Side-by-side comparison
| Parameter | ASK | FSK |
|---|---|---|
| Modulation Parameter | Amplitude | |
| Bandwidth (for bit rate Rb) | Rb (minimum) | |
| Noise Immunity | Poor — amplitude fades easily | |
| Bit Error Rate | Highest BER at same SNR | |
| Complexity | Simplest — ON/OFF keying | |
| Power Efficiency | Low | |
| Typical Application | RFID, simple 433 MHz modules | |
| Coherent Detection Required | No |
Key differences
ASK collapses under multipath fading because any amplitude drop looks like a zero — a serious problem at 315 MHz unlicensed bands. FSK sidesteps that by shifting between two discrete frequencies (e.g., 1070 Hz and 1270 Hz in Bell 103), making it resilient to amplitude noise but hungry for bandwidth. PSK packs the best BER using phase, but demands a phase-coherent carrier reference at the receiver — hardware that ASK and non-coherent FSK skip entirely. For the same Eb/N0 of 10 dB, BPSK delivers roughly 10× lower BER than OOK-ASK.
When to use ASK
Use ASK when hardware simplicity and cost dominate the design constraint. A low-cost 433 MHz garage door remote using the PT2262 encoder IC is the textbook case.
When to use FSK
Use PSK when the channel is noisy and bandwidth is limited, and power efficiency matters. Wi-Fi 802.11b uses BPSK and QPSK on its 22 MHz channels precisely for this reason.
Recommendation
For most placement interviews and GATE problems, choose PSK when asked which scheme gives the best BER at a fixed SNR. For a simple short-range sensor link with no DSP budget, choose ASK — but never for anything safety-critical.
Exam tip: Examiners frequently ask you to sketch the PSD of each scheme and compare bandwidth: remember FSK bandwidth = 2Δf + 2Rb while BPSK bandwidth equals 2Rb — getting these formulas right earns full marks.
Interview tip: A core-company interviewer expects you to explain why BPSK outperforms OOK in terms of Euclidean distance on the constellation diagram, not just say "PSK is better."