Concept Notes
1. What is Modulation?
- Modulation is the process of varying a carrier wave parameter (amplitude, frequency, or phase) according to the message signal.
- In PM (Phase Modulation), the phase of carrier is varied proportional to the instantaneous amplitude of the modulating signal.
2. Phase Modulation (PM) Definition
- The instantaneous phase of carrier signal is varied linearly with the modulating signal.
Carrier signal:
PM signal:
Where,
= carrier amplitude
= carrier angular frequency
= phase sensitivity (radians/volt)
= message signal
3. Phase Deviation
- The maximum change in phase due to modulation is phase deviation.
Where 
= peak amplitude of message signal.
4. Bandwidth of PM Signal
Using Carson’s Rule:
But in PM, frequency deviation depends on modulating frequency:
So, bandwidth of PM is generally larger than FM for high frequencies.
5. Relation between PM & FM
- Both are angle modulation techniques.
- FM: frequency varies with message signal.
- PM: phase varies with message signal.
Conversion:
- PM can be obtained from FM by integrating the message signal.
- FM can be obtained from PM by differentiating the message signal.
⚙️ Important Formulas
PM Signal:
Phase Deviation:
Instantaneous Phase:
Instantaneous Frequency (PM):
Carson’s Rule (approximate bandwidth):
🔢 Example
If 
, message signal 
.
- Maximum phase deviation:
Bandwidth (Carson’s Rule):
Message frequency 
.
So, 
🔟 10 MCQs
Q1. In PM, which parameter of carrier is varied?
a) Amplitude
b) Frequency
c) Phase
d) Both amplitude & phase
Q2. The equation of a PM signal is:
a) 
b) 
c) 
d) 
Q3. The phase deviation in PM is given by:
a) 
b) 
c) 
d) 
Q4. PM is classified as:
a) Amplitude modulation
b) Angle modulation
c) Pulse modulation
d) None
Q5. Carson’s Rule gives:
a) Power of signal
b) Bandwidth of signal
c) Phase deviation
d) Noise performance
Q6. PM and FM belong to:
a) Analog modulation
b) Digital modulation
c) Both
d) None
Q7. In PM, instantaneous frequency depends on:
a) Amplitude of m(t)
b) Derivative of m(t)
c) Integral of m(t)
d) Constant
Q8. PM can be derived from FM by:
a) Differentiation of m(t)
b) Integration of m(t)
c) Using AM techniques
d) None
Q9. If 
and 
, then phase deviation is:
a) 1 rad
b) 5 rad
c) 10 rad
d) 100 rad
Q10. The bandwidth of PM signal is generally:
a) Smaller than FM
b) Equal to FM
c) Larger than FM
d) Zero
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | c |
| Q2 | c |
| Q3 | a |
| Q4 | b |
| Q5 | b |
| Q6 | a |
| Q7 | b |
| Q8 | a |
| Q9 | c |
| Q10 | c |
🧠 Explanations
- Q1: PM → carrier phase varies.
- Q2: PM signal equation = .
- Q3: Phase deviation = .
- Q4: PM is angle modulation.
- Q5: Carson’s Rule estimates bandwidth.
- Q6: FM & PM are analog modulation techniques.
- Q7: Instantaneous frequency in PM depends on derivative of m(t).
- Q8: FM ↔ PM relation: differentiation/integration of message.
- Q9:
. - Q10: PM bandwidth > FM for high f.
🎯 Motivation
Phase Modulation is the backbone of digital communication (used in PSK, 4QAM, etc.).
If you master PM basics, ECET numericals on modulation index, bandwidth, and deviation become very easy.
👉 Remember: “Strong basics in PM = confidence in advanced modulation schemes.”
📲 CTA
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