vary f and you have FSK, where each symbol is a different tone. Vary phi and you have PSK, where each symbol is a different phase of the same steady tone: two phases for BPSK (one bit per symbol), four for QPSK (two bits per symbol). Vary amplitude and phase together and you get QAM, which is how you pack four or more bits into one symbol.
Why it matters
The first question about an unknown signal is always which family it belongs to, because the answer decides which demodulator you point at it and whether a replay will work at all. It also explains a lot of protocol design: cheap sub-GHz remotes use OOK because the transmitter is one switched oscillator, BLE and most sensor links use GFSK because a fade changes amplitude and FSK does not care about amplitude, and anything that needs spectral efficiency moves to PSK or QAM and pays for it with a receiver that must track phase.
How the three compare
| Family | What varies | Typical use | Weakness |
|---|---|---|---|
| ASK / OOK | amplitude | 315/433/868 MHz remotes, cheap sensors | fades and interference hit amplitude directly |
| FSK / GFSK | frequency | BLE, sub-GHz telemetry, pagers | needs more bandwidth than PSK for the same rate |
| PSK (BPSK, QPSK) | phase | satellite, ADS-B, modern links | receiver must recover the carrier phase, and phase is ambiguous |
| QAM | amplitude and phase | high-rate links | needs a much better SNR |
Reading a constellation
Plot each received symbol as a point whose position is the IQ pair at the sampling instant, and you get a constellation diagram. Ideal BPSK is two dots on the real axis, QPSK is four in a square, 16QAM is a 4 by 4 grid. Noise smears each dot into a cloud; a frequency error rotates the whole picture; a clock error smears the points radially. It is the fastest visual diagnosis in digital RF: you see at a glance both what the modulation is and how healthy the link is.
Symbol rate versus bit rate
A symbol rate (in baud) is symbols per second; a bit rate is bits per second. They are equal only when a symbol carries exactly one bit, as in OOK, 2-FSK and BPSK. QPSK carries two bits per symbol and 16QAM carries four, so the same 100 kbaud signal is 100 kbit/s in BPSK and 400 kbit/s in 16QAM. Occupied bandwidth follows the symbol rate, not the bit rate, which is exactly why higher-order modulations exist.
Pitfalls
- Baud is not bitrate. Mixing them is the vocabulary error that ruins the most analyses, and it makes every bandwidth estimate wrong by the number of bits per symbol.
- PSK phase is ambiguous. Without a known preamble or differential encoding, a BPSK demodulator can lock 180 degrees out and hand you a perfectly clean, fully inverted bitstream.
- OOK is not always amplitude only. Many cheap remotes send Manchester or PWM-coded symbols on top of OOK, so recovering the envelope is only half the job; you still have to decode the line coding.
- "It is FSK" is not enough to replay. Deviation, symbol rate, and the exact preamble all have to match before a transmitter reproduces something the receiver accepts.