---
title: "ASK, FSK and PSK modulation"
subject: "Data Communication"
chapter: 5
key: "semester_5/data_communication:ask-fsk-psk"
source: https://www.bctengineer.com/q/semester_5/data_communication/ask-fsk-psk
site: BCT Engineering
---

# ASK, FSK and PSK modulation

Data Communication · Chapter 5: Data Encoding and Modulation

- Asked in: 2076 Asoj, 2 marks; 2069 Chaitra, 7 marks; 2069 Asar, 3 marks (asked 3 times)
- Video: [ASK FSK PSK Modulation / Digital Modulation Techniques / Amplitude, Frequency and Phase Shift Keying](https://www.youtube.com/watch?v=01zv0thaAUw) — Engineering Made Easy
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/ask-fsk-psk
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-5

## Question

Explain ASK, FSK and PSK digital-to-analog modulation techniques with mathematical expression, example and generation circuit; compare them.

## Short answer

ASK varies amplitude (on-off keying), FSK varies frequency between two tones, PSK varies phase (0° or 180° for BPSK). PSK has best noise immunity, ASK simplest but poorest. FSK needs more bandwidth. Example: data 10110 yields different waveforms.

## Answer

These are the basic digital-to-analog modulation schemes: the binary data changes one parameter (amplitude, frequency or phase) of a sine carrier.

#### ASK (Amplitude Shift Keying)
- Expression: s(t) = A·m(t)·cos(2πfct), with m(t) = 1 for bit 1 and 0 for bit 0 (on-off keying).
- Generation: a product modulator (multiplier), or a switch that passes the carrier when the bit is 1.
- It is sensitive to noise; bandwidth B = (1 + r)·Rb.

#### FSK (Frequency Shift Keying)
- Expression: s(t) = A cos(2πf₁t) for bit 1, A cos(2πf₂t) for bit 0.
- Generation: two oscillators (f₁, f₂) with an electronic switch driven by the data, or a voltage-controlled oscillator (VCO) driven by the bit stream.
- Better noise immunity than ASK; needs more bandwidth, B = (1 + d)·Rb + (f₂ - f₁), where d is between 0 and 1.

#### PSK (Phase Shift Keying)
- Expression (BPSK): s(t) = +A cos(2πfct) for bit 1 and s(t) = -A cos(2πfct) = A cos(2πfct + π) for bit 0 (phase 0° or 180°).
- Generation: a balanced modulator (multiplier) with the carrier and the bipolar data signal (+1/-1).
- Best noise performance of the three; needs a coherent receiver (carrier recovery). QPSK sends 2 bits per symbol.

#### Example: data 1 0 1 1 0
```text
Data :  1      0      1      1      0
ASK  : /\/\/\ ______ /\/\/\ /\/\/\ ______
FSK  : /\/\/\ /-\_/-\_ /\/\/\ /\/\/\ /-\_/-\_
PSK  : /\/\/\ \/\/\/ /\/\/\ /\/\/\ \/\/\/
(FSK: more cycles for 1; PSK: 180-degree flip for 0)
```

#### Comparison
| Point | ASK | FSK | PSK |
|---|---|---|---|
| Varies | Amplitude | Frequency | Phase |
| Noise immunity | Poor | Good | Best |
| Bandwidth | Low (about Rb) | High | Low (about Rb) |
| Complexity | Simplest | Moderate | Complex (coherent detection) |
| Power efficiency | Poor | Moderate | Good |
| Use | Optical fibre, RFID | Low-speed modems | Wi-Fi, satellite, cellular |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. ASK, FSK, PSK vary respectively:
   A. Phase, amplitude, frequency
   B. Amplitude, frequency, phase
   C. Frequency, phase, amplitude
   D. Amplitude, phase, frequency
   Answer: B. By name. (Key point: Parameters)

2. Which has best noise performance?
   A. ASK
   B. FSK
   C. PSK
   D. All same
   Answer: C. ASK is poor, FSK good. (Key point: Noise)

3. BPSK represents bits by:
   A. Two frequencies
   B. Phase of 0° or 180°
   C. On/off carrier
   D. Two amplitudes
   Answer: B. +A or -A cos. (Key point: BPSK)

4. FSK can be generated by:
   A. A balanced modulator only
   B. An on-off switch
   C. A low-pass filter
   D. Two oscillators and a switch, or a VCO
   Answer: D. Frequency switching. (Key point: FSK generation)

## Related in Data Communication

- [Compare analog and digital communication](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adigital-vs-analog-comparison) — Data Communication, chapter 1, asked 4×
- [PCM and analog-to-digital conversion](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Apcm-working) — Data Communication, chapter 5, asked 4×
- [Digital vs analog spread spectrum](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-digital-vs-analog) — Data Communication, chapter 6
- [Multiplexing techniques; analog one explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Alist-mux-techniques) — Data Communication, chapter 6
- [Advantages of digital over analog systems](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adigital-vs-analog-advantages) — Data Communication, chapter 1, asked 5×
- [Define analog and digital signals](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aanalog-digital-signal-define) — Data Communication, chapter 1

## Related in other subjects

- [ASK and FSK modulators/demodulators; PSK with envelope detector](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aask-fsk-mod-demod-psk-envelope) — Communication Systems, chapter 7
- [Need for shift keying; ASK vs FSK vs PSK](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashift-keying-ask-fsk-psk) — Communication Systems, chapter 7
- [Digital vs analog modulation; GMSK](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adigital-modulation-gmsk) — Wireless Communication, chapter 4
- [Coherent ASK and PSK error; ASK needs double power](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aask-psk-error-double-power) — Communication Systems, chapter 9