Why This Topic is Important for ECET
Digital communication is the backbone of modern electronics – from mobile networks to IoT devices. Amplitude Shift Keying (ASK) and Frequency Shift Keying (FSK) are the most basic digital modulation techniques. In ECET exams, questions are asked on formulas, bandwidth, bit rate, and advantages/disadvantages.
Mastering these helps in solving direct numerical problems quickly and strengthens fundamentals for advanced modulation (PSK, QAM, OFDM).
📘 Concept Notes
1. What is Digital Modulation?
- In digital modulation, a digital bitstream (0s and 1s) is transmitted by varying a property of the carrier wave (amplitude, frequency, or phase).
- Common schemes: ASK, FSK, PSK.
2. Amplitude Shift Keying (ASK)
- Definition: Amplitude of the carrier changes with data bit.
- Binary ASK (On–Off Keying):
- Bit ‘1’ → carrier present.
- Bit ‘0’ → carrier absent.
Advantages:
- Simple, low cost.
Disadvantages:
- Very noise sensitive.
- Not suitable for long-distance high-speed transmission.
Example: If data = 1010, ASK signal alternates between carrier (for 1) and no carrier (for 0).
3. Frequency Shift Keying (FSK)
- Definition: Frequency of the carrier is changed according to the bit.
- Bit ‘1’ → higher frequency f1f_1f1.
- Bit ‘0’ → lower frequency f0f_0f0.
Advantages:
- Better noise immunity than ASK.
Disadvantages:
- Requires larger bandwidth than ASK.
Example: For bitstream 101, carrier shifts between f1,f0,f1f_1, f_0, f_1f1,f0,f1.
4. Bandwidth Considerations
- ASK requires bandwidth nearly equal to bit rate.
- FSK requires bandwidth twice the frequency deviation + bit rate.
⚙️ Formulas
- Bit Rate:
ASK Bandwidth:
FSK Bandwidth (Carson’s Rule):
Frequency deviation in FSK:
Carrier Signal in ASK:
Carrier Signal in Binary FSK:
🔟 10 MCQs
Q1. ASK represents digital data by varying:
a) Frequency
b) Amplitude
c) Phase
d) Time
Q2. FSK requires bandwidth approximately equal to:
a) RbR_bRb
b) 2Δf+Rb2\Delta f + R_b2Δf+Rb
c) Δf\Delta fΔf
d) fcf_cfc
Q3. If bit rate = 2 kbps, what is ASK bandwidth?
Q4. For FSK, if f1=2100Hzf_1 = 2100 Hzf1=2100Hz and f0=1900Hzf_0 = 1900 Hzf0=1900Hz, find frequency deviation.
Q5. In binary ASK, bit ‘0’ is usually represented by:
a) Higher amplitude
b) Lower amplitude
c) No carrier
d) Double frequency
Q6. Which modulation is more immune to noise?
a) ASK
b) FSK
c) Both same
d) None
Q7. For bit rate = 3 kbps, frequency deviation = 500 Hz, find FSK bandwidth.
Q8. ASK is also called:
a) Frequency modulation
b) Amplitude modulation
c) On-Off Keying
d) Phase modulation
Q9. In FSK, two frequencies are separated by 1.2 kHz. What is the deviation?
Q10. Which digital modulation needs a wider bandwidth but gives better error performance?
a) ASK
b) FSK
c) PSK
d) AM
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | b |
| Q2 | b |
| Q3 | 2 kHz |
| Q4 | 100 Hz |
| Q5 | c |
| Q6 | b |
| Q7 | 4 kHz |
| Q8 | c |
| Q9 | 600 Hz |
| Q10 | b |
🧠 Explanations
- Q1: ASK changes amplitude of carrier → (b).
- Q2: FSK bandwidth ≈ 2Δf+Rb2Δf + R_b2Δf+Rb → (b).
- Q3: Bandwidth of ASK ≈ bit rate = 2 kHz.
- Q4: Δf = (2100–1900)/2 = 100 Hz.
- Q5: In ASK, bit 0 = no carrier → (c).
- Q6: FSK has better noise immunity than ASK → (b).
- Q7: B=2×500+3000=4000Hz B = 2×500 + 3000 = 4000 HzB=2×500+3000=4000Hz.
- Q8: ASK is also called On–Off Keying → (c).
- Q9: Δf = 1200/2 = 600 Hz.
- Q10: FSK uses more bandwidth but better error resistance → (b).
🎯 Motivation / Why Practice Matters
In ECET 2026, questions from digital modulation are scoring and formula-based.
- Practicing ASK & FSK ensures you can quickly calculate bandwidth, bit rate, frequency deviation.
- These topics connect directly to advanced topics like QPSK, OFDM, 5G.
- With speed + accuracy, you can secure marks others may leave.
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