Comparison Frequency modulation�and Phase modulation

Comparison Frequency modulation�and Phase modulation

1. Comparison Frequency modulation
and Phase modulation
•. Types of Modulation(Analog)
•Phase-Frequency Relationships
•FM andPM basics
•the Generation of FM andPM
•Comparing Frequency Modulation
to PhaseModulation

2. Types of Modulation(Analog)
1. Angle Modulation:
In the angle modulation, again there are two
different types of modulations.
•Frequency modulation
•Phase modulation.
2. Amplitude Modulation:

3. Angle Modulation
Gives PM andFM
d(t)
dt

i (t)
tt
i
t
and(t)

i()d

Angle
Modulation
Phase
Modulation
Frequency
Modulation
Frequency modulation and phase modulation are closelyrelated!
3

4. 
0
timet
i
i
C
dt
tt
Slope:(t) 
d(t)

Phase-Frequency Relationship When Frequency isConstant
(t) Acos((t))
(t) is generalizedangle
(t) Acos(
Ct
0 )
(t)

Ct
0
4

5. Concept of InstantaneousFrequency
(t) is generalizedangle
(t) Acos((t))
Angle
Modulation
0
(t) Acos(
Ct
0 )
(t)

Ct
0
(t)
d(t)
dt
tt
i
Slope: 
i (t)
Figure 5.1from
Lathi & Ding;
Page 253

C
timett
i
5

6. Phase Modulation(PM)
Equation(5.3b)
Lathi & Ding;
Page 254
(t) 
Ct k
pm(t) Generally we let 
0 0.

PM(t)Acos(
Ctk
pm(t))
The instantaneous angular frequency (in radians/second)is
k
m(t)

i C p C p
dt dt
(t) 
d(t)
 km
.
(t)
6
In phase modulation (PM) the instantaneous angularfrequency

i varies linearly w
.
ith the derivative of the message signal m(t)
(denoted here bym(t)).
k
p is phase-deviation (sensitivity) constant. Units: radians/volt
[Actually in radians/unit of the parameterm(t).]
Let 
0 0

7. Frequency Modulation(FM)
But in frequency modulation the instantaneous angularfrequency

i varies linearly with the modulating signalm(t),

i 
C k
fm(t)
t
)d


 





FM (t) AcosCtk
f
m(
Equation(5.5)
Lathi & Ding;
Page 254
7
k
fm())d
Ctk
f
t
(t) 
(
C
t

m()d
 
k
f is frequency-deviation (sensitivity) constant. Units:radians/volt-sec.
Then
FM and PM are very much related to each other.
In PM the angle is directly proportional tom(t).
In FM the angle is directly proportional to the integral of m(t), i.e., 
m(t)dt

8. Summary
Angle
Frequency
PhaseModulation
(t)
Ctk
pm(t)
t
(t)
Ctk
f
m()d

k
dm(t)
i C p
dt

i 
C k
fm(t)
FrequencyModulation
In phase modulation m(t) drives the variation of phase.
In frequency modulation m(t) drives the variation of frequencyf.
8
d(t)
i
(t)
dt
Definition: Instantaneous frequencyis

9. A Pictorial Way to View the Generation of FM andPM
Phase
Modulator
Frequency
Modulator
d
dt
m(t)
m(t) m(t)
..
PM
(t)
FM
(t)
t

m()d

FrequencyModulator
9
Phase Modulator
We require that H(j) be a reversible (or invertible)operation
so that m(t) isrecoverable.
H(j) =1/j
H(j) =j

10. Comparing Frequency Modulation to
PhaseModulation

11. Comparing Frequency Modulation to PhaseModulation

12. 12
# Frequency Modulation(FM) Phase Modulation(PM)
1Frequency deviation is proportional to
modulating signalm(t)
Phase deviation is proportional
to modulating signalm(t)
2Noise immunity is superior to PM (and of
courseAM)
Noise immunity better than AM
but not FM
3Signal-to-noise ratio (SNR) is better than
inPM
Signal-to-noise ratio (SNR) is
not as good as inFM
4FM is widely used for commercial
broadcast radio (88 MHz to 108MHz)
PM is primarily for some
mobile radioservices
5Modulation index is proportional to
modulating signal m(t) as well as
modulating frequencyf
m
Modulation index is
proportional to modulating
signalm(t)
Comparing Frequency Modulation to PhaseModulation

13. FM OR PM?
Both must occur whenever either form of angle modulation
isperformed.
FM PM
Instantaneous frequency of the carrier
is varied from its reference value by
an amount proportional to the
modulating signalamplitude
Freq. carrier ---> directly varied
Phase carrier ---> indirectlyvaried
Phase angle of the carrier is
varied from its reference value
by an amount proportional to
the modulating signalamplitude
Phase carrier ---> directly varied
Freq. carrier---> indirectlyvaried