Concept Notes
1. Introduction to Amplitude Modulation (AM)
- Amplitude Modulation is a technique where the amplitude of the carrier wave is varied according to the instantaneous amplitude of the message signal while the frequency remains constant.
- Carrier:
- Message signal:
2. AM Wave Equation
The general AM wave is:
![s(t) = \left[ A_c + m(t) \right] \cos(2 \pi f_c t)](https://learnnewthings.fun/wp-content/ql-cache/quicklatex.com-30cc9f2bd52bb42f1e63bc615df929c0_l3.png "Rendered by QuickLaTeX.com")
Where:
= Carrier amplitude
= Carrier frequency- = Message signal (modulating signal)
For sinusoidal message 
, AM wave becomes:
![s(t) = A_c \left[ 1 + \frac{A_m}{A_c} \cos(2 \pi f_m t) \right] \cos(2 \pi f_c t)](https://learnnewthings.fun/wp-content/ql-cache/quicklatex.com-bbf8468cf3e8ea76eadd33ae3f2c4bfa_l3.png "Rendered by QuickLaTeX.com")
3. Modulation Index (m)
- It measures the extent of modulation:
Types:
- Under-modulation:
→ safe, no distortion - 100% modulation:
→ maximum amplitude without distortion - Over-modulation:
→ distortion occurs
4. AM Wave Components
- Carrier:
- Upper sideband (USB):
- Lower sideband (LSB):
Total AM power:
Where:
→ carrier power
→ modulation index
5. Advantages of AM
- Simple generation & demodulation
- Compatible with existing broadcast systems
Disadvantages:
- Low power efficiency (carrier consumes 2/3 of power at 100% modulation)
- Susceptible to noise
⚙️ Formulas
AM Wave:
![s(t) = A_c \left[ 1 + m \cos(2 \pi f_m t) \right] \cos(2 \pi f_c t)](https://learnnewthings.fun/wp-content/ql-cache/quicklatex.com-2b735742b4a9a57dcf8b1cad23bcaf32_l3.png "Rendered by QuickLaTeX.com")
Modulation Index:
AM Power:
Carrier Power:
Sideband Power:
Total AM Power:
🔟 10 MCQs
Q1. In AM, the frequency of the carrier:
a) Varies with message signal
b) Remains constant
c) Becomes zero
d) Increases linearly
Q2. The modulation index is:
a) 
b) 
c) 
d) 
Q3. Total AM power for 100% modulation is:
a) 
b) 
c) 
d) 
Q4. AM wave contains:
a) Only carrier
b) Carrier + 1 sideband
c) Carrier + 2 sidebands
d) Only sidebands
Q5. For over-modulation:
a)
b)
c) 
d) 
Q6. Upper sideband frequency:
a) 
b) 
c) 
d) 
Q7. AM is most susceptible to:
a) Amplitude noise
b) Phase noise
c) Frequency variation
d) Thermal noise
Q8. Carrier power:
a) 
b) 
c) 
d) 
Q9. Sideband power in AM:
a)
b) 
c) 
d) 
Q10. Advantage of AM:
a) High efficiency
b) Simple generation and detection
c) Immune to noise
d) No carrier needed
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | b |
| Q2 | b |
| Q3 | a |
| Q4 | c |
| Q5 | c |
| Q6 | b |
| Q7 | a |
| Q8 | b |
| Q9 | b |
| Q10 | b |
🧠 Explanations
- Q1: Carrier frequency remains constant in AM → (b)
- Q2:
→ (b) - Q3:
- Q4: AM has carrier + 2 sidebands → (c)
- Q5: Over-modulation → → (c)
- Q6: Upper sideband = → (b)
- Q7: AM affected by amplitude noise → (a)
- Q8: Carrier power = → (b)
- Q9: Sideband power = → (b)
- Q10: AM is simple to generate & detect → (b)
🎯 Motivation / Why Practice Matters
- AM forms the basis of analog communication.
- ECET 2026 often tests: modulation index, AM power, sidebands.
- Mastering AM formulas ensures fast numerical solving and conceptual clarity in the exam.
📲 CTA
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